Stearic acid and palmitic acid are two common consumed saturated fatty acids found in foods. Palmitic acid is associated with increases in blood cholesterol, whereas stearic acid is generally considered neutral with respect to blood cholesterol. However, the mechanisms underlying these different effects are not fully understood. The goal of this clinical trial is to determine how replacing dietary palmitic acid with stearic acid affects microorganisms living in the gastrointestinal tract (gut microbiome) and the metabolites they produce, including secondary bile acids, and whether these changes influence heart and metabolic health in postmenopausal women. The main question it aims to answer is whether the cholesterol lowering effect of dietary stearic acid, compared to dietary palmitic acid is due to changes in the composition and function of the gut microbiome and bile acid metabolism. Participants will consume a controlled diet high in palmitic acid for a 10-day run-in period, followed by a controlled diet high in stearic acid for 28 days. During the study, participants will provide biological samples, including blood samples collected in both fasting and non-fasting states, for assessment of cardiometabolic outcomes. Participants will also swallow mini-pills designed to collect samples from different regions of the gastrointestinal tract to assess the gut microbiome and related metabolites. Results will contribute to understanding the relationship between stearic acid mediated microbiota effects on cardiometabolic heath.
Postmenopausal women (defined as complete natural cessation of menses for >12 months or a bilateral oophorectomy), who are not currently using hormone replacement therapy (HRT) or gender-affirming hormone treatments
Age >50 years
BMI >20 to <35 kg/m2, inclusive
Non-smoker or former smoker who quit > 6 months
Allergy/intolerance/religious reasons to avoid study meals or food ingredients
Regular use of dietary supplements, pre/probiotics, laxatives or fiber supplements (except multivitamins) within the past 3 months
Use of steroids (excluding topical) within the past 3 months
Regular use of aspirin (except baby aspirin), NSAIDS, or anti diarrheal medication
Single group
High Palmitic Acid Diet
Other interventionThe main source of palmitic acid that will be used to prepare the meals will include blends of palm oil and safflower oil, which are commercially available. These blends will be incorporated into foods such as muffins, cookies, sauces, and mixed dishes (e.g., casseroles).
High Stearic Acid Diet
Other interventionThe main source of stearic acid that will be used to prepare the meals will include blends of cocoa butter, fully hydrogenated soybean and safflower oil, all of which are commercially available. These blends will be incorporated into foods such as muffins, cookies, sauces, and mixed dishes (e.g., casseroles).
Relative Abundance of Gut Microbial Taxa (%)
Gut microbial composition in gastrointestinal luminal and stool samples will be assessed using whole-genome shotgun metagenomic sequencing and reported as relative abundance (%). Alpha and beta diversity will also be evaluated as part of the microbiome analyses.
Relative Abundance of Gut Microbial Functional Pathways (%)
Gut microbial functional pathways in gastrointestinal luminal and stool samples will be assessed from whole-genome shotgun metagenomic sequencing data using bioinformatic pathway analysis and reported as relative abundance (%). Analyses will include pathways related to bile acid metabolism as well as other microbial metabolic pathways.
Concentrations of Bile Acids in Gastrointestinal Luminal Samples (nmol/mL)
Primary and secondary bile acids, including conjugated and unconjugated bile acids, will be quantified in gastrointestinal luminal samples using targeted mass spectrometry and reported as nmol/mL.
Concentrations of Bile Acids in Plasma (nmol/mL)
Primary and secondary bile acids, including conjugated and unconjugated bile acids, will be quantified in fasting and postprandial plasma using targeted mass spectrometry and reported as nmol/mL.
Plasma Metabolite Concentrations (µmol/L)
Plasma metabolites will be measured using targeted metabolomic profiling and reported as concentrations in µmol/L.
Fecal Metabolite Concentrations (nmol/mg dry weight)
Fecal metabolites will be measured using targeted metabolomic profiling, normalized to fecal dry weight, and reported as nmol/mg dry weight.
Serum Cholesterol metabolism markers
Cholesterol synthesis markers (squalene, desmosterol, lathosterol) and cholesterol absorption markers (phytosterols) will be measured using gas-liquid chromatography, normalized to total cholesterol, and reported as 10² mmol/mol total cholesterol.
Fibroblast growth factor 19 (FGF19) Concentration (ng/mL)
Fibroblast growth factor 19 (FGF19) concentrations in distal ileum contents and plasma will be measured using a human FGF19 ELISA and reported as ng/mL.
Plasma Lipid Concentrations (mg/dL)
Plasma HDL cholesterol and triglyceride concentrations will be measured using enzymatic assays. LDL cholesterol will be calculated using the Friedewald formula. Individual lipid measures will be reported in mg/dL.
Serum Glucose Concentration (mg/dL)
Serum glucose concentration will be measured using an enzymatic assay and reported in mg/dL.
Serum Insulin Concentration (µIU/mL)
Serum insulin concentration will be measured using radioimmunoassay (RIA) and reported in µIU/mL.
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