Not all saturated fats behave the same. A small trial looked at why one of them spares cholesterol

Saturated fat is usually treated as one thing on a food label, but it is a family of different fatty acids. Two of the most common are palmitic acid, the main saturated fat in palm oil and a large part of dairy and meat fat, and stearic acid, plentiful in cocoa butter and beef fat. Earlier feeding studies have found that stearic acid does not raise LDL cholesterol compared with palmitic acid, and behaves much like oleic acid, the main fat in olive oil. Why has been less clear.

A paper published online in The American Journal of Clinical Nutrition on 6 October 2026, from the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, went back to a controlled crossover trial to look for an explanation. Seventeen postmenopausal women with mildly raised cholesterol ate three diets with the same calories, each enriched in stearic, palmitic or oleic acid, for five weeks at a time, with two-week breaks in between. At the end of each diet the team measured gut bacteria, bile acids in blood and stool, markers of how much cholesterol the body made and absorbed, and the activity of related genes.

Gut bacterial diversity barely changed between diets. The clearer differences were in bile acids, which the liver makes from cholesterol. Compared with the palmitic acid diet, both the stearic and the oleic acid diets lowered fasting levels of primary bile acids, raised the activity of two genes (FXR and SHP) that act as the body's bile acid sensors three- to four-fold, and lowered the ratio of cholesterol made to cholesterol absorbed by 22 to 24%. Stearic acid also shifted a few specific gut microbes and bile acids of its own. The authors read this as a shared liver pathway, with an extra gut effect for stearic acid.

This is a small secondary analysis in one group of women, measuring markers rather than heart attacks or strokes, so it explains a known result rather than offering new diet advice. It does add to the case that the type of fat matters, not only the label 'saturated', and that swapping palm-heavy fats for unsaturated oils remains the better-tested choice.