Soybean oil

Research suggests LDL cholesterol falls when linoleic-acid-rich oils such as soybean oil replace saturated fat, one of the better-replicated findings in nutrition and the basis for long-standing dietary guidance. This is evidence about a substitution within the diet, not about a property of soybean oil that applies when it is simply added.

What may be going on: Polyunsaturated fatty acids upregulate hepatic LDL receptors and lower circulating LDL relative to saturated fatty acids of the same energy.

Moderate evidence. Several human studies point the same way, with real caveats.

The literature here is genuinely divided. Pooled analyses of trials replacing saturated fat with polyunsaturated oils generally report fewer coronary events, some individual trials from the same era point the other way once their unpublished data were recovered, and the question of whether very high omega-6 intakes carry costs remains open. The honest summary is that substitution appears favourable and the ceiling of that benefit is unknown.

Limited evidence. Early human work, or mostly lab and animal research.

Evidence is limited, and the popular claim that dietary omega-6 raises inflammation is not well supported by human data. Controlled trials increasing linoleic acid intake have generally not shown rises in C-reactive protein or other circulating inflammatory markers, but the trials are short and the marker set is narrow.

Insufficient evidence. Not enough human research to say anything useful yet.