Differential effects of stearic, palmitic, and oleic acid–enriched diets on gut microbiome, bile acid, and cholesterol metabolism in mildly hypercholesterolemic postmenopausal females: A secondary analysis of a randomized controlled trial

Background: Stearic acid (18:0), a saturated fatty acid, does not raise plasma low-density lipoprotein cholesterol concentrations compared with palmitic acid (16:0) and has similar effects to oleic acid (18:1), but underlying mechanisms remain unclear. Objectives: To determine if the hypocholesterolemic effects of dietary 18:0 and 18:1 relative to 16:0 are mediated by alterations in gut microbiome, bile acid (BA), and cholesterol metabolism. Methods: This study is a secondary analysis of a randomized controlled crossover trial in mildly hypercholesterolemic postmenopausal females (n = 17) who consumed isocaloric diets enriched in 18:0, 16:0, or 18:1 for 5 wk each, with 2-wk washouts. Gut microbiome composition, plasma and fecal BA profiles, cholesterol absorption and synthesis markers, and related gene expression were assessed at the end of each dietary phase. Diet effects and multiomics associations were evaluated using mixed-effects and multivariate models, accounting for repeated measures. Results: Fecal microbiome diversity was stable across diets, with modest species-level differences. Both 18:0 and 18:1 diets resulted in lower fasting total primary BAs [mean difference (95% confidence interval [CI]): –159.1 (–313.0, –5.7) and –200.2 (–353.0, –47.7), respectively] and higher nonfasting unconjugated primary BA concentrations [2.7 (1.1, 6.4) and 2.4 (1.1, 5.6), geometric mean ratio (95% CI), respectively] compared with the 16:0 diet. The 18:0 diet resulted in lower fecal secondary BAs compared with the 18:1 diet [–15.2 (–27.8, –2.6), mean difference (95% CI)] and higher nonfasting conjugated BAs compared with the 16:0 diet [1.4 (0.8, 2.3), geometric mean ratio (95% CI)]. Farnesoid X receptor and small heterodimer partner expression were 3- to 4-fold higher (P < 0.01), and the cholesterol synthesis:absorption ratio was 22%–24% lower, after both 18:0 and 18:1 compared with the 16:0 diet (P < 0.05). Multimomics analyses identified moderate-to-strong correlations (r = 0.43–0.72) among microbes, BAs, gene expression, cholesterol metabolism markers, and lipid profiles. Conclusions: The cholesterol-lowering effects of 18:0 and 18:1 relative to 16:0 involved shared hepatic regulation of BA and cholesterol metabolism, whereas 18:0 uniquely modulated specific microbial taxa and BAs, suggestive of altered enterohepatic BA recycling.This trial was registered at clinicaltrials.gov as NCT02145936.