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Seed oils are bad through calories
The case against
The case against them rests on epidemiology, where higher seed oil intake goes with worse outcomes but is tied up with many other behaviors, and on a plausible mechanism: polyunsaturated fats have multiple double bonds that can oxidize with heat, which could cause inflammation.
What trials show
The trials look different. Substituting polyunsaturated fat for saturated fat leaves inflammation basically neutral, and effects on cardiovascular disease markers tend to be neutral or positive. Based on those trials, monounsaturated and polyunsaturated fat in place of saturated fat is probably the better trade.
Why buckets mislead
Lumping fats into buckets hides differences. Stearic acid, a saturated fat, doesn't tend to raise LDL cholesterol, while saturated fat as a whole does. Seed oils have hurt health mainly because oil became a big addition to diets over the last 20 or 30 years, and that raised the overall calorie load.
Why do heated polyunsaturated oils oxidize?
Polyunsaturated fatty acids have hydrogen atoms sitting between double bonds, and free radicals pull them off easily. That starts a chain reaction called lipid peroxidation, which breaks the fat into smaller reactive compounds. Aldehydes such as 4-hydroxynonenal form when linoleic acid breaks down at high heat. Repeated heating, as in deep frying, speeds the process.
Why does stearic acid act differently from other saturated fats?
The liver converts a large share of stearic acid into oleic acid, a monounsaturated fat, using an enzyme called stearoyl-CoA desaturase. Feeding studies by Scott Grundy and others in the 1980s found that stearic acid did not raise LDL cholesterol the way palmitic and myristic acids did. Chain length decides much of how a saturated fat behaves in the body.
Why is epidemiology weak on diet questions?
Observational studies rely on food questionnaires, which depend on memory. People who eat more of one food also differ in income, smoking, exercise, and many other habits. Statistical adjustment cannot remove all of this, a problem nutrition researchers call residual confounding. Randomized trials avoid it by assigning the diet.
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