Western diet linked to changes in gut fungi and metabolism

The microbes that live in our guts, or microbiota, are known to play important roles in how our bodies metabolize the food we eat and many other aspects of our health.

To date, however, the majority of studies have focused on bacteria, and few have looked at viruses. This means that research has largely ignored the other kingdoms of organisms we play host to, such as protists, archaea, and fungi.

Recent research in humans and mice does suggest that fungi influence their hosts’ metabolism, either directly or via their effect on bacteria.

However, their role remains unclear. This is partly because of the difficulty in distinguishing between fungi that are temporary guests — after ingestion in food or from other environmental sources, for example — and those that make their home in the gut.

Researchers at the University of Tennessee Health Science Center in Memphis addressed this question by studying fungi from laboratory mice with the same genetic backgrounds but from four different suppliers.

They fed the mice either a diet rich in purified carbohydrates — which reflects an ultra-processed, Western diet — or a more balanced, standard lab chow.

The scientists then investigated changes in the abundance and diversity of fungi in part of the small intestine called the jejunum. This is known to host the most diverse fungal populations in the intestines of mice.

 

Permanent residents

Their first discovery was that the gut mycobiome, which is the collective genome of fungi in the gut, varied dramatically between mice from different suppliers.

However, when the researchers analyzed fungi in the food pellets provided by the suppliers and those in the pellets they used in their own experiments, they found no evidence to suggest that these were a major source of the fungi in the animals’ guts.

This strongly suggests that the fungi were permanent residents in their intestines.

Next, they discovered that when the animals ate a processed diet, it reduced the diversity of fungi living in their jejunum, compared with the normal diet.

This, in turn, correlated with unhealthy changes in the metabolisms of male mice. There were increases in the amount of fat deposited in their livers, for example.

In addition, changes in the mycobiome in response to a processed diet were linked to increases in serum levels of triglycerides and various hormones involved in metabolism, including insulin, leptin, and ghrelin.

Leptin helps regulate the amount of body fat, whereas ghrelin boosts appetite.

Specifically, increases in these markers of unhealthy metabolism correlated with increased abundance of a fungal genus called Thermomyces and decreased abundance of another genus, called Saccharomyces.

 

Implications for future research

The authors note that other researchers who study the microbiome and its effects on health often only analyze bacteria in fecal samples.

By ignoring the abundance and diversity of fungi in the gut, they may overlook an important hidden variable that contributes to their results.

One limitation of the new study was that it only revealed correlations between diet, fungi, and metabolism, rather than direct, causal links. There is a possibility that diet drives changes in bacterial communities, for example, which can, in turn, influence metabolism and the mycobiome.

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