High-Fat Diet Bad for Gut Microbiome?
Society’s recent fascination (read: obsession) with the gut microbiome has only been matched, and possibly outpaced, with its interest (again read: obsession) with high-fat diets.
A new study, however, shows that pop culture’s latest two love interests might be at odds with one another.
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Researchers have known for quite a while that a person’s diet has a significant impact on the quantity and diversity of the gut microbiome. The latest study, published in the journal Gut, involved 217 Chinese men and women, ages 18-35 years old with an average BMI < 28.
Scientists divided the 217 test subjects into three groups and had each group consume one of the following isocaloric diets:
- Low-fat diet: 20% fat / 66% carbohydrates / 14% protein
- Moderate-fat diet: 30% fat / 56% carbs / 14% protein
- High-fat diet: 40% fat 46% carbs / 14% protein
To account for any impact that differences in dietary fiber intakes might have, all three diets maintained a minimum fiber intake of ~14 g/day.
For more detailed info about each group’s diets, here a chart breaking down the exact plan each group followed. 
All food (and most of the beverages) for each of the three diet plans was provided to subjects during the 6-month trial, and subjects were also asked to complete a daily diary in which they recorded whether they had consumed all the provided food as well as any non-provided foods they may have eaten as well.
Researchers were examining the effects of each diet on the subjects’ gut microbiota, fecal metabolomics and plasma inflammatory factors.
Following the 6-month study, individuals in the low-fat diet group experienced increases in the amount of “good” gut bacteria strains Blautia and Faecalibacterium compared to their levels at the beginning of the trial.
Individuals in the high-fat diet group saw decreased levels of these beneficial bacteria.
Why is this important?
Well, Blautia and Faecalibacterium bacteria are involved in the production of butyrate, a short-chain fatty acid known to offer a broad range of beneficial effects to human health, including anti-inflammatory properties. 
Stool samples collected by researchers showed the low-fat diet group had increased levels of butyrate (likely due to the increased levels of good gut bacteria) while the high-fat diet group had lower levels that at the start of the trial.
Furthermore, individuals following the high-fat diet also saw increases in the amount of Alistipes and Bacteroides, two types of bacteria linked with Type 2 diabetes. 
And, to make matters just a bit worse, that high-fat diet group also saw a rise in levels of several long-chain fatty acids, including arachidonic acid. Subsequently, researchers also identified elevations in various markers of inflammation in the blood of high-fat group members.
Based on the findings, researchers concluded:
"Higher-fat consumption by healthy young adults whose diet is in a state of nutrition transition appeared to be associated with unfavorable changes in gut microbiota, fecal metabolomic profiles and plasma proinﬂammatory factors, which might confer adverse consequences for long-term health outcomes.” 
Now, based on the study’s findings and the way various headlines have read, you might begin to think that all high-fat diets are inflammatory, reduce good gut bacteria, and should be avoided at all costs, but, there are a few things (“issues”) worth noting:
First, only 14% of daily calories in each of the diets came from protein. Assuming you’re consuming ~2000 calories per day (like the subjects in this study), this equates to a daily protein intake of 70g of protein, an amount woefully low for most active individuals.
Second, the fat used to increase the dietary fat content of the moderate and high-fat diet groups came primarily from soybean oil (the most widely consumed edible oil in Asia).
Now, there’s a great deal of debate regarding how “healthful” soybean oil is, especially by the keto/paleo community, due to the oil’s high omega-6 to omega-3 ratio which many in the low-carb community believe could lead to more inflammation.
Various observational studies have noted an association between high intakes of omega-6 fatty acids to increased risks of arthritis, obesity, heart disease, and inflammatory bowel disease (IBD).
However, it’s important to remember that associational studies merely show correlations (or relationships between two things), not causation.
Studies investigating the effects of feeding individuals omega-6 fatty acids typically do not support the notion that omega-6 fats lead to an increase in inflammation. 
Third, fiber intake was relatively low (~14 grams per day). While there is no clear-cut amount of fiber every individual needs, most experts recommend a total dietary fiber intake of 25 to 30 grams per day, and the latest U.S. Dietary Guidelines recommend an intake of 14 grams per 1000 kcal. 
Since fiber provides food for gut bacteria, this could have an impact on which strains of bacteria survived during the course of the 6-month trial. However, many people that consume the “standard” American Diet fall woefully short of the daily fiber recommendation, having the subjects consume a diet high in omega-6 fats coupled with a low fiber and protein intake is a fair approximation of the average Westerner’s diet.
Fourth, the study was relatively small (217 people) and consisted of individuals from China (The study was conducted at the People's Liberation Army General Hospital in Beijing and Zhejiang University in Hangzhou, China).
While our understanding of the gut microbiome is in its infancy and continues to evolve with the release of each new study, researchers do know that different ethnicities have differing gut microbiomes. 
Perhaps feeding people a diet different from what their historically used to eating negatively impacts their gut bacteria and overall health.
So, what can we take away from this latest study?
First, this study is not a nail-in-the-coffin for high-fat diets. It’s pitifully low in protein and fiber, and likely micronutrients too.
Had the subjects been given more MCT or coconut oil and fiber-rich vegetables in place of soybean oil, perhaps they would have fared better at the end of the trial and had more good gut bacteria and less inflammatory markets. This is due to the simple fact that short-chain fatty acids can mitigate the potential inflammatory effects of long-chain fatty acids.
What this study does show us is that feeding people a crappy diet adversely impacts their gut microbiome and sets them on the road for potential health complications should they adhere to it long-term.
Finally, we don’t know how this same high-fat diet would affect people of different origins. Gut microbiomes differ from one race to another, so it’s possible this soybean oil-heavy diet might be completely fine for other people, though that is probably a stretch.
What do you think of the study and its findings?
Are high-fat diets inherently bad?
Or, is it more the composition of the high-fat diet that makes it potentially hazardous for health?
Leave your thoughts below.
1) Wan Y, Wang F, Yuan J, et al. Effects of dietary fat on gut microbiota and faecal metabolites, and their relationship with cardiometabolic risk factors: a 6-month randomised controlled-feeding trial. Gut. Published Online First: 19 February 2019. doi:10.1136/gutjnl-2018-317609
2) Rios-Covian D, Ruas-Madiedo P, Margolles A, Gueimonde M, de Los Reyes-Gavilan CG, Salazar N. Intestinal short chain fatty acids and their link with diet and human health. Front Microbiol. 2016;7:185. doi: 10.3389/fmicb.2016.00185.
3) Canani RB, Costanzo MD, Leone L, Pedata M, Meli R, Calignano A. Potential beneficial effects of butyrate in intestinal and extraintestinal diseases. World J Gastroenterol. 2011;17(12):1519–1528. doi: 10.3748/wjg.v17.i12.1519.
4) Whang, A., Nagpal, R., & Yadav, H. (2019). Bi-directional drug-microbiome interactions of anti-diabetics. EBioMedicine, 39, 591–602. https://doi.org/https://doi.org/10.1016/j.ebiom.2018.11.046
5) Patterson E, Wall R, Fitzgerald GF, Ross RP, Stanton C. Health implications of high dietary omega-6 polyunsaturated Fatty acids. J Nutr Metab. 2012;2012:539426.
6) Harris, W. S. (2006). The omega-6/omega-3 ratio and cardiovascular disease risk: uses and abuses. Current Atherosclerosis Reports, 8(6), 453–459.
7) "Appendix 7. Nutritional Goals for Age-Sex Groups Based on Dietary Reference Intakes and Dietary Guidelines Recommendations - 2015-2020 Dietary Guidelines - Health.gov." Home of the Office of Disease Prevention and Health Promotion - Health.gov, health.gov/dietaryguidelines/2015/guidelines/appendix-7/.
8) Andrew W. Brooks, Sambhawa Priya, Ran Blekhman, Seth R. Bordenstein. Gut microbiota diversity across ethnicities in the United States. PLOS Biology, 2018; 16 (12): e2006842 DOI: 10.1371/journal.pbio.2006842
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