Lipid Hypothesis

The lipid hypothesis, also known as the diet-heart or cholesterol hypothesis, is the theory that cholesterol in the blood causes atherosclerosis, and that consuming polyunsaturated oils lowers blood cholesterol while saturated fats raise it, thereby preventing heart disease. 1 Ray Peat argued that this hypothesis was constructed without valid evidence, serving instead to create a market for the seed oil industry after their products were displaced from paints and plastics by petroleum-based compounds. 1 He noted that the hypothesis left behind any concept of nutritional essentiality, allowing industry and academic supporters to promote polyunsaturated oils as having drug-like therapeutic properties, with larger amounts supposedly being more protective. 1 Peat maintained that there was no basis for a connection between high cholesterol, saturated fat, and heart disease, a position he adopted after reading John Yudkin’s work in 1972, which showed that sugar, not fat, increased blood lipids. 2

Peat and others have detailed how the lipid hypothesis was refuted by multiple lines of evidence. Uffe Ravnskov demonstrated that none of the hypothesis’s arguments—linking dietary saturated fat to blood cholesterol, blood cholesterol to atherosclerosis, or atherosclerosis to heart disease mortality—had evidential support. 4 A veterans’ study in the 1960s found that replacing saturated fats with liquid oils caused more deaths from heart disease and significantly more from cancer, creating early problems for the theory. 4 Danny Roddy notes that Chris Masterjohn and others have thoroughly picked apart the flaws in the hypothesis, vindicating saturated fats in the process. 7 Georgi Dinkov has pointed out that the association between high cholesterol and cardiovascular disease is not causative, and that only oxidized cholesterol is problematic, with oxidation occurring only under conditions of excessive reactive oxygen species. 9 Peat emphasized that cholesterol is actually a protective substance, increasing at locations of injury and being necessary for healthy cell division, DNA replication, nerve function, and learning, functioning as the body’s most basic anti-stress protective substance. 4

Peat’s framework reinterpreted elevated cholesterol as an adaptive response to stress and low thyroid function rather than a dietary pathology. He observed that cholesterol increases during stress to provide the raw material for steroid synthesis, with levels rising before exams and normalizing afterward. 2 Elevated cholesterol is closely related to low thyroid function, and together with thyroid hormone and vitamin A, it is used to synthesize the “youth-associated” steroid precursor pregnenolone. 2 In this context, Peat saw cholesterol not as a toxin to be lowered, but as a crucial structural and functional molecule. He described protoplasm as a complex solution in which cholesterol acts as both a lubricant and a stabilizer, decreasing cell rigidity by increasing protein mobility. 2 Experimentally, lowering the cholesterol content of red blood cells makes them more rigid and fragile, while restoring normal cholesterol restores their flexibility, directly contradicting the membrane fluidity arguments of the lipid hypothesis. 10

The actual drivers of atherosclerosis, in Peat’s view, are the polyunsaturated fats that the lipid hypothesis promoted as protective. He documented that lipid peroxidation products from polyunsaturated fats, such as hydroxynonenal, malondialdehyde, and acrolein, attract macrophages that accumulate in plaques, while the age pigment lipofuscin, derived largely from PUFA, accumulates iron and catalyzes oxidation, creating local hypoxia and inflammation. 5 Furthermore, polyunsaturated fatty acids degrade the protein ABCA1, which is responsible for removing excess cholesterol from arteries, thereby impairing the cholesterol secretory pathway in macrophages. 5 Peat traced the origins of the “essential fatty acid” concept that underpinned the lipid hypothesis to the flawed 1929 experiments of George and Mildred Burr, whose fat-free diet produced a condition that was actually a vitamin B6 deficiency caused by the diet’s high sugar content accelerating metabolic rate and nutritional requirements. 3 He noted that animals fed a diet lacking these so-called essential fatty acids consumed oxygen at a very high rate, had unusually stable mitochondria, resisted immunological rejection of transplanted tissues, and were extremely hard to kill by trauma and toxins. 8