High-Fat Diets Linked With Changes in Gut

Higher fat diets were linked with changes in gut microbiota, as well as adverse effects on certain biomarkers in young adults, Chinese researchers found.

Compared to a lower-fat diet, a higher-fat diet was associated with changes in gut microbiota, fecal metabolomic profiles, and proinflammatory factors for healthy young adults, reported Duo Li, MB, PhD, of Qingdao University in Qingdao, China, and colleagues, in the journal Gut.

“These findings provide confirmatory evidence that nutritional guidelines in countries in a state of nutrition transition should advise against increasing intakes of dietary fat,” Li and associates wrote. “The results might also have relevance in developed countries in which fat intake is already high.”

Gut flora diversity has been associated with improved health in the elderly, while reduced diversity has been linked to increased risk of gastrointestinal diseases and proinflammatory features. Previous research has shown that high-fat intake promotes intestinal inflammation.

This study examined fecal and blood samples from healthy, non-obese adults ages 18 to 35 in the Optimal Dietary Macronutrient Distribution in China trial, which randomized young adults to a traditional lower-fat, higher-carbohydrate diet, or one of two others with more fat content, in a design paralleling the country’s current transition from a traditional Chinese diet to a higher-fat Western-style regimen.

Over 6 months, participants followed one of three isocaloric fat-escalating diets:

• A lower-fat diet (fat 20% energy)
• A moderate-fat diet (fat 30% energy)
• A higher-fat diet (fat 40% energy)

In this study, Li and colleagues examined data from 217 participants. About 52% were women, with a mean age of about 23, and a BMI of approximately 22.

Mean baseline carbohydrate intake in all groups was 55% of energy, but during the intervention accounted for 67% in the lower-fat group, 57% in the moderate-fat group, and 48% in the higher-fat group. Mean dietary fat consumption in all was 32% of energy at baseline, changing to about 20%, 30%, and 40% in the lower-fat, moderate-fat, and higher-fat groups, respectively. Protein intake remained stable at 13% to 14% of energy.

All three groups lost weight and waist circumference, with the greatest losses occurring in the lower-fat diet group. Not surprisingly, reduction in waist circumference, total cholesterol, high-density lipoprotein cholesterol (HDL-C), LDL-C, and non-HDL-C for the lower-fat group were significantly greater than the high-fat group, the authors noted.

Overall, there were no major changes in the global gut microbiota, they wrote, although they cited “overall unfavorable effects on several important biomarkers.” Compared with the lower-fat diet, the higher-fat diet significantly increased abundance of Firmicutes and decreased the abundance of Bacteroides, with the ratio of Firmicutes to Bacteroides significantly decreased in the moderate and high-fat groups after the intervention.

Li and colleagues found the lower-fat group was associated with significant increases in the abundance of the genera Blautia and Faecalibacterium, whereas the higher-fat group was linked with significant increases in concentration of Alistipes and Bacteroides and reduced concentrations of Faecalibacterium.

Faecalibacterium and Blautia are involved in the production of butyrate, a short-chain fatty acid linked to anti-inflammation, intestinal barrier function enhancement, and mucosal immunity. Reduced levels of this microbe are found in people with inflammatory gastrointestinal diseases, the authors noted.

There were significant shifts in fetal metabolomic profiles, with stratified analyses showing significant alterations in amino acid metabolites and long-chain fatty acids in the higher-fat group, with significant reductions in total short-chain fatty acids in the higher-fat group versus other groups.

The cometabolites p-Cresol and indole, which are associated with metabolic disorders, were reduced in the lower-fat group, while palmitic acid, stearic acid, and arachidonic acid were significantly increased after the higher-fat diet. The main saturated fatty acids in food and tissues, palmitic and stearic acids are thought to stimulate inflammatory signaling in macrophages, hepatocytes, adipocytes, and myocytes, the authors explained.

Calling the study “interesting,” Purna C. Kashyap, MBBS, of the Mayo Clinic in Rochester, Minnesota, told MedPage Today that the change in Firmicutes to Bacteroides ratio described in the study is consistent with prior findings. Kashyap was not involved with the research.

“The study also found intriguing associations between the dietary intervention and bacterial metabolites, especially the indole compounds, but it still remains to be determined if these metabolites are responsible for the metabolic improvement in patients or simply an adaptation by microbes to changes in diet,” Kashyap said.

Kashyap added that as in previous findings from smaller dietary studies, the magnitude of change in the human microbiome following dietary intervention is much lower than that observed in mouse studies.

In terms of promoting circulating proinflammatory factors, the higher-fat diet was tied to increased plasma high-sensitivity C-reactive protein and thromboxane B2 versus its lower-fat counterpart, while leukotriene B4 and prostaglandin E2 in the lower-fat diet decreased.

A limitation of this study was that fecal sampling was done only at baseline and trial’s end. In addition, all three groups lost weight, but weight loss differed among groups, prompting the authors to ask whether weight loss led to the changes in gut microbiota and fecal metabolomics or vice versa. Another limitation was the study’s restricted generalizability since participants were healthy, young, non-obese adults and results for this cohort might not apply to people at high risk of cardiometabolic diseases.