The amount of carbohydrates in a diet matters. But new Clemson University research suggests the variety of those carbohydrates is important, too.
Biological sciences Ph.D. student Clara Flores and assistant professor Anna Seekatz found that reducing the number of carbohydrate sources in the diet of mice changed their gut microbial community, even when the total amount of carbohydrates and calories stayed the same. Mice fed less-diverse carbohydrate diets had lower microbial diversity. Some key bacterial groups also had fewer overlapping species that could perform the same functions, according to the study.
The findings point to a simple idea with potentially broad implications: gut microbes may benefit not only from enough food, but from a range of different foods.
Flores likened the idea to planting a pollinator garden.
“The more different varieties of plants you have in your garden, the more different pollinators are going to come,” she said. “I think about the gut in a similar way. More diversity in the diet may help support more diversity in the gut.”
The mice were fed a high-complexity carbohydrate diet that included simple sugars plus starches from corn, wheat and potato for four weeks.
Then, for another four weeks, the mice either stayed on that diet or were switched to a mid-complexity or low-complexity diet. The diets were designed so the major nutritional categories such as calories, total carbohydrate content, protein, fat and fiber were the same in all of the diets. What changed was the number and type of digestible carbohydrate sources.
The mid-complexity diet included the same simple sugars and starch from corn. The low-complexity diet included only simple sugars as digestible carbohydrate sources.
Cellulose, a non-digestible fiber source, stayed constant across all diets.
“During the first four weeks, we were trying to give ourselves an even playing field,” Flores said. “That way, when we changed the diets, we were really testing how limiting the carbohydrates from that initial high-complexity diet influenced things.”
Common in human diets
Flores said corn, wheat and potato starch were selected because they are common in human diets and differ in structure. Potato starch, for example, contains more resistant starch, which behaves more like fiber because the host does not fully digest it. That leaves more material available for gut microbes.
“Those starches are not all the same. They have different structures and different levels of resistant starch, and that means they can provide different resources for the microbes,” said Flores, who expects to graduate with her doctoral degree in December.
Simple sugars such as glucose, fructose, sucrose and lactose are more readily absorbed by the host. Complex carbohydrates, including starches and resistant starches, are more likely to reach the colon, where gut bacteria can ferment them.
Researchers tracked food consumption by cage and weighed the mice throughout the study.
The lower-complexity diet mice remained outwardly healthy. The lower-complexity diets did not significantly change total weight gain, body composition, gastrointestinal transit time or intestinal barrier integrity.
But inside the gut, the microbial community shifted.
The researchers found that mice fed the mid- and low-complexity carbohydrate diets showed an overall decrease in fecal microbial diversity after the diet switch and in cecal microbial diversity at the end of the study.
Clear shifts
“I thought maybe our shifts in diet weren’t going to be big enough to make a clear difference,” Flores said. “But we did see clear shifts in the microbiota.”
One bacterial genus, Akkermansia, increased in mice fed the lower-complexity diets. The study found significantly higher relative abundance of Akkermansia in mice fed mid- and low-complexity diets than in mice maintained on the high-complexity diet, and an increase in absolute abundance in the low-complexity group.
“Akkermansia is interesting because it lives along the mucus lining in the gut,” Flores said. “It can use mucin as a carbon source, and people are studying it now as a potential next-generation probiotic.”
At the same time, bacteria in the Lachnospiraceae family decreased under lower-complexity diets. Flores said Lachnospiraceae stood out because members of that family can make metabolites considered beneficial to the host, including short-chain fatty acids.
Flores said the study measured which microbes were present, but not what all of them were doing. Taxonomic redundancy is the presence of multiple related microbial types within a broader group. Redundancy matters because ecosystems with more organisms capable of filling similar roles may be more resilient when disturbed. Flores said reduced taxonomic redundancy could reflect reduced functional redundancy, which could leave ecosytems more vulnerable to functional losses.
Healthy in the short term
The study did not show that the lower-complexity diets made the mice sick. In fact, Flores said one surprise was that the microbial shifts did not come with measurable changes in the host tests used in the study. She had expected that body weight might not change, but thought more sensitive measures, such as intestinal permeability, might show differences. They did not.
“I was a little surprised that we didn’t see significant changes in the permeability assays,” Flores said. “It was interesting to see clear shifts in the microbiota, but not see those physical characteristics change yet.”
That may mean the mice were still healthy in the short term, even though the microbial community had changed. Flores said the effects could become clearer over a longer period or after a disturbance, such as antibiotic treatment or exposure to a pathogen.
Relevance to humans
The findings may be relevant to modern diets, which often rely on processed foods and a narrower set of rapidly digestible carbohydrate sources. The study notes that many modern diets are calorically sufficient but can be dominated by processed and ultra-processed products rich in rapidly digestible carbohydrates from limited sources.
“I think diversity in carbohydrate sources is going to help keep that diversity of gut bacteria stable,” she said. “From this study, we can’t say exactly what functions we’re gaining or losing, but we can say that changing the variety of carbohydrates changes what is there.”
The next step is to move beyond identifying which microbes are present and examine what they are doing, Flores said.
“That’s where a lot more of the story is going to come out,” she said.
Details of the study were published in the journal Microbiology Spectrum in an article titled “Decreased carbohydrate sources reduce microbial diversity and taxonomic redundancy in the murine gut.”
The study was supported by the National Science Foundation and the National Institutes of Health.
