Seed oils

Last update: 13 December 2021

(For the archival version of the linoleate hypothesis article, see here.)

Whether the seed oils (also called “vegetable fats”, “n-6 polyunsaturated fats” and similar) are the cause or significant contributor to modern human chronic disease, including obesity, diabetes, coronary heart disease and cancer.

Proposition 1.

It would seem that seed oils cause obesity. In the US, BMI increases have tracked closely with the proportion of added vegetable fats in the food supply1,3,4. Linoleic acid especially seems to be obesogenic14.

PROPOSITION 1a.

It would seem a low proportion of n-3 fatty acids and high proportion of n-6 fatty acids in body tissues (rather than simply n-6 fatty acids per se) cause obesity8.

Proposition 2.

It would seem that seed oils cause diabetes13,17 and oils rich in n-6 fatty acids cause more diabetes than those lower in n-6 fatty acids11. The rise of diabetes in the US is closely associated with the rise of vegetable fats in the diet1,4,18.

Proposition 3.

It would seem that vegetable fats rich in n-6 cause coronary heart disease7, via the oxidation of linoleic acid10.

Proposition 4.

It would seem that linoleic acid is required for cancer to occur24, therefore it causes cancer.

On the contrary,

the Epidemiologist says2, “[…] the cause-and-effect interpretation of our data should not seriously conflict with the generally known facts of the natural history and biology of the disease – in the expression of the Advisory Committee to the Surgeon-General it should have coherence.”

I answer that,

Association doesn’t automatically imply causation, and extrapolation from animal experiments to humans should be done very cautiously. In the case of the seed oils, the association of them and bad things appears to be a historical accident in the United States (in particular), which is notably absent in some other places. Experiments in animals don’t find translation in humans, and even mechanistic studies on human subjects fail to find confirmation in long-term human studies with outcomes of interest.

All this is not to say that polyunsaturated n-6 vegetable fats have no health effects whatsoever (whether positive or negative) – there are many further hypotheses that should be tested – but their impact is vastly overstated by proponents of them as the sole or dominant factor in modern health.

Reply to Proposition 1.

Randomized controlled trials that tested increases in n-6 fats in the diet don’t back up the claim that n-6 fats increase body fatness5,6,7,28.

Reply to proposition 1a.

Randomized controlled trials regarding n-3 supplementation don’t find any difference in adiposity between arms, despite substantial increases in n-3 fatty acids in body tissues in intervention groups9.

Reply to Proposition 2.

Randomized trials find no difference in diabetes incidence or markers when increasing polyunsaturated fats (n-6 and others)23. Higher tissue concentrations of linoleic acid are associated with less diabetes than lower ones19,25. While this last item is not unequivocal evidence that n-6 PUFA protect against diabetes, due to marker competition with effects of diabetes (increased de novo sythesis of non-polyunsaturated fatty acids)20,21, it makes largely untenable the idea that they cause diabetes. Given the known etiology of diabetes progression and remission22, this is also reinforced by the non-obesogenicity of seed oils (shown above).

Reply to proposition 3.

While it is possible to construct a meta-analysis of a small number of trials that shows a trend towards harm7, pooling everything finds very little effect of n-6 on heart disease (including specifically CHD)6. The high variability in study quality and heterogeneity don’t reinforce any solid position of either benefit or harm. Higher tissue and circulating levels of n-6 fatty acids are associated with slightly less cardiovascular disease (including CHD) than lower concentrations, further degrading the proposition that these fats cause such illness12. Finally, coronary heart disease can exist in abundance even in absence of substantial n-6 polyunsaturated intake15,16.

Reply to proposition 4.

There is very little randomized trial data concerning the interaction of n-6 fats and cancer, all of it low quality26. Higher tissue concentrations of linoleic acid are associated with a less cancer mortality than lower ones27, which does not lend much credence either idea that linoleic acid is markedly carcinogenic or that there’s some threshold effect (that applies to humans) at very low intakes.

References

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  2. HILL AB. THE ENVIRONMENT AND DISEASE: ASSOCIATION OR CAUSATION?. Proc R Soc Med. 1965;58(5):295-300.
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