Overall score
Scientific accuracy
Reference accuracy
Healthfulness
How hard would it be to apply the book's advice? Fairly difficult
The Proof is in the Plants, by Simon Hill, makes the case that a minimally-processed, plant-predominant diet helps to maintain a healthy weight and can reduce the risk of heart disease and some cancers.
Key points from our review
- The claim that plant-predominant diets can help maintain a healthy weight had the strongest evidence behind it.
- There is a moderate amount of evidence that plant-predominant diets can prevent cancer and heart disease.
- The book scored well in reference accuracy.
- The book’s diet advice is likely to be very healthy.
- Following the diet strictly would be expensive and challenging.
Bottom line
The Proof is in the Plants is a well-researched nutrition book that makes a strong, mostly evidence-based case for incorporating more whole plant foods into your diet.
Book published in 2021
Published by Penguin Life
First Edition, Paperback
Review posted November 7, 2022
Primary reviewer: Seth Yoder
Peer reviewer: Mario Kratz
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Table of contents
Introduction
The Proof is in the Plants by Simon Hill was not on our radar until recently, when its review was funded during Red Pen Reviews’ most recent funding drive.
The book recommends a whole-foods plant-based diet to maintain a healthy body weight, and to reduce the risk of disease (the text focuses on cardiovascular disease, cancer, and dementia), live longer, and protect the environment. For the sake of brevity, this review focuses on the claims about body weight, cancer, and cardiovascular disease.
The dietary pattern advanced in this book is a plant-based, whole-foods approach. The Proof is in the Plants defines plant-based rather broadly in the text to include a strict whole-foods vegan diet, but also allows for the inclusion of some processed and animal foods. This guidance could therefore also be implemented in the form of a vegetarian, pescatarian, flexitarian (mostly vegetarian, but occasionally including meat), Mediterranean, or DASH diet. The author suggests consuming at least 85% of total calories from whole plant foods.
The whole-foods aspect of the diet is not explicitly defined, but is understood to mean minimal processing, few ingredients, typically higher in fiber, and foods closer to their natural state.
Although a wide variety of eating patterns may be consistent with the book’s recommendations, the ideal diet outlined in the book is a minimally processed vegan diet. That said, The Proof is in the Plants isn’t very dogmatic about how far the reader takes this, as long as they eat more whole plants and less meat and other animal foods. The book suggests that eating mussels and oysters may be acceptable to some in an otherwise vegan diet.
Scientific Accuracy
The Proof is in the Plants makes several claims about how plant-based diets impact health: how they help maintain a healthy body weight, reduce the risk of cardiovascular disease and cancer, protect brain health, and enable a longer life span. The Proof is in the Plants also discusses how diet impacts environmental health. Each of these benefits is outlined to make the argument for a whole foods, plant-based diet. We chose to examine the first three claims related to body weight, cardiovascular disease, and cancer more closely in this review.
In general, all three claims fared well in scoring. The first claim that a plant-based diet is an effective way to maintain a healthy weight had the strongest scientific support, and therefore scored the highest. The next two claims (cardiovascular disease and cancer) still scored well, but are not as strongly supported by evidence as the first claim.
There are certainly studies showing that plant based diets are associated with a lower risk of cardiovascular diseases or cancer, but others fail to support this. Although in most studies, even in those that don’t show a statistically significant association or effect, there’s usually a trend suggesting that plant-based diets are beneficial.
The claims made in the text were not wildly overstated or exaggerated. The language used to describe the evidence for the claims was appropriate, and the supporting evidence used was from reputable sources, such as peer-reviewed scientific journals or respected institutions like the CDC or government health statistics.
Claim 1
A whole-foods, plant-based diet is an effective way to lose weight or maintain a normal weight.
Supporting quote(s) and page number(s)
Page 78: “If weight loss or maintenance is your goal, the evidence across all levels of science supports a diet rich in whole plant foods that minimises calories from ultra-processed foods, alcohol and animal foods, as you’ll see in this chapter.”
Page 79: “The primary mechanism behind how a plant-based diet works for weight loss is increased satisfaction with fewer calories.”
Page 89: “What’s clear is that a diet centred on plant-based foods in their whole or minimally-processed form is one that lends itself to a healthy body weight.”
Page 100: “Ultimately, if we want to lose weight it’s clear there is a powerful advantage to eating more whole plants – they keep us full with fewer calories […]”
Criterion 1.1. How well is the claim supported by current evidence?
4 out of 4
This claim received a score of 4, indicating that the claim is strongly supported by current evidence.
The first claim we address is that a whole-foods, plant-based diet is effective for losing weight. It might be worth noting that The Proof is in the Plants does not claim that this kind of diet is the only way to lose weight. In fact, the text states on page 77 that “it’s possible to lose weight with many different diets. Any diet that helps you consume fewer calories than you expend will promote weight loss, but not all diets are necessarily health-promoting or sustainable in the long-term.” The Proof is in the Plants does not even explicitly state that it’s the best diet for losing or maintaining weight, although that is clearly the implication since the entire focus of the book is on a plant-based diet. Instead, statements about diet are usually framed in terms of plant-based diets having an “advantage” or lending itself to a healthy body weight. The strongest statement made might be that “science supports” such a diet for weight loss or maintenance.
In support of the suggestion to prefer whole-foods over highly processed foods, The Proof is in the Plants prominently refers to a widely cited trial by Dr. Kevin Hall and colleagues comparing diets of ultra-processed foods and unprocessed foods. The results of the study demonstrate that participants consumed about 500 kcal more per day, and gained a small amount of weight while on the ultra-processed diet for two weeks. (It may be worth noting that both diets in this study contained meats.) Indeed, it appears that those consuming a diet high in ultra-processed foods are at a greater risk of overweight and obesity. A 2020 systematic review and meta-analysis concluded that those that consume ultra-processed foods are at a 40% greater risk for having a high waist circumference and of being overweight compared to those that consume the least processed foods. A similar meta-analysis published at the same time that included more studies also showed a positive association between ultra-processed food and overweight and obesity.
With regard to the plant-based portion of the claim in the book, the text cites several observational studies showing that those following a plant-based diet have lower BMIs than non-vegetarians: Adventist Health Study-2, EPIC-Oxford study, the Heidelberg cohort, and Flemish vegetarians. The results of these studies do indicate that those following plant-based diets, such as veganism or lacto-ovo vegetarianism, have lower BMI than do non-vegetarians. However, the BMI data from the Heidelberg study cited in The Proof is in the Plants may not be very strong as the BMI data was calculated from self-reported height and weight, and the study itself was focused on mortality and not weight or BMI. The book also uses BMI data from a 1999 publication analyzing other observational studies such as an earlier Adventist study and an Oxford vegetarian study. In each case the study publication is focused on mortality rather than weight and states that BMI tended to be lower in vegetarians versus nonvegetarians, but it’s not clear if any kind of statistical analysis was applied to the BMI data in these studies directly comparing these two groups. In other words, these latter studies were probably not the most robust from which to make conclusions about BMI.
Nevertheless, The Proof is in the Plants does go on to mention a 2016 meta-analysis of RCTs that provides strong evidence that vegetarian diets, and vegan diets in particular, are beneficial for weight loss. Additionally, a 2022 meta-analysis of randomized controlled trials (RCTs) with participants having type 2 diabetes lasting between 2 and 24 months indicates that those following a plant-based diet during the trials significantly reduced their body weight, BMI, and waist circumference. In contrast, a more recent meta-analysis was performed involving the connection between body composition (waist circumference and BMI) and a plant-based diet. This meta-analysis did not include randomized controlled trials, but rather cross-sectional studies and cohort studies, which are less useful at demonstrating a cause-effect relationship. The study showed a trend toward a lower BMI and lower waist circumference for those that ate diets higher on the “plant-based diet index” but the results were not statistically significant. (The higher the plant-based diet index score, the more closely it conforms to a whole-foods, plant-based diet.)
Relatedly, a recent randomized controlled trial was published in the Journal of the American College of Nutrition where participants were assigned to either a low-fat vegan diet or a Mediterranean diet. No limits were placed on how much they could eat, but the participants assigned to the low-fat vegan diet ended up eating less calories than the Mediterranean group and ended up losing more weight. The Proof is in the Plants doesn’t advocate for a low-fat vegan diet or counting/restricting calories. Rather, the text states that characteristics of a whole-foods, plant-based diet such as higher fiber content and lower energy density compared to diets containing animal products and ultra-processed foods will spontaneously help adherents feel more satisfied with fewer calories.
In summation, there is a small amount of evidence that doesn’t support the contention that consuming a whole foods, plant-based diet will promote a healthy body weight, but that evidence is weak. Based on this review, the strongest evidence that is currently available supports this book’s claim.
Criterion 1.2. Are the references cited in the book to support the claim convincing?
4 out of 4
The book’s references received a score of 4, indicating that references are very convincing. The text cites a variety of study types (randomized controlled trials, meta-analyses, government reports, cohorts) from a variety of reputable, peer-reviewed journals.
Criterion 1.3. How well does the strength of the claim line up with the strength of the evidence?
4 out of 4
The claim received a score of 4, indicating that the strength of the claim aligns well with strength of the evidence.
Although the position of the text is clearly in favor of plant-based diets, the actual claims that are made are never overstepping the strength of the scientific evidence. The book is also noteworthy for providing a balanced view of weight loss diets, explicitly stating that many diets can lead to weight loss.
Overall (average) score for claim 1
4 out of 4
Claim 2
A whole-foods, plant-based diet is beneficial for preventing or slowing the progression of cardiovascular disease.
Supporting quote(s) and page number(s)
Page 104: “The research, as you’ll see, is clear: if you want to avoid developing [cardiovascular] disease in the first place or you want to stop or slow the progression of the disease if you have it already, you’ll want to adopt an anti-inflammatory diet that’s low in saturated and trans fats, cholesterol, refined carbohydrates, salt and heme iron, and rich in unrefined carbohydrates, fibre, unsaturated fats, plant protein and antioxidants. In other words, a plant-predominant or plant-exclusive diet based on whole foods.”
Page 136: “Several large observational studies and meta-analyses have identified that animal protein itself, independent of the saturated fats that it comes packaged with, appears to heighten the risk of dying from cardiovascular disease.”
Page 136: “While a plant-based diet is naturally low in saturated fats, dietary cholesterol, trans fat, refined carbohydrates, sodium and heme iron – dietary components that we’ve seen increase CVD risk – plant foods have a few more tricks up their sleeve.”
Page 137: “Plant-based foods contain a high number of the antioxidant molecules vitamin E, vitamin C, beta-carotene, lycopene and polyphenols, which are thought to prevent the damage that leads to atherosclerosis.”
Criterion 1.1. How well is the claim supported by current evidence?
3 out of 4
This item received a score of 3, indicating moderate support for the claim. The evidence for the claim that a plant-based diet protects against heart disease is a bit murkier than that for weight loss because cardiovascular disease is a more complex condition that requires long-term trials to obtain conclusive evidence.
The Cochrane Collaboration doesn’t have a review on whole food, plant-based diets and the prevention of cardiovascular disease, but they do have one on vegan diets and cardiovascular disease, which is pretty close. The Cochrane Review states that there just is not enough evidence available to make a conclusion because there are no clinical trials that look at clinical outcomes of cardiovascular disease (for example: heart attack, stroke, death). Information on vegan diets and CVD risk factors exist from short and medium-term trials, but the authors state the results are mixed. There is a Cochrane review on the effects of increased fruit and vegetable consumption for the prevention of CVD, which states “[t]he limited evidence suggests advice to increase fruit and vegetables as a single intervention has favourable effects on CVD risk factors but more trials are needed to confirm this.” Additionally, there is another review on saturated fat consumption and CVD which states that reducing saturated fat intake will reduce the risk of cardiovascular disease. Of course, these reviews are not of a particular diet, but high consumption of fruits and vegetables and limited consumption of saturated fats are characteristics of the plant-based diet recommended in the book.
Let’s begin with the clinical trial evidence in favor of the claim. One dietary pattern that’s mentioned in the book is the Portfolio Diet, which is a diet designed to lower cholesterol levels by promoting consumption of a “portfolio” of cholesterol-lowering foods like nuts, fiber, soy protein, and plant sterols. It’s not an explicitly vegetarian or vegan diet, but it shares numerous characteristics with the recommended whole-foods, plant-based diet. A meta-analysis of seven trials concluded that the Portfolio diet was effective at improving risk factors for heart disease, such as lowering LDL cholesterol, total cholesterol, apolipoprotein B, etc. Most of these trials only lasted for four weeks, which is clearly not enough time to see if heart disease develops or not, which is why these trials (and most others) only measure risk factors for heart disease, such as blood lipids, blood pressure, biomarkers of inflammation, body weight, and similar measurements. Another meta-analysis examining trials of vegan diets lasting longer than 12 weeks had mixed effects on cardiovascular disease risk factors: vegan diets significantly reduced body weight, BMI, glycated hemoglobin, LDL, and total cholesterol; however, vegan diets did not meaningfully change blood pressure, triglycerides, or HDL cholesterol. In something of a confirmation of this, another meta-analysis focusing on diets and blood pressure indicated a strict vegan diet did not significantly reduce blood pressure. Interestingly, vegetarian diets that included eggs and dairy did produce a significant reduction in blood pressure, along with the DASH diet and the Healthy Nordic diet. The latter two diets are certainly not plant exclusive, but do emphasize fruits, vegetables, and whole grains, with the Nordic diet also placing an emphasis on fatty fish.
An additional trial that may be of interest is a six month trial comparing a control diet against a mediterranean diet and a “green” mediterranean diet with less meat, more plants, green tea, and a daily green protein shake called Mankai. The green Mediterranean diet performed similarly to the Mediterranean diet in terms of weight loss; however, the green diet achieved greater improvement in cardiovascular risk factors and therefore had a better Cardiovascular Risk Score than the traditional mediterranean diet. One aspect to note is the inclusion of a green protein powder in the trial, which probably cannot be called a “whole food” considering all the processing that surely is involved in creating this product.
Overall, it’s therefore important to note that no RCT thus far has specifically tested the impact of the recommended whole-food plant-based diet on incident CVD, or CVD risk factors. Nevertheless, the available data from intervention studies do moderately support the claim made.
Moving on to observational data, a recent meta-analysis indicated that a vegetarian diet was associated with a lower risk of ischemic heart disease mortality compared to a non-vegetarian diet, although no benefit was found with respect to cerebrovascular diseases. Similarly, another meta-analysis focusing on risks of strokes with vegetarians found no benefit with the diet. Again, we note that these diets are not identical to the one recommended in the book.
A 2021 meta-analysis of prospective cohort studies published in Frontiers in Cardiovascular Medicine had some interesting, if somewhat muddled, results. That study indicated that compared to meat-eaters, those following a vegetarian diet did not have a reduced risk of death from cardiovascular disease, but there was a reduced risk of a cardiovascular disease event among vegetarians. In other words, vegetarians are less likely to develop cardiovascular disease than meat eaters, but if CVD is established both vegetarians and meat-eaters have about the same chance of dying from it. The study also found that unhealthful plant-based diets (i.e. those with more sugar and less fiber) were associated with increased risk of death from CVD, but not with simply having CVD. Conversely, healthy plant-based diets were associated with decreased CVD incidence, but not death from CVD. Comparable findings were reported in a very similar meta-analysis published that year in the journal Nutrients.
There is not a lot of information that directly examines the “whole foods” aspect of the diet and cardiovascular disease. However, there is one meta-analysis that does investigate whole grain consumption and cardiovascular disease risk. The results from that analysis state that whole grain consumption is associated with lower body weight and lower C-reactive protein (a measure of inflammation and a risk factor for heart disease), but no significant differences in cholesterol, blood pressure, and other risk factors. However, if we examine the other side of the processing spectrum and look at the CVD outcomes of consuming ultra-processed foods we find that these types of foods are clearly associated with an increased CVD risk. A couple of recent meta-analyses of epidemiological studies demonstrate that consumption of ultra-processed foods increase a range of poor health outcomes, including death from heart disease.
A meta-analysis of prospective trials indicated that replacing red meat with other protein sources such as poultry, dairy, eggs, nuts, or legumes reduced the risk of coronary heart disease. This was more an indictment of red meat than anything since it showed that poultry or eggs can also lower risk compared to red meat; however, the furthest risk reduction of these alternative protein sources were seen with the plant-based protein sources nuts and legumes.
Taken together, there is no conclusive direct evidence that the recommended whole-food plant-based diet prevents cardiovascular disease, simply because no study has specifically investigated this dietary pattern. However, numerous lines of evidence suggest that the recommended whole-food plant-based diet is very likely to be beneficial for cardiovascular health. These include the fact that the recommended diet is low in saturated fat, dietary cholesterol, and trans fatty acids, which strong evidence has linked to elevated LDL-cholesterol concentrations. The recommended diet would also be very high in fruit and vegetables, leading to a high fiber and phytochemical intake, which are associated with a reduced risk of CVD in observational studies. Similarly, the focus on sources of mono- and polyunsaturated fatty acids is likely beneficial, as suggested by associations with reduced CVD risk in the observational study literature. Therefore, we scored this claim 3 out of 4, indicating moderate support in the literature for the claim.
Criterion 1.2. Are the references cited in the book to support the claim convincing?
4 out of 4
The book’s references received a score of 4, indicating that references are very convincing. Again, The Proof is in the Plants cites reputable, peer-reviewed journals to support the claims within the text.
Criterion 1.3. How well does the strength of the claim line up with the strength of the evidence?
4 out of 4
The claim received a score of 4, indicating that the strength of the claim aligns well with the strength of the evidence. The Proof is in the Plants is generally careful in the language that is used and refrains from exaggerating or misrepresenting the evidence that is presented. Broadly, the evidence that is presented is aligned with the statements that are made in the text.
Overall (average) score for claim 2
3.7 out of 4
Claim 3
Whole foods, plant-based diets can reduce your risk of some types of cancer.
Supporting quote(s) and page number(s)
Page 45: “[T]he research shows that eating more whole grains, fruits, legumes, vegetables, nuts and seeds and eating less animal protein and highly processed foods is associated with lower risk of developing chronic diseases, such as cardiovascular disease, type 2 diabetes and colorectal cancer, and is a clear commonality among people who live long, healthy, relatively disease-free lives.”
Page 68: “You’ll soon see how plants can prevent, slow or stop the progression of, and in some cases reverse, chronic disease: obesity, type 2 diabetes, heart disease, cancer and dementia.”
Page 141: “There is evidence to suggest that regular consumption of processed meat, red meat, ultra-processed foods and excessive salt is associated with a higher risk of developing various types of cancer. […] On the other hand, whole plant foods, and specific compounds they contain, have been consistently shown to have anti-carcinogenic effects – that is, they protect against certain forms of cancer.”
Page 153: “There are mountains of evidence supporting consumption of many different whole plant foods to reduce the risk of various forms of cancer.”
Page 156: “[P]erhaps you’re wondering if people who adopt vegetarian and vegan diets fare better when it comes to cancer. In short, yes they do.”
Criterion 1.1. How well is the claim supported by current evidence?
3 out of 4
This item received a score of 3, indicating moderate support for the claim. Again, evidence for this particular claim is not exactly definitive, but a substantial body of evidence suggests that the claim is likely correct.
This is another one of those areas of nutrition where it’s very difficult, if not downright impossible, to conduct randomized clinical trials that examine the link between diet and the development of cancer. Cancer, much like heart disease, is a malady that takes many years to develop. Therefore, much of what we know about the link between diet and cancer comes from epidemiological studies, animal studies, and shorter-term trials assessing dietary impacts on risk factors.
Let’s begin with what might be the most comprehensive aggregation of evidence on lifestyle (including diet) and cancer, known as the Third Expert’s Report, published in 2018. The findings of this report are aligned with the main thrust of this claim. The report states that the evidence that processed meat causes colorectal cancer is convincing, while the link between red meat and colorectal cancer risk is considered probable. Meanwhile, those foods that probably decrease risk of cancer include whole grains (for colorectal cancer), foods containing fiber (for colorectal cancer), and non-starchy vegetables and fruits (for digestive cancers). Dairy consumption was also weakly associated with prostate cancer. However, it is worth pointing out that the report noted dairy was also found to probably decrease colorectal cancer risk. This is mentioned in The Proof is in the Plants on page 146 “[T]here is strong evidence to suggest that dairy consumption reduces the risk of developing colorectal cancer and perhaps even premenopausal breast cancer.” The report also found limited evidence that red and processed meats lead to other cancers, and also that non-starchy fruits and vegetables might reduce the risk of other cancers besides colorectal.
If we consider the components of any plant-based diet, it would almost certainly include a fair amount of phytochemicals that have bioactive properties, such as carotenoids, lignans, isothiocyanates, circumin, and others. There is evidence that, for example, lignans can play a role in the prevention of breast cancer or colon cancer, and curcumin may help reduce the risk of various cancers (particularly breast cancer). The Proof is in the Plants mentions the anti-cancer properties of the polyphenols in green tea and coffee. The major limitation to a lot of this research is that it is conducted on animal models or cell cultures; high-quality clinical trials on these dietary components is minimal to non-existent.
Most of the available evidence on the issue of plant-based dietary patterns and cancer come from epidemiological evidence. One observational study referred to in The Proof is in the Plants on pages 148 and 156 is the Adventist Health Study-2 where it is mentioned that vegan men demonstrated a lower risk of prostate cancer and vegetarian women demonstrated a lower risk for female-specific cancers, such as breast, uterine, and ovarian). However, a systematic review of several observational trials and some controlled trials (most lasting a few months and examining a biomarker, prostate-specific antigen) revealed mixed results when it comes to the link between plant-based diets and prostate cancer. Many studies included in the review showed beneficial effects of a plant-based diet on prostate cancer risk, but several studies also showed no benefit over a nonvegetarian diet.
Several meta-analyses have been published on the topic of plant-based diets and cancer. Common among all of the studies is that plant-based diets tend to have a protective effect; however, in most cases the findings are not statistically significant. For example, a 2017 meta-analysis on vegetarian and vegan diets and multiple health outcomes indicated that both vegetarian and vegan diets significantly reduced the risk of the incidence of cancer generally. But when evaluating these diets against incidence of specific cancers such as breast, colon, lung, or prostate the results were not statistically significant. Another meta-analysis published that same year looking specifically at prostate, breast, and colon cancers concluded that pescatarian diets (vegetarian diet plus fish) and semi-vegetarian diets (very low meat consumption) were protective against colon cancer but not prostate cancer or breast cancer. Interestingly, the same study showed stricter vegetarian diets offered no significant protection against any of the three cancers.
In more recent studies, a 2022 meta-analysis of observational studies focusing on cancers of the digestive system found that plant-based diets reduced the risk of several types of cancers (including pancreatic, colon, and rectal). The analysis included anything from strict vegan diets without any animal product to diets that included dairy, eggs, fish, and occasionally other meats as plant-based diets. In something of a contrast, a large prospective cohort study from the UK indicated that vegetarian diets and pescatarian diets were both protective against the development of cancer overall, but when specifically examining breast, colon, lung, and prostate cancers there was no significant protective effect, although a trend toward these plant-based diets being beneficial was clear.
What about your diet after a cancer diagnosis? Will eating a plant-based diet help improve your chances of survival? According to a 2022 review of the topic the evidence is mixed, however, plant-based diets tended to improve cancer prognosis, particularly “healthy” or unprocessed plant-based diets. That is, diets containing whole grains, nuts, and fiber. Plant-based diets containing a high intake of refined grains or fruit juice fared poorer.
Criterion 1.2. Are the references cited in the book to support the claim convincing?
4 out of 4
This item received a score of 4, indicating the references are very convincing. The Proof is in the Plants cites many studies to support its claims in the text that are either from reputable scientific journals or reputable institutions, such as the CDC or the WHO.
Criterion 1.3. How well does the strength of the claim line up with the strength of the evidence?
4 out of 4
The claim received a score of 4, indicating that the strength of the claim aligns well with strength of the evidence. The Proof is in the Plants doesn’t really overstate the connection between plant-based diets and cancer, using precise language when describing the results of studies (such as “associated with lower risk of developing these diseases”). The text also mentions some of the evidence that might contradict the main claim, such as when it is mentioned that dairy consumption may in fact reduce the risk of developing breast or colorectal cancer.
Overall (average) score for claim 3
3.7 out of 4
Overall (average) score for scientific accuracy
3.8 out of 4
Reference Accuracy
In general, The Proof is in the Plants fared reasonably well in reference accuracy. Most references provided clear and strong support for the statements in the text. Two references out of ten were mostly irrelevant to what was being discussed in the text, which brought down the score. There were also cases where the source didn’t fully support the book’s claim but these were in the minority.
Reference 1
Reference
Chapter 7, reference 2. Dementia Australia. What is dementia? Dementia Australia.
Associated quote(s) and page number(s)
Page 161: “People with dementia may experience difficulty with memory, comprehension, calculation, learning capacity, judgement, language and performing everyday tasks, and may become confused or experience personality change.”
Criterion 2.1. Does the reference support the claim?
4 out of 4
This reference received a score of 4, indicating that it offers strong support for the claim. The statement from the chapter cites this and another text from the WHO as evidence for the claim. The Dementia Australia fact sheet explicitly supports everything listed with the exception of perhaps calculation and learning capacity, which are only implicit in the text. However, the additional text from the WHO explicitly mentions calculation and learning capacity in their section on dementia.
Reference 2
Reference
Chapter 4, reference 46. Mozaffarian, D. and Ludwig, D. S. (2015). The 2015 US dietary guidelines: Lifting the ban on total dietary fat. JAMA, 313(24), 2421–2.
Associated quote(s) and page number(s)
Page 84: The Proof is in the Plants mentions that we have increased our energy consumption since the 1980s by 200-450 calories, comparing the increase in calories with an extra cheeseburger and soda every day. Then states “It’s foods like these, along with others rich in heavily refined carbohydrates, such as desserts, pasta, tacos, and alcohol – foods that we know don’t adequately satisfy our hunger – that make up these increased calories.”
Criterion 2.1. Does the reference support the claim?
2 out of 4
This reference received a score of 2, indicating that it offers weak support for the claim. The article itself is almost irrelevant to the above quote, with the exception of a couple of statements, such as the following: “[R]efined grains continue to represent the largest category of calories in the US food supply, including white bread, white rice, and most chips, crackers, cereals, and bakery desserts.” But the cited text is not a research article but an opinion piece (JAMA calls them Viewpoints), and the bulk of the article is essentially an argument that the USDA should remove the limits on total fat in the 2015 Dietary Guidelines.
Of note, the book cites three other papers as well in support of this statement, and they may well provide support for the statement. Independent of this, however, the citation evaluated here does not provide support of the statement, and should therefore not have been cited here.
Reference 3
Reference
Chapter 1, reference 86. King, L., Hebden, L., Grunseit, A., Kelly, B. and Chapman, K. (2013). Building the case for independent monitoring of food advertising on Australian television. Public Health Nutrition, 16(12), 2249–54.
Associated quote(s) and page number(s)
Page 36-37: Following a discussion on industry self-regulation, The Proof is in the Plants mentions the Responsible Children’s Marketing Initiative code developed by the Australian Food and Beverage Industry and states “Despite this particular self-regulated code, and others existing in Australia, multiple studies have identified that marketing to children on TV remains largely unchanged.”
Criterion 2.1. Does the reference support the claim?
4 out of 4
This reference received a score of 4, indicating that it offers strong support for the claim. The cited text, along with another study cited with this statement do indicate that advertising has remained unchanged between the time periods that were examined.
Reference 4
Reference
Chapter 9, reference 96. Stokstad, E. Fishing fleets have doubled since 1950 – but they’re having a harder time catching fish. Science. 27 May 2019.
Associated quote(s) and page number(s)
Page 225: “Thanks to a doubling of fishing fleets since 1950, humans have been taking out many more fish than those that remain can replace […]”
Criterion 2.1. Does the reference support the claim?
3 out of 4
This reference received a score of 3, indicating that it offers moderate support for the claim. The cited source is a Science magazine article, itself a commentary on a research article published in PNAS. The Science article does state that the PNAS article found that fishing vessels have more than doubled from 1950 to 2015. However, the article does not fully support the claim that humans have been taking more fish out than can be replaced. The article does state that 20% less fish are being caught with the same effort expended in 1950 and does express some concern about fisheries management if current trends continue, but also quotes a fisheries ecologist who says that “We haven’t reached the peak of intensive fishing.”
Reference 5
Reference
Associated quote(s) and page number(s)
Page 139: After discussing some research suggesting that vegans and vegetarians have a lower risk of dying from heart disease, The Proof is in the Plants states “More recently a new meta-analysis has been performed with updated data from almost 250,000 subjects, again finding that vegetarians, including vegans, have better cardiovascular health – this time a 25% lower risk of developing and dying from heart disease.”
Criterion 2.1. Does the reference support the claim?
4 out of 4
This reference received a score of 4, indicating that it offers strong support for the claim. The cited study is a meta-analysis of several observational studies on the topic of vegetarianism and risk of chronic diseases and mortality. The results showed a 25% lower risk of ischemic heart disease among vegetarians compared to nonvegetarians.
Reference 6
Reference
Chapter 8, reference 68. Prasanth, M. I., Sivamaruthi, B. S., Chaiyasut, C. and Tencomnao, T. (2019). A review of the role of green tea (Camellia sinensis) in antiphotoaging, stress resistance, neuroprotection, and autophagy. Nutrients, 11(2), 474.
Associated quote(s) and page number(s)
Page 188: “If you’re not a fan of coffee, the good news is that polyphenols in tea have been shown to induce autophagy.”
Criterion 2.1. Does the reference support the claim?
2 out of 4
This reference received a score of 2, indicating that it offers weak support for the claim. The cited text is a review article examining what we know of the protective properties of green tea. The article does mention several studies that indicate that compounds in green tea contribute to autophagy, which certainly supports the claim in The Proof is in the Plants. However, it should also be noted that the studies mentioned in this review article are largely cell and tissue studies with one study discussed involving rats. The quote from the book doesn’t explicitly mention autophagy in rats or humans or isolated tissues, but although mice are mentioned prior to this quote, it seems reasonable that the expectation is that drinking tea will induce autophagy in a human body which is not conclusively supported by this review article.
Reference 7
Reference
Part 1, reference 5. Australian Bureau of Statistics. Media release: Consumption of added sugars exceeds recommendations. 27 April 2016.
Associated quote(s) and page number(s)
Page 20: “It is no wonder then that most of us consume more saturated fats and salt than the national recommendations and 50% of us exceed the World Health Organization’s recommendation for added (or ‘free’) sugars.”
Criterion 2.1. Does the reference support the claim?
4 out of 4
This reference received a score of 4, indicating that it offers strong support for the claim. The cited source is an Australian government survey stating that 52% of Australians exceed the WHO recommendation “free sugars” make up less than 10% of caloric intake. The first part of the claim in the quoted sentence was addressed in a previous reference.
Reference 8
Reference
Chapter 7, reference 29. Green, R. C. et al. (2003). Depression as a risk factor for Alzheimer disease: The MIRAGE Study. Archives of Neurology, 60(5), 753–9.
Associated quote(s) and page number(s)
Page 164: [As part of a discussion on risk factors for dementia] “Other risk factors which should be considered as part of a holistic approach to reducing the chance of developing dementia are: depression, sleep deprivation, chronic stress, excessive alcohol consumption, and physical inactivity, as well as traumatic brain injury, such as could be developed by playing contact sports.”
Criterion 2.1. Does the reference support the claim?
4 out of 4
This reference received a score of 4, indicating that it offers strong support for the claim. This reference is one of several cited sources that The Proof is in the Plants uses for this statement. This reference in particular deals with the depression risk factor. It is a cross-sectional study that examines depression as a risk factor for the most common type of dementia: Alzheimer’s disease. Depression symptoms were significantly associated with the disease.
Reference 9
Reference
Chapter 4, reference 103. Uribarri, J. et al. (2015). Dietary advanced glycation end products and their role in health and disease. Advances in Nutrition, 6(4), 461–73.
Associated quote(s) and page number(s)
Page 97: [As part of a larger discussion on diabetes and weight] “Various studies have also implicated animal protein, heme iron (top sources in a typical Western diet include red meat and poultry), advanced glycation end products (AGEs, which occur in high levels in cooked meat), isolated fructose and sucrose (table sugar) in the development of insulin resistance and type 2 diabetes.”
Criterion 2.1. Does the reference support the claim?
4 out of 4
This reference received a score of 4, indicating that it offers strong support for the claim. Again, this sentence cites several sources. This source addresses the advanced glycation end products (AGEs). The review supports the text’s claim that AGEs play a role in diabetes and insulin resistance and also mentions that “foods rich in both protein and fat, mostly of animal origin, and cooked at high and dry heat, such as in broiling, grilling, frying, and roasting, tend to be the richest dietary sources of AGEs.”
Reference 10
Reference
Chapter 10, reference 50. American Heart Association. (2018). Dietary recommendations for healthy children. American Heart Association.
Associated quote(s) and page number(s)
Page 272: In answer to the question of who should pay particular attention to cooking oils in their diet, The Proof is in the Plants states: “Groups that may specifically benefit from the inclusion of oils in their diet include young children, who have a higher fat requirement to support their growth and a lower appetite, and people who are trying to put on weight who are still struggling to achieve a calorie surplus.”
Criterion 2.1. Does the reference support the claim?
2 out of 4
This reference received a score of 2, indicating that the reference offers weak support to the claim. The cited source is almost irrelevant to the claim being made. It’s a set of American Heart Association dietary guidelines for children and families. There is one bullet point, however, that does mention oils when discussing dietary fats, recommending that most fat come “from sources of polyunsaturated and monounsaturated fatty acids, such as fish, nuts and vegetable oils.”
Overall (average) score for reference accuracy
3.3 out of 4
Healthfulness
In general, the diet recommended by The Proof is in the Plants is likely to improve the health of most people who choose it. A transition to a whole foods, plant-based diet, particularly from a standard American diet or typical Western diet, will likely help a person maintain a normal weight, lower heart disease risk, and reduce the risks of at least some cancers.
That said, we think this diet would be difficult and expensive for most people to fully follow. The first few chapters of the book discuss how the food landscape is “rigged” against eating such a diet: junk food is cheaper, more convenient, and engineered to taste good.
It’s hard to find unprocessed and/or vegan entrees in most restaurants or break rooms. Most of the shelves at your local supermarket would also be off limits, as most of your groceries would need to come from the edges of the store where produce and chilled items reside, along with some dry food aisles for things like dried grains, lentils, beans, nuts, and oils. Most of your meals would need to be prepared at home, which would take some time and effort.
This would also be a more expensive diet than a typical Western diet. Not necessarily due to the lack of animal foods (as some studies have demonstrated that a vegetarian diet might even be cheaper than a vegan or omnivorous diet), but rather consumption of minimally processed foods: in one of the studies mentioned in the book, the authors state that the diet of processed foods given to participants cost $106 per week, while the unprocessed foods cost $151 for the week. Adam Drewnowski of the University of Washington has done much work on the subject of food prices and his work demonstrates that ultra-processed foods are typically cheaper than other foods. Drewnowski has found that nutrient-poor but energy dense foods are cheaper on a cost-per-calorie basis than nutrient-rich foods, such as fresh fruits and vegetables. Therefore, unless the plant-based diet is one consisting mainly of grains, beans, and pulses, one is likely to spend more when incorporating fresh produce. Drewnowksi’s work focuses on food costs in the United States, but this seems to be true of other countries like France and the UK as well. However, one study from Germany showed that a highly-processed dietary pattern was more expensive than a freshly-cooked omnivorous diet or a vegetarian diet.
Another drawback is the risk of a nutrient deficiency. Vegetarian and especially vegan diets are at higher risk of low intakes of certain vitamins and minerals like vitamin B12, calcium, iron, selenium, and zinc. It’s worth noting that low intakes of a micronutrient are not necessarily associated with increased risk of disease. For example, a lower zinc status among adult vegetarians (in high-income countries) does not appear to have adverse health effects, presumably due to some kind of homeostatic adaptation. On the other hand, individuals with a B12 deficiency may be more susceptible to cardiovascular disease if the B12 deficiency translates to elevated homocysteine levels. Nevertheless, The Proof is in the Plants recommends ways to ensure adequate intake of these nutrients, like supplements and specialty foods.
Summary of the health-related intervention promoted in the book
The key recommendation of the book is to consume a plant-predominant, minimally-processed diet (at least 85% of total calories from whole plant foods).
Condition targeted by the book, if applicable
The main condition targeted by The Proof is in the Plants is a general reduction in disease burden from common non-communicable diseases: obesity, heart disease, cancer, dementia. There are also arguments within the book for a diet that reduces greenhouse gas emissions and promotes longevity.
Apparent target audience of the book
The target audience of the book seems to be a general audience, although The Proof is in the Plants does state that the book’s advice is mainly directed at the Western world, and not necessarily applicable to developing countries with populations experiencing food scarcity.
Criterion 3.1. Is the intervention likely to improve the target condition?
4 out of 4
This item received a score of 4 because there is good evidence that the recommended diet will improve the conditions that are reviewed in this piece. The strongest evidence is for the obesity claim, discussed elsewhere in this review. The evidence available that a whole foods, plant-based diet can improve or prevent cancer or cardiovascular disease is less compelling due to a lack of long-term clinical trials, but the evidence that is available strongly suggests that this type of diet can reduce the development of these diseases.
Criterion 3.2. Is the intervention likely to improve general health in the target audience?
4 out of 4
This intervention received a score of 4 indicating it is likely to greatly improve health. Of course this depends greatly on the diet pattern one is currently consuming and may also depend on the type of plant-based diet one were to exchange it with. Compared to a standard Western diet that is high in ultra-processed foods, saturated fats, processed red meats, refined carbohydrates (white flour and sugars), and low in fiber most any type of whole foods, plant-based diet is likely to greatly improve health.
In addition to what is discussed in this review, there is evidence that plant-based diets can reduce inflammation, reduce the risk of diabetes in older adults, reduce the risk of stroke, and improve insulin sensitivity in obese individuals.
Criterion 3.3. Does the diet portion of the intervention promote an adequate nutrient intake for general health in the target audience?
1 out of 4
This diet received a score of 1 indicating it is likely somewhat nutritionally inadequate. It should be noted that this score heavily depends on whether or not one chooses to include animal foods as well as vitamin and mineral supplements.
For example, those that follow a lacto-ovo-vegetarian diet (that includes eggs and dairy) or a pescatarian diet (that includes fish and marine life) are at a reduced risk for developing a deficiency than stricter vegan diets.
Most of the literature surrounding nutrient deficiency among those adhering to plant-based diets is focused on cobalamin (vitamin B12) deficiencies. A 2014 review article indicated that vegans were at higher risk for this deficiency than lacto-ovo-vegetarians, but that both communities had a high prevalence of deficiency for B12.
In addition to B12, vegans are also at risk of a deficiency of minerals such as zinc, calcium, and selenium. A 2021 review article indicates vegans may also be at risk for deficiencies of omega-3 fatty acids and vitamin D. Pescatarians, however, appeared to have much higher intakes of both, even more so than meat-eaters. This is likely due to intakes of fatty fish, which are high in both of these nutrients.
It’s worth emphasizing that this score reflects the available evidence for the overall plant-based dietary pattern, and does not reflect additional supplementation. The deficiency risks are addressed in the last chapter of The Proof is in the Plants with a discussion on nutrients of focus, and how to achieve nutritional adequacy with either plant-based food sources of the nutrient or a recommended dietary supplement.
Overall (average) score for healthfulness
3 out of 4
Most unusual claim
The Proof is in the Plants does not contain any claims that stand out as radical or unusual in most respects. However, the concept of nutrient timing is presented in the book and it might be useful to discuss that here.
There is a section in the last chapter of the book that claims that not only what you eat is important, but also when you eat. It was accompanied by a phrase I hadn’t encountered before: chrononutrition, which seems to be a relatively recently coined term that refers to how meal timing affects health. This statement from page 360 sums it up: “When we can sync our meals with these natural circadian rhythms, there are health benefits up for grabs.”
In short, The Proof is in the Plants recommends avoiding late-night meals and eating more calories in the morning than the evening. To support this, the book discusses a crossover trial where people were fed diets containing the same calories at either 6pm or 10pm. According to the study results, glucose clearance and fatty acid oxidation were both lower after the later meal.
Another crossover trial is discussed where normal weight men were fed a low-calorie breakfast and a high-calorie dinner, and later fed a high-calorie breakfast and a low-calorie dinner. The results show that regardless of whether a high calorie or low calorie meal is consumed, diet-induced thermogenesis (the calories expended to digest the meal) was 2.5 times higher in the morning than the evening. The authors conclude that one should therefore eat a higher-calorie breakfast to prevent obesity, as more calories from the breakfast will be burned following the meal.It might be worth noting that other researchers have criticized the conclusion as perhaps being overstated.
Nevertheless, an earlier trial of overweight and obese women seems to support it, as it demonstrated that women fed a diet with a 700 calorie breakfast and a 200 calorie dinner lost more weight after 12 weeks than those fed a 200 calorie breakfast and a 700 calorie dinner.
Several review articles have been published in this area that explore nutrient timing and metabolic regulation, and the potential mechanisms at work. However, very few human studies have been done in this area.
This is a similar idea to time-restricted feeding (TRF), which is also discussed in the book. TRF is the idea that food is to be consumed only during certain hours instead of at any point during the day. There is some evidence that this type of restriction can help reduce body weight: a 2020 meta-analysis concluded that TRF can lead to a significant reduction in weight and fat mass. However, a 12-week clinical trial and a large 12-month clinical trial published since then demonstrated no weight loss benefit compared to a control group. Another meta-analysis indicated very little difference in metabolic profiles between TRF groups and control groups.
Other
Conclusion
The Proof is in the Plants is a well-researched book that argues that a whole foods plant-based diet is optimal for human health and the planet. We reviewed three of the health claims in the book, namely that a well-planned whole-foods plant-based diet promotes a healthy weight, reduces the risk of several types of cancer, and reduces the risk of cardiovascular disease. There is quite a bit of evidence in the literature that plant-based diets can play a role in each, although much of the evidence is in the form of observational studies or animal models, or short- to medium-term human studies assessing the impact of dietary interventions on risk factors rather than hard clinical outcomes. More long-term clinical trials with hard disease outcomes would need to be conducted to come to definitive conclusions.
The diet is likely to be very healthy compared to a typical Western diet, although extra care will need to be taken to reduce the risk of some common nutrient deficiencies that can occur on diets that don’t contain animal foods. In addition, the diet will likely also be expensive and difficult to follow in the long term.