The Good Gut: Taking Control of your Weight, Your Mood, and Your Long-term Health

Overall score

Scientific accuracy

Reference accuracy

Healthfulness

How hard would it be to apply the book's advice? Fairly easy

Book published in 2015

Published by Penguin Press

First Edition, Hardcover

Review posted February 21, 2019

Primary reviewer: Stephan Guyenet

Peer reviewer: Maggie Burhans

Table of contents

1. Introduction
2. Scientific accuracy
3. Reference accuracy
4. Healthfulness
5. Most unusual claim
6. Other
7. Conclusion
8. Updates

Claim 1

Summary of claim

The gut microbiome may impact the risk of immune disorders such as allergies, asthma, and autoimmune disease.

Supporting quote(s) and page number(s)

Page 67: “Although there is still ongoing debate about the complex interplay of factors and mechanisms behind the hygiene hypothesis, it’s clear that the prevalence of autoimmune diseases in a population tracks with how effectively that population reduces its exposure to microbes.”

Page 69: “Mounting evidence suggests that decreases in total combined microbial exposure, either from a low-diversity microbiota or an ultraclean environment, can translate into immunological problems.”
Page 69: “It is important to note that these studies have not established a causative role for antibiotics and the development of immune-mediated disease…”

Criterion 1.1. How well is the claim supported by current evidence?

4 out of 4

This claim received a score of 4, meaning it is strongly supported by current evidence.  Peer-reviewed review papers written by other experts in the field tend to support the claim.  It is supported by animal studies and human observational studies, yet the human evidence appears rather indirect.  The idea is plausible since the microbiota play a key role in “training” the immune system to behave appropriately, defending the body against credible threats while not overreacting to small threats or attacking tissues.  There does not appear to be direct evidence in humans, but the authors acknowledge this.

Criterion 1.2. Are the references cited in the book to support the claim convincing?

4 out of 4

References received a score of 4, indicating that they are very convincing.  The Good Gut cites a number of studies supporting its claim, including rodent studies and human observational studies.  These studies are straightforwardly consistent with the book’s claim.

Criterion 1.3. How well does the strength of the claim line up with the strength of the evidence?

4 out of 4

This claim received a score of 4, indicating that its strength aligns well with the strength of the evidence.  The Good Gut is fairly cautious, which is appropriate given the lack of direct evidence in humans.

Overall (average) score for claim 1

4 out of 4

Claim 2

Summary of claim

The gut microbiome has a significant impact on body weight in humans.

Supporting quote(s) and page number(s)

Page 57: “These studies illustrate that antibiotic use early in life can have a direct impact on the composition of the microbiota and, what’s more disturbing, on long-lasting weight gain and adiposity years after the antibiotic onslaught.”
The book also discusses microbiota and weight on pages 128-129 and pages 31-32, and implies in the title that a healthy microbiome helps control body weight: “The Good Gut: Taking control of your weight, your mood, and your long-term health”.

Criterion 1.1. How well is the claim supported by current evidence?

2 out of 4

This claim received a score of 2, meaning it is weakly supported by current evidence.  Some experiments in rodents suggest that the gut microbiome can impact body weight, although other findings in rodents have not found that it plays an important role in the development of obesity, and not all of the ostensibly supportive findings are very convincing when closely examined (one seminal finding may have resulted from a statistical pitfall called regression toward the mean).  Some human observational studies report associations between antibiotic use and higher body weight in infancy and childhood.  The most direct evidence is a human study that reported that doing a fecal microbiota “transplant” from lean people to people with obesity did not cause detectable weight loss over 6 or 18 weeks (this study was published after The Good Gut).  This study doesn’t offer definitive evidence that the gut microbiome has no impact on obesity in humans, but the overall evidence seems mixed, and direct evidence in humans is unsupportive so far.

Criterion 1.2. Are the references cited in the book to support the claim convincing?

3 out of 4

These references received a score of 3, indicating that they are moderately convincing.  The Good Gut cites a handful of studies to support this claim, including rodent studies and observational studies in humans.  These studies are consistent with the book’s claim but do not offer direct evidence that the microbiome impacts body weight in humans.

Criterion 1.3. How well does the strength of the claim line up with the strength of the evidence?

2 out of 4

This claim received a score of 2, indicating that it is moderately overstated.  Since the claim that the gut microbiome impacts body weight in humans is implied in The Good Gut more often than it is stated explicitly, it’s hard to get a handle on exactly how strong it is.  However, given the lack of direct evidence that the microbiome impacts body weight in humans, and limited direct evidence suggesting that it doesn’t (to be fair, published after The Good Gut), the claim seems at least moderately overstated.

Overall (average) score for claim 2

2.3 out of 4

Claim 3

Summary of claim

A healthy gut microbiome defends against harmful gut infections, including sometimes deadly types such as Salmonella and Clostridium difficile.

Supporting quote(s) and page number(s)

Supporting quote(s) and page number(s): Page 92: “Probiotic bacteria, through their ability to reinforce the intestinal border and prime the immune system, should be effective allies in the fight against gastrointestinal infections.”

Page 173: “Our bodies are constantly exposed to pathogenic bacteria like Salmonella, but because of the resistance afforded by our microbiota, it takes many pathogenic microbes to make us sick.”

Page 174: “Antibiotics disrupt connections within the complex food web, leaving a resource gap that Salmonella and C. difficile can exploit.”

Criterion 1.1. How well is the claim supported by current evidence?

4 out of 4

This claim received a score of 4, meaning it is strongly supported by current evidence.  A rigorous meta-analysis (a study that pools together data from many studies on a particular topic) concluded that “probiotics appear to be safe and have clear beneficial effects in shortening the duration and reducing stool frequency in acute infectious diarrhoea.”  In addition, several studies report that fecal microbiota “transplants” from healthy people are remarkably effective for treating Clostridium difficile.  A 2013 meta-analysis reported that the treatment is effective in about 88 percent of cases, although no randomized trials (high-quality controlled studies) had been published at that time.  Since then, there have been several successful randomized controlled trials, which together are very compelling.

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, meaning they strongly support the claim.  The Good Gut cites a rigorous meta-analysis concluding that “probiotics appear to be safe and have clear beneficial effects in shortening the duration and reducing stool frequency in acute infectious diarrhoea.”  It also cites a rigorous randomized controlled trial demonstrating that fecal microbiota transplants are more effective for treating Clostridium difficile infection than powerful antibiotics.  While the book could have cited more studies, those that it cited are sufficient to establish its claims.

Criterion 1.3. How well does the strength of the claim line up with the strength of the evidence?

4 out of 4

This claim received a 4, indicating that the strength of the claim aligns well with the strength of the evidence.  The authors’ claims are fairly strong, but so is the evidence.

Overall (average) score for claim 3

4 out of 4

Overall (average) score for scientific accuracy

3.4 out of 4

Reference 1

Reference

Chapter 9, reference 2.  Ip et al. Evid Rep Technol Assess 153:1. 2007.

Associated quote(s) and page number(s)

Page 212: “Although admittedly a lot of work, the effort you invest to establish breast-feeding with your child will be rewarded with a lower risk of allergies, asthma, obesity, and even diabetes.”

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 quoted statement clearly suggests that breastfeeding protects against allergies, asthma, obesity, and diabetes.  The reference, in contrast, is a sweeping review of mostly observational studies, which are limited for establishing cause-and-effect relationships.  Although the review did find that breastfeeding is associated with a lower risk of the conditions in question, its abstract states “Because almost all the data in this review were gathered from observational studies, one should not infer causality based on these findings.”

Reference 2

Reference

Chapter 1, reference 9.  Eckburg et al. Science 308:1635. 2005.

Associated quote(s) and page number(s)

Page 29: “When fecal bacteria are compared to bacteria coming straight from the colon (as collected during a colonoscopy) the two samples are very similar.”

Criterion 2.1. Does the reference support the claim?

3 out of 4

This reference receives a score of 3, indicating that it offers moderate support for the claim.  The study in question, among other things, compares the composition of the microbiome adhering to the mucosal layer of the colon (collected by colonoscopy) vs. the fecal microbiota.  However, the accuracy of the quote in The Good Gut comes down to how we interpret the phrase “very similar”.  As claimed, the authors found similarities between the mucosal and fecal microbiota for each individual, but they also reported apparently notable differences.  Figure 3A of the paper, and the following quotes, illustrate this. “We postulate that the fecal microbiota represents a combination of shed mucosal bacteria and a separate nonadherent luminal population.”  “Differences between individuals were significantly greater than intrasubject differences, with the exception of variation between stool and adherent mucosal communities.”

Reference 3

Reference

Chapter 3, reference 3.  Wlasiuk et al. Curr Opin Allergy Clin Immunol 12:461. 2012.

Associated quote(s) and page number(s)

Page 67: “The hygiene hypothesis has progressed to incorporate the finding that children raised on farms have fewer allergies than children living in very clean, affluent households.”

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 Good Gut cites a review paper on the association between farm living and lower allergy risk, which supports the book’s claim.  However, this was a close call because the reference does not refer to “very clean, affluent households”, but rather simply “nonfarm” children.

Reference 4

Reference

Chapter 3, reference 7.  Herbst et al.  Am J Respir Crit Care Med 184:198. 2011.

Associated quote(s) and page number(s)

Page 69: “Germ-free mice, raised in the extreme conditions of no microbial exposure, develop severe airway responses similar to asthma when allergens are present.  Mice that are colonized by a full microbiota are protected.”

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 study demonstrated that germ-free mice have greater immune responses to an allergen than germ-free mice that have been recolonized with a normal gut microbiota.

Reference 5

Reference

Chapter 2, reference 6.  Alfaleh et al. Cochrane Database Syst Rev 1:Cd005496. 2008.

Associated quote(s) and page number(s)

Page 40: “Premature infants given beneficial bacteria, such as those belonging to the Lactobacillus family, are much less likely to develop necrotizing enterocolitis than infants that receive no bacterial therapy.”

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 Good Gut cites a rigorous meta-analysis (quantitative study of studies) reporting that probiotics reduce the risk of severe necrotizing enterocolitis by 68 percent, which I feel is consistent with the phrase “much less likely” in the associated quote.

Reference 6

Reference

Chapter 1, reference 3.  Yatsunenko et al. Nature 486:222. 2012.

Associated quote(s) and page number(s)

Page 18: “The microbiota from individuals living a traditional agrarian lifestyle similar to how humans lived ten thousand years ago also contains a more diverse collection of microbes than Westerners typically house.”

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 paper compares the fecal microbiota of US residents and two nonindustrial cultures from South America and Africa.  It reports that “The fecal microbiota of USA adults was the least diverse compared to the two other populations.”

Reference 7

Reference

Chapter 8, reference 14.  Cuervo et al. Nutr Res 33:811. 2013.

Associated quote(s) and page number(s)

Page 205: “In fact, increased fiber consumption among seventy-six to ninety-five-year-olds is directly related to elevated [short-chain fatty acid] production.”

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 title of the paper says it all: “Fiber from a regular diet is directly associated with fecal short-chain fatty acid concentrations in the elderly.”

Reference 8

Reference

Chapter 8, reference 6.  Evans et al. PLoS One 9:e92193. 2014.

Associated quote(s) and page number(s)

Page 195: “Studies in laboratory mice, which allow more independent control of diet and exercise, show that each factor on its own can improve the state of the microbiota.”

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 study does demonstrate that diet and exercise both impact (body weight and) the microbiota in mice, however since the microbiota changes induced by exercise were distinct from those of diet (i.e., exercise didn’t make the microbiota of an animal eating an unhealthy diet look like that of an animal eating a healthy diet), it’s not obvious to me that exercise “improved” the microbiota.

Reference 9

Reference

Chapter 6, reference 1.  Neufeld et al. Neurogastroenterol Motil 23:255. 2011.

Associated quote(s) and page number(s)

Page 140: “Scientists noted that microbe-free mice have a personality that is distinct from mice with a normal microbiota.”

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.  Quote from the paper: “We conclude that the presence or absence of conventional intestinal microbiota influences the development of behavior, and is accompanied by neurochemical changes in the brain.”

Reference 10

Reference

Chapter 5, reference 14.  Ridaura et al. Science 341:12412. 2013.

Associated quote(s) and page number(s)

Page 129: “In 2013 Washington University’s Dr. Jeffrey Gordon led a study to examine the microbiota from twins that were discordant for obesity, meaning one twin was lean and the other obese.  When they transferred a relatively low-diversity, or poor, microbiota from an obese twin into mice, those mice got fat. A transplant from the higher-diversity, or rich, microbiota from the lean twin resulted in lean mice.”

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 study did show that microbiota from twins with obesity caused fat gain relative to microbiota from lean people, but the effects were quite modest.  Microbiota from twins with obesity increased body fat by about 10 percent in mice (figure 1D), and this appears to be a relative rather than absolute increase.  In absolute terms this is equivalent to a person going from 12 percent body fat (lean) to 13.2 percent body fat (still lean). This seems less impressive than what is implied by the phrase “those mice got fat”.

Overall (average) score for reference accuracy

3.5 out of 4

Summary of the health-related intervention promoted in the book

Increasing fiber intake, diversifying fiber intake, eating less meat, limiting saturated fat intake, and increasing intake of bacteria-rich fermented foods like yogurt and sauerkraut.  Feeding babies breast milk if possible, exposing oneself to dirt and/or pets, avoiding unnecessary antibiotics, and taking probiotic supplements at particularly sensitive times (such as after a course of antibiotics).

Condition target by the book, if applicable

The Good Gut focuses primarily on reducing the risk of infections, immune-related disorders like allergies, mood disorders, and obesity.

Apparent target audience of the book

The Good Gut appears to be intended for a general audience.

Criterion 3.1. Is the intervention likely to improve the target condition?

3 out of 4

The Good Gut received a score of 3, indicating that its health intervention is likely to moderately improve the targeted conditions.  The health claims in the book are broad, and there is no direct evidence available on The Good Gut’s diet and lifestyle recommendations as a package, but there is enough evidence to gain some clues about its likely effectiveness.

For example, there is reasonably strong evidence that probiotic supplements prevent or treat upper respiratory tract infections (like the common cold) and diarrhea, supporting the book’s advice.  Although there is less strong evidence supporting the idea that a healthy microbiome reduces the risk of immune-related disorders like allergies, there is a lot of indirect evidence from rodent studies and human observational studies.  There is also evidence that probiotic supplements reduce the risk of depression, but perhaps not anxiety, partially supporting the book’s claims.  As for obesity, I am not convinced that the microbiota play a major role in humans or even in animals, but the diet recommended by The Good Gut is likely more slimming than the average diet for other reasons.  High-fiber foods are more satiating per calorie than lower-fiber foods, and the simple, home-cooked, whole-food-based approach is likely to result in some degree of fat loss among people who carry excess fat.

Criterion 3.2. Is the intervention likely to improve general health in the target audience?

3 out of 4

The Good Gut received a score of 3, indicating that its health intervention is likely to moderately improve general health in the target audience.  In general it is hard to find fault with the book’s recommendations. Since the health claims of the book are broad, criterion 3.1 and 3.2 overlap and the rationale for their score is similar.  It’s also worth noting that the whole-food diet recommendations in The Good Gut will likely reduce cardiovascular disease risk.  Cardiovascular disease is the single most common cause of death worldwide.

Criterion 3.3. Does the diet portion of the intervention promote an adequate nutrient intake for general health in the target audience?

4 out of 4

The Good Gut’s recommended diet received a score of 4, meaning it is likely substantially more than nutritionally adequate.  Given the omnivorous nature of the diet, its inclusion of abundant and diverse plant foods, and its reduction of nutrient-poor refined carbohydrate like sugar and white flour, it is likely to supply adequate amounts of all essential nutrients (except vitamin D, which can be obtained from sunlight or supplements) and substantially more nonessential health-promoting nutrients (e.g., fiber, polyphenols) than what most people commonly eat.

Overall (average) score for healthfulness

3.3 out of 4