A saturated fat from beef fat and cocoa that's notably neutral on LDL.
Mechanisms, studies and evidence grades for each claim above.
"Saturated fat" covers several fatty acids that the body handles differently. The ones that raise LDL in feeding studies are lauric (C12), myristic (C14) and palmitic (C16). Stearic acid (C18:0) is the exception.
The feeding trials. In a 1988 metabolic-ward study, men ate liquid diets matched for calories, each high in one fat: palmitic, stearic or oleic. Compared with the palmitic diet, the stearic diet lowered LDL by 21%, slightly more than oleic acid (15%). All three fats were almost completely absorbed, so stearic acid's effect wasn't from poor absorption.1 A meta-analysis of 60 controlled feeding trials confirmed it: calorie for calorie, stearic acid had essentially no effect on LDL compared with carbohydrate and slightly lowered the total-to-HDL cholesterol ratio, while palmitic and myristic raised LDL.2
Why it behaves differently. Two reasons are usually given. First, the body quickly converts a share of stearic acid to oleic acid (see below), so it partly acts like olive oil fat. Second, stearic-rich fats such as cocoa butter and tallow put stearic acid mostly on the outer positions of the triglyceride molecule, where pancreatic lipase releases it as a free fatty acid. Free stearic acid tends to bind calcium in the gut into an insoluble soap, so some is excreted rather than absorbed. That matters more with whole foods than in the liquid-formula trials.
Whether LDL concentration is the right target is a separate question, taken up in Statins & LDL and Linoleic Acid. Either way, stearic acid doesn't count against tallow, butter or dark chocolate on that measure.
↑ back to summaryThe enzyme stearoyl-CoA desaturase (SCD1), mostly in the liver and fat tissue, adds a double bond in the middle of stearic acid's 18-carbon chain, turning it into oleic acid (C18:1), the main fat in olive oil. This is the body's main source of its own monounsaturated fat, which it uses to keep membranes fluid and to build triglycerides and cholesterol esters.
In humans fed deuterium-labelled fats, about 9% of stearic acid was converted to oleic acid, compared with about 4% of palmitic acid converted to its own monounsaturated form. The conversion was lower when the diet was high in linoleic acid, which suppresses SCD1.3 People varied a lot in how much they converted, which may help explain why blood cholesterol responds differently to the same saturated fat from person to person. The conversion is real but modest. Most dietary stearic acid is burned as fuel or stored as stearic acid, not turned into oleic acid.
↑ back to summaryStearic acid also acts as a signal. In cells, it attaches to the transferrin receptor (TFR1), which switches on a pathway that makes mitochondria fuse into longer, more connected networks. Fused mitochondria generally burn fat more efficiently. In a human study, a single drink containing stearic acid caused mitochondrial fusion in blood cells within 3 hours and lowered circulating long-chain acylcarnitines, which suggests more fat was being burned.4 It's a short-term study of a marker, not an outcome, but it's one more way stearic and palmitic acid differ despite both being "saturated fat."
↑ back to summaryThere's no requirement for stearic acid. The body can make it from other fats and from carbohydrate. The richest foods: