Author qualifications

There were 83 unique authors among the top 100 diet and nutrition-focused books as some authors had multiple best-seller hits among the top 100. Additionally, two books had no single attributed individual author and are listed as being a collective edition. Of the 83 authors, 33.7% had a medical degree and 6.0% had a Ph.D degree, while about half of the authors had no M.D., Ph.D or other graduate degree (see Fig. 1a). The current and previous occupations of the authors revealed that a third of them were at some point practicing physicians, and a great variety of other occupations were also represented, including editors, entrepreneurs, personal trainers, nutritionists, actors, bloggers, reality television stars, a fire fighter, and a professional pool player (see Fig. 1b).

Fig. 1: Education and occupations of best-selling authors. The highest levels of educational attainment (a) and the occupations (b) of the 83 best-selling authors based on available information. The most common highest degree was a medical degree and the most common occupation was physician. M.D. Doctor of Medicine, N.M.D./N.D. Doctor of Naturopathic Medicine, Ph.D Doctor of Philosophy. Full size image

Purported health benefits

We collected and categorized the Google Books summary of each of the top 100 diet and nutrition-related books. Eighty of the summaries mentioned weight loss or weight management, and of the summaries that specified a program length and/or the number of pounds that could be lost, the median was 21 days (n = 45, interquartile range (IQR) 14–30) and 15.5 pounds (n = 20, IQR 10–20), respectively. Apart from weight loss, one of the most common claims of the summaries was that the nutritional and diet advice in the books could cure or prevent disease (n = 31). The most commonly listed diseases were diabetes (n = 15) and heart disease (n = 12). Other commonly listed diseases that could be treated or prevented using these books were cancer (n = 11), dementia (n = 8), arthritis (n = 6), autoimmune disorders (n = 5), Parkinson’s disease (n = 3), autism spectrum disorder (n = 3), and depression (n = 3). Two additional common claims in the summaries were that following the advice in the book could increase energy levels (n = 13) and lifespan (n = 7).

Incongruent nutritional recommendations

The 100 summaries of best-selling books showed very little consistency in terms of what were the key components of diet that would achieve all these benefits. Many summaries included strong views surrounding the consumption of carbohydrates, as six summaries suggested a low or no carb diet and six suggest being gluten or grain free. However, four summaries did support eating grains or bread and one suggested a high-carb diet. Other summaries disagreed about the consumption of dairy, as two summaries insisted that dairy products should not be consumed, while three claimed that dairy products should be consumed. Of those that suggested that dairy products should be consumed, two said that non or low-fat dairy products are preferable, and one said full fat dairy products are preferable. Summaries also included differing views on which food groups should be emphasized in one’s diet, as nine summaries suggested a high-plant diet, seven suggested a high-fat diet, and four suggested a high-protein diet. Several summaries promoted popular diets, including four summaries that suggested a Mediterranean diet, three that supported a paleo diet, and two promoted a ketogenic diet. Other summaries promoted replacing meals altogether, as five mentioned replacing meals with smoothies and two suggested replacing meals with bone broth. Three summaries suggested alternating dieting strategies to confuse or boost one’s metabolism. A number of additional nutritional suggestions were made in the summaries, including eating anti-inflammatory foods (n = 3), reducing processed foods (n = 6), reducing or replacing sugar consumption (n = 8), eating nutrient dense foods (n = 3), eating a vegetarian diet, (n = 3), eating whole foods (n = 5), and eating foods with a low glycemic index (n = 2).

There was lack of consensus on some simple issues, e.g., whether calories are important and should be counted. Based on the book summaries, one summary suggested counting calories, while eight books advised their readers not to count calories. One summary suggested reducing portions/calories, four advised not to reduce portions/calories, three suggested cycling size of portions/calories consumed, and two encouraged their readers to eat more food. Given the large, almost wild, diversity in nutritional opinions and claims, it may be difficult to set a threshold of what would qualify as disputable and unsubsantiated versus not. Regardless, many claims seemed very puzzling to us and based on our knowledge of the scientific literature we would not be able to even remotely endorse them. Same examples appear in Box 1.

Authors with university appointments

We decided to further investigate the credentials of these best-selling authors to see how many of them had faculty appointments in universities. Of the 83 authors, we found a total of 20 authors with some mention of a faculty appointment, either currently or in the past. Of those 20, seven were currently on the faculty of some university and the information could be validated in the website of the institution by searching their name. Several of these authors seemed to have auxiliary appointments at adjunct or “voluntary” positions (unclear what that latter faculty title means). Most of the current professors were in biomedical (including nutrition-related) fields, but one was on the faculty of non-fiction writing. Fifteen authors with some identified or claimed faculty appointment had at least one publication with 100 or more citations as first or last authors in Google Scholar (often opinion pieces rather than full research papers and papers with data).

We also more thoroughly evaluated the profile, career and claims of the 20 authors associated with universities. University-affiliated authors are embedded by definition in academic environments where scientific publications, as well as scrutiny of the publications and research practices may be expected.

One author (Aggarwal) had 28 of his papers retracted and another ten had expressions of concern while another 17 had corrections as of 9 January, 2020 due largely to the reuse of images in figures, e.g., see several references for discussion of these issues (Prasad et al., 2016a; Sung et al., 2016a; Kannappan et al., 2016; Sung et al., 2016b; Gupta et al., 2016; Prasad et al., 2016b; Ackerman, 2016). An updated list of papers with retractions, expressions of concern, and corrections along with full details on each of them can be found in the Retraction Watch database.Footnote 1

We also noted that several other authors had attracted fierce criticism for some of their claims. For example, one author (Perlmutter) was listed by Quackwatch in a list of “Promoters of Questionable Methods and/or Advice”.Footnote 2 He has received severe criticism from scientists about his claims that Alzheimer’s disease, depression, Parkinson’s disease, and attention deficit hyperactivity disorder (ADHD) can be prevented by avoiding grains, or that the link between autism and vaccines is “ill-defined” (Levinovitz, 2015). Other scientists have made claims that some books made assertions that had no or little scientific foundation, as in the case of South Beach Diet proposed by Agatston (Goff et al., 2006) and Wheat Belly proposed by Davis (Brouns et al., 2013). We refer readers to the published criticisms where they can judge for themselves the validity of the original claims made in the books and the counterclaims and we encourage them to also take into account the original authors’ potential rebuttals of any criticisms.

Finally, the enthusiasm of several of the authors about a new discipline of “functional medicine” was very controversial among some scientific circles but was enthusiastically endorsed by others. Even the definition of functional medicine is non-standardized and it includes different elements of holistic, alternative, and traditional medicine.

Text-mining of full books

To determine the extent to which nutritional claims were backed up with evidence from the scientific literature, the full text of seven of the ten best-selling were searched for the terms “systematic review”, “meta-analysis” and “randomiz(s)ed” in the setting of controlled/clinical trials. One of the seven referenced meta-analyses of the nutritional literature (“The 20/20 Diet”), and three referenced randomized controlled or clinical trials (“Master Your Metabolism”, “Flat Belly Diet!”, and “The 20/20 Diet”), though only one of these referenced more than five (“The 20/20 Diet”). The majority of appearances of the search terms occurred in article names cited at the end of each book (25 out of 28). However, “Master Your Metabolism” and “The 20/20 Diet” did not include in-text citations, making it difficult to tell where the information from the scientific articles was incorporated, and whether any claim made in the text corresponded to a specific study.