The category “saturated fat” encompasses several individual saturated fatty acids, which are often grouped together on nutrition information. Dietary guidelines advise consumers to limit their intake of saturated fats, since sources such as butter, dairy, and coconut oil can raise risk of cardiovascular disease.
However, recent research has shown that the body treats individual saturated fatty acids differently from each other, indicating the potential for more nuanced dietary advice in the future.
In particular, the saturated fatty acid stearic acid, which is found in cocoa butter, meat, and animal fats, does not raise LDL-cholesterol as much as palmitic acid, another common saturated fatty acid. A new study by researchers at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University points to a potential biologic explanation for this distinction in the gut microbiome and bile acid metabolism.
The research, published in the American Journal of Clinical Nutrition, used blood and stool samples to examine how diets enriched in different saturated and unsaturated fatty acids affect the gut microbiome, bile acids, cholesterol production and absorption, and genes regulating these processes in post-menopausal women.
“To our knowledge, this is the first human study to examine how diets enriched in these specific fatty acids may influence all of these pathways together,” explained Wen Zhang, N26, first author of the study. “Looking at these systems collectively allowed us to build a more complete picture and provide a specific hypothesis for future studies to test more directly.” Zhang recently earned her Ph.D. from the Gerald J. and Dorothy R. Friedman School of Nutrition Science and Policy.
Relative to palmitic acid, stearic acid and the unsaturated fatty acid oleic acid both lowered LDL-cholesterol. Diets enriched in these fatty acids led to similar changes in markers of cholesterol production and absorption, and in genes involved in regulating cholesterol and bile acid metabolism. However, the researchers found more granular differences at the intersection of the gut microbiome and bile acid metabolism.
Eating triggers the gallbladder to release primary bile acids into the small intestine to digest and absorb fats. Eventually, most of these bile acids are reabsorbed and returned to the liver. Some, though, move on to the large intestine, where specialized microbes convert them into secondary bile acids. A first set of microbes uses an enzyme called bile salt hydrolase to remove an amino acid from the primary bile acids, a step called deconjugation. Then, another group of bacteria further processes these unconjugated primary bile acids into secondary bile acids.
Women who consumed diets enriched in stearic acid (compared to oleic acid) had higher levels of microbes capable of the first step, without an increase in microbes performing the second step. The women also had higher levels of unconjugated primary bile acids in blood samples taken after eating a meal, suggesting that stearic acid may alter how bile acids are modified and recycled between the gut and liver.
This nuance provides a concrete signal for researchers to investigate further. However, since most bile acids are reabsorbed, stool samples miss a key source of information about gut metabolism. Historically, invasive endoscopes were the only way to probe the gut microbiome further. But Nirupa Matthan , scientist 1 at the HNRCA and associate professor at the Friedman School, is working with the Tufts School of Engineering to develop an ingestible "mini-pill" that takes samples along the intestine. Matthan hopes the mini-pill will help uncover differences in bile acid metabolism between dietary fatty acids in greater detail.
The team’s research may also clarify a long-standing scientific debate around saturated fats. “Some epidemiology studies show saturated fats are not as bad, but randomized controlled trials consistently show they are unhealthy,” Matthan said. “One of the reasons we are seeing this discrepancy may be that researchers are grouping saturated fatty acids together without accounting for the nuances between them.”
This research further fills a key gap between scientific understanding and industrial practice. Food manufacturers have reformulated some products with stearic acid as a replacement for trans fats, which have been largely banned due to their association with cardiovascular disease.
“Stearic acid might be a good substitute in terms of palatability and texture, but we need to better understand its health effects,” Matthan explained. “We don't want to make the same mistake we made with trans fatty acids.”
Ultimately, this growing understanding of how our bodies treat different fatty acids may lead to future dietary guidelines that provide more detailed recommendations about the broad category of saturated fats.