The effects of starch supplementation on gut microbiota sterol and bile acid pathways

One of the most prevalent forms of dietary fiber is resistant starch, which has been previously shown to improve glucose metabolism and lower blood cholesterol levels. These beneficial effects on host metabolic health are mediated by gut microbes. Resistant starch consumption is known to affect microbial transformation of bile acids, which affect blood cholesterol levels. However, there is a gap in knowledge regarding whether resistant starch consumption promotes another microbial process that affects serum cholesterol levels – the cholesterol:coprostanol transformation pathway. Individuals who possess gut microbes with intestinal sterol metabolism A genes can transform cholesterol to coprostanol and have been shown to have lower serum cholesterol. We conducted a seven-week crossover study with 59 individuals completing the three treatments. Participants consumed resistant starch type 2 (RS2), resistant starch type 4 (RS4), and a digestible starch, for ten days each with five-day washout periods in between. We collected fecal samples at the beginning and end of each treatment and extracted DNA from the samples to generate a shotgun metagenomics dataset. We also measured the concentrations of bile acids, and the sterols, cholesterol and coprostanol, in the stool samples. RS supplementation significantly affected microbiome composition. In agreement with previous studies, we found that resistant starch supplementation also affected total and conjugated bile acids. RS supplementation did not significantly affect sterol levels. However, we observed robust and consistent correlations across timepoints, and longitudinally within subjects, between the abundance of some species level genome bins (SGBs) and sterols levels. In particular, SGBs taxonomically assigned to the genus, Oscillibacter, were negatively correlated with cholesterol and positively correlated with coprostanol. Due to its ability to transform cholesterol, Oscillibacter is thought to benefit lipid metabolism and cardiovascular health. Our findings suggest that resistant starch does not affect the ability to transform cholesterol to coprostanol.