Low-carb diets radically alter the balance of microbes in the gut with changes depending on whether fats or proteins are used as substitutes, preclinical research suggests
The canine study, reported in PNAS, suggests that low-carbohydrate (LC) diets extensively reprogram the gut, restructuring its microbial community and metabolic activity in different ways
These diets therefore not only change nutrients directly available the host but also the microbially derived metabolites available, with potentially secondary effects on the gut
“Overall, our work shows that LC consumption has a dramatic influence on the gut microbiome and its underlying metabolism, with only some microbes dependent on the replacement energysures,” reported the authors
“This helps to inform both studies of the impact of diet on host and the microbiome generally, as well as specific strategies for improved human health outcomes
Replacing carbohydrates in the diet with fats or protein is associated with positive health outcomes, with studies showing it can aid weight loss, epilepsy control, and reduce inflammation
However, the impact of these dietary changes on the gut—and how its microbiome mediates any health benefits accrued—remains poorly understood
It remains unclear how the microbiome impacts health benefits from low-carbohydrate diets
Jacob Nearing, PhD, from the Harvard Chan Microbiome in Public Health Center, and co-workers therefore examined how the gut microbiome responded to reduced carbohydrate intake in a study involving 35 dogs
The animals were fed a high-carbohydrate diet for four weeks before being switched to low-carb diets for five weeks then switched again to the alternative low-carb diet for another five weeks. Fecal and blood samples were collected after each interval
The team found that switching from a high-carb diet dramatically remodeled microbiomes and metabolomes
Both low-carb diets changed the abundance of bacterial species in the gut, consistently and significantly reducing Bifidobacteriumspecies whereas many species from Firmicutes increased
Several microbially associated metabolites—including tryptophan derivatives, secondary bile acids, and short-chain fatty acid fermentation—shifted in abundance in response to the differing low-carb foods
Biochemical signatures also suggested increased microbial protein catabolism on the low-carb diets, even if fat was replacing the carbohydrates
Fecal samples showed a strong trend toward decreased amino acids with the low-carb diets, particularly the high-fat version. Together with decreased fecal dipeptide levels and increased fecal ammonia, this indicated a broad metabolic shift within the gut microbiome, potentially towards protein catabolism
Several sugars decreased in the feces of dogs fed the low-carb diets, as did levels of butyrate and β-Hydroxybutyric acid, the authors noted
“This suggests lower levels of fermentable carbohydrates, a primary substrate of Bifidobacterium metabolism,” they explained
“Thus, the loss of the bifidobacterial niche within the colon may have ultimately led to growth of other bacteria capable of utilizing alternative carbon
“For example, we saw several increases in bacteria from the Clostridiaceae family, a group of microbes known for their ability to metabolize amino acids.”
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