Linoleate

Last update: 29 August 2021

Here is a summary of the linoleic acid chronic disease hypothesis. Please let me know if you have supporting or opposing data that may substantially alter the conclusions (find my email here).

Hypothesis

Consumption of linoleic acid (a type of exogenous polyunsaturated fatty acid) is the a major contributor to chronic disease experienced in the westernized world34,35,36,40. Unnaturally high (as compared to pre-agricultural times) consumption of this lipid, well beyond ancestral intakes, causes or contributes to the so-called diseases of civilization – chiefly cardiovascular disease, cancer, diabetes and age-related neurodegeneration – via oxidative stress and metabolic dysregulation.

Intake observations

Ancestral intake of polyunsaturated n-6 fatty acids, in particular of linoleic acid, is low1, even as recently as the early 20th century3. As far as I know, there is only one exception among hunter-gatherers, in the !Kung people2, but this intake may be of relatively recent origin4. Agricultural societies, such as the ancient Egyptians noted for presence of atherosclerosis6, may have had greater than paleolithic intakes of linoleate, but it is far from clear. It is clear that they had an extensive vegetable oil production industry52.

In the United States of America, intake of of linoleic acid (via vegetable oils) has greatly increased over the last sixty years5, and has likely been rising ever since cottonseed oil was introduced as a food product in the late 19th century7.

Biomarkers

Higher linoleic acid concentrations in tissues are associated with lower mortality44,45. However, this cannot be taken to mean that it is causally protective because these are confounded by at least three factors – carbohydrate intake46 (which can’t be assumed to be neutral), diabetes48 (which dilutes linoleate47), and genes49.

Post-war New Zealand exists as a data point against the hypothesis, having considerable heart disease53 yet low adipose tissue linoleate28.

Disease associations

The omega-3 index is strongly associated with cardiovascular disease30,39, but marine oil supplementation studies show very little effect31. Since n-3 and n-6 compete for enzymes32, this suggests that the excess of n-6 fatty acids is the problem, not the lack of n-3 fatty acids.

There is very little data on association of n-6 fatty acids and cancer33.

Consumption of polyunsaturated oils is associated with age-related macular degeneration34.

Diabetic retinopathy is associated with lower n-3 erythrocyte fatty acids and lower n-3 to n-6 ratio38.

Adipose tissue inflammation in obesity has been linked to polyunsaturated fatty acid peroxidation41.

Experimental data – animals

Linoleate (or its parent vegetable oils) has been shown to:

  • contribute to obesity (and by extension – diabetes)8,10,15,17,
  • cause heart failure9,17,
  • induce mitochondrial dysfunction11,17,
  • induce cancer20,
  • permit alcoholic damage to liver42,
  • dysregulate hypothalamic gene expression43.

Of particular note is the fact that, at least for cancer, there is a threshold value (approximately 4.4% energy) above which there is no further increase in incidence20.

Removal of linoleic acid metabolites is accomplished, apparently, primarily through beta-oxidation12,13, which suggests a low-carbohydrate diet can increase an organism’s tolerance for high linoleic acid intake. This is consistent with data on time-restricted feeding, which also increases beta-oxidation14.

Experimental data – humans

Consistent with the proposed mechanism, reducing linoleate intake also reduces its metabolites16. Reducing linoleate to less than 4% of total energy intake cures non-alcoholic fatty liver disease18, even just replacing cooking oil helps29. Replacing saturated fat with polyunsaturated vegetable fat has been shown to significantly increase mortality in at least one study19 (there are also some that showed non-significant increases19,37) and in total no benefit to the intervention. In general, added n-6 interventions don’t show much benefit50,54, and what they do is probably a result of simultaneously added n-351.

Consistent with animal studies20, but inconclusive here, is that cancer is not greatly affected by high linoleic intake21, possibly due to the threshold effect.

Inconsistent with the hypothesis, insulin resistance is better with more polyunsaturated fat (usually mostly linoleate)55,57. The case is similar with liver fat56, at least when compared to saturated (palmitate rich) fat.

My conclusions

It seems possible that excess linoleate consumption is a major problem, especially in the context of a high-carbohydrate diet, that has been largely missed due to a preoccupation with cholesterol, and threshold effects of intake. Ceasing to consume excessive (>4%) linoleic acid should hypothetically eventually (years) resolve these problems, as body fat stores22 and other tissues23 reconstitute with less of it, but some effects may be palpable much sooner, as the bulk of the adipose is long-term storage22, and various tissues have different lifespans.

On a practical level, linoleic acid is very hard to avoid, even if one does not consume these oils or processed foods (where vegetable oils are added). Unprocessed animal foods often contain high amounts of it24,25,26, as monogastric animals are fed high-linoleate feed, which shows up in their products. Beef linoleate, at least, is hardly affected by grain or grass-feeding27. If you’re a farmer, you might want to lower the amount of linoleic acid you feed your animals, since it affects their lipids28 (and probably their health).

Increasing beta-oxidation by restricting carbohydrate and intermittent fasting will probably help to tolerate greater-than-optimal linoleate intake.

Regardless, as of writing, this remains a (rather fringe) hypothesis, and is far from solid. I, personally, used to be far more convinced it is true than I am right now.

Refutation

A good hypothesis is falsifiable. Therefore, I propose the following methods to potentially refute linoleate’s effects:

  • An ad-libitum, inpatient feeding trial matched for protein, carbohydrate and fat, and possibly other relevant factors (like sugar and fiber) but different in levels of n-6 and especially linoleic acid. One arm should have typical western levels (~10 g/d), the other ancestral levels (~1 g/d). Primary outcome – ad libitum calorie intake. Secondary outcome – change in body fat.
  • As above, except isocaloric and eucaloric. Primary outcome – change in liver fat from baseline.
  • Long-term dietary modification for cardiovascular disease patients, similar to early vegetable oil interventions. Assign people an amount of butter (forbid marganines and mixes) to eat per day, forbid the use of any cooking oils (to reduce the impact of adulteration), track compliance by adipose tissue fatty acid changes over time. Primary outcome – all-cause mortality. Secondary outcome – major adverse cardiovascular events.

Sources

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