Overview
Linoleic acid is the so-called essential fatty acid and the most abundant polyunsaturated fat in the modern food supply, but it is not actually essential for human health. The 1929 Burr studies that established its essential status were invalidated in the 1940s when researchers showed that the symptoms blamed on linoleic acid deficiency were actually caused by vitamin B6 deficiency on the impoverished experimental diet. Linoleic acid is anti-thyroid, pro-cancer, and pro-inflammatory. It blocks thyroid hormone at every step from secretion to cellular response, converts to arachidonic acid which fuels inflammatory prostaglandins, and accumulates in tissues for years where it gets liberated under stress to drive vicious cycles of stress hormones and fat breakdown. The rise of linoleic acid in the diet, primarily from soy and corn oil pushed since the 1940s, tracks the rise in obesity, diabetes, heart disease, and cancer.
Key Points
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Linoleic acid is not an essential nutrient. The 1929 Burr study that established the doctrine was invalidated by 1946. George and Mildred Burr fed rats a purified diet missing linoleic acid and observed scaly skin, infertility, and slowed growth, which they called Burr disease. Roger Williams' lab at the University of Texas later reproduced Burr's exact diet and cured the same symptoms with vitamin B6 alone. The animals on the fat-free diet had a 50 percent higher metabolic rate, so their nutritional needs simply exceeded what the deficient diet could provide. Two years before Burr's experiment, German researchers had already shown that a fat-free diet prevented almost all spontaneous cancers in rats.
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Linoleic acid blocks thyroid function at every stage from secretion to cellular response. A French series of studies in the 1980s showed that polyunsaturated fats inhibit the thyroid system at multiple points: the proteolytic enzymes that release thyroid hormone from the gland, the proteins that transport it in the blood, and the cellular response to the hormone. The blocking effect scales with the number of double bonds, so linoleic acid is intermediate, linolenic acid is worse, and fish oils with five or six double bonds are almost total inhibitors. Even progesterone production is blocked in proportion to the number of double bonds in the fatty acids present.
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Linoleic acid converts to arachidonic acid, which fuels inflammatory prostaglandins. Even if you don't eat arachidonic acid directly, linoleic acid will be turned into it by enzymes in the body. Arachidonic acid becomes one of the most toxic stored fats in both phospholipids and triglyceride storage. When liberated under stress, it forms prostaglandins, which are the biggest single source of inflammation in the body, driving swelling, edema, bone loss, allergy, and progressive tissue damage.
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Linoleic acid was originally a livestock fattening agent before being marketed to humans as heart-healthy food. In the late 1940s, the agricultural industry used chemical thyroid suppressors to fatten pigs on less feed. When those chemicals were found carcinogenic, the industry switched to corn and soy, which contain enough linoleic acid to suppress thyroid the same way. The animal's fat becomes chemically equivalent to the vegetable oil it ate. When the seed oil industry needed a market beyond paint and animal feed, they sold the same oils to humans on the basis that they lowered blood cholesterol.
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The toxic threshold is roughly four grams per day, about one teaspoon of polyunsaturated fat. Above that level, the breakdown of polyunsaturated fats begins to show measurable increases in cancer and degenerative disease. Coconut oil contains about 1 percent polyunsaturates. The dangerous concentrations are found in grains, seeds, nuts, vegetable oils, and the meat of non-ruminant animals (chicken, pork, fish), which reflect their feed almost exactly.
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Linoleic acid stored in body fat is liberated under stress, activating a hormone cascade that releases more of it. When fatty acids are released from fat stores, they activate adrenaline, ACTH, cortisol, glucagon, and TSH. All of these in turn cause more fatty acid release, creating vicious circles. Only the polyunsaturated fatty acids do this. Stearic acid, the saturated fat in butter, has the opposite effect: it turns off adrenaline, ACTH, and cortisol. Hans Selye showed that purified linoleic acid causes spots of cardiac tissue to die, and that adding stearic acid from cocoa butter neutralized the heart damage.
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The metabolites of linoleic acid are mutagens and carcinogens. Malondialdehyde, which represents about 80 percent of the aldehydes produced from PUFA breakdown, is officially listed by the NIH as a known human carcinogen. 4-hydroxynonenal is now recognised as a biomarker for Alzheimer's, Parkinson's, and multiple sclerosis. Acrolein, another aldehyde from PUFA peroxidation, is classified as a known human carcinogen in essentially every medical textbook. Saturated fats produce none of these aldehydes during metabolism.
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Linoleic acid is the precursor to all the major inflammatory mediators. Arachidonic acid, prostaglandins, leukotrienes, and thromboxanes are all synthesised from linoleic acid. There is no other significant pathway through which chronic inflammation is generated in the body. The more PUFA in the system, the higher baseline inflammation, which then drives cortisol upward, suppresses thyroid, activates aromatase, increases serotonin synthesis, and locks the cell into the field of stress hormones.
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Linoleic acid is the precursor of lipofuscin, the brown age pigment. When polyunsaturated fats break down in tissues, they form lipofuscin, which acts as an enzyme equivalent that wastes oxygen and energy and produces hydrogen peroxide. The brain accumulates DHA and other highly unsaturated fats with age, slowing metabolism and contributing to Alzheimer's. Mink fed too much fish or polyunsaturated-fed horse meat developed yellow fat disease, hindquarter paralysis, and death from this same pigment buildup.
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Body fat takes about four years to fully turn over after dietary changes. Even after stopping linoleic acid intake, the stored polyunsaturated fats are gradually released over years, with about half replaced after two years. Half an ounce of coconut oil produces a burst of thyroid-like activity within an hour as it competes against the stored polyunsaturates, but lasting tissue change is slow. After about two years on a clean diet, the metabolism stabilizes at a much higher level.
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Average intake is now 12 to 13 percent of calories, when ancestral levels were under 2 percent. Carcinogenic territory begins around 4 to 5 grams a day, and the typical Western diet delivers 12 to 13 percent of calories from linoleic acid, with some people reaching 20 percent. The target should be below roughly 1.3 percent of total calories, around 3 grams per day. Mead acid is the standard blood test for essential fatty acid deficiency; being in the bottom 10 percent of that range is desirable, not concerning.
Notable Quotes
"Linoleic acid is the anti-thyroid, pro-cancer, pro-inflammatory, so-called essential fatty acid"
[Ray Peat — Milk, Calcium and Hormones]
"Above the equivalent of about a teaspoonful of the polyunsaturated fats per day, four grams a day is where the breakdown of these fats starts showing an increased incidence of cancer and other degenerative diseases"
[Ray Peat — The Science Behind the Dangers of Polyunsaturated Fats]
"Hans Selye gave purified linoleic acid, a major component of rapeseed (canola) oil, and showed that was the actual toxic agent to the heart"
[Ray Peat — Fats (PolSci, July 2008)]
"Linoleic Acid binds directly to the estrogen receptors and acts like a full estrogen, 100%."
[Georgi Dinkov — Mercola Interview on Estrogen and Serotonin]
Important Things To Consider
Typical American linoleic acid intake is many times the toxic threshold. A single restaurant meal cooked in vegetable oil can deliver 20 grams or more, against a tolerance of about 4 grams per day. Bread, salad dressings, fried foods, processed snacks, mayonnaise, and the meat of grain-fed animals all stack on top of each other. The cumulative dose is what matters because linoleic acid stays in body fat for years.
Animal fats reflect exactly what the animal ate. Pigs and chickens fed corn and soy have body fat that is chemically equivalent to those oils, making them just as toxic and fattening as the oils themselves. Ruminant animals (cattle, sheep, goats) are protected because rumen bacteria hydrogenate the polyunsaturated fats from grass before they reach the meat. This is why grass-fed dairy and beef are safer than chicken, pork, or farmed fish.
Conjugated linoleic acid (CLA) from dairy is protective, not harmful. CLA is a natural component of dairy and ruminant fat, formed when rumen bacteria partially hydrogenate the polyunsaturates in grass. It blocks the action of linoleic acid and has anti-inflammatory and metabolism-stimulating effects. The small amount of natural dairy trans fats are intermediate forms in this protective hydrogenation, which is why they should not be confused with industrial trans fats from partially hydrogenated vegetable oils.
Even partially hydrogenated vegetable oils retain enough linoleic acid to be toxic. Industrial hydrogenation does not remove enough polyunsaturates to make these fats safe. Margarine has been blamed on its trans fat content, but the underlying problem is that linoleic acid is still present in significant amounts. Coconut oil and butter remain the safest cooking fats, with olive oil acceptable as a garnish in small amounts (a teaspoon or so) because it is only about 10 percent polyunsaturated.
Aspirin and niacinamide help by blocking the release and inflammatory conversion of stored linoleic acid. Niacinamide inhibits the lipase that liberates fats from fat stores. Aspirin acts on multiple lipase enzymes including the adipose hormone-sensitive lipase, and it blocks the conversion of arachidonic acid into inflammatory prostaglandins. Both compounds break the stress cycle that linoleic acid storage perpetuates.
The placenta protects the developing baby from linoleic acid. Newborns from healthy, well-fed mothers have very low polyunsaturated fat content in the brain, which conventional science wrongly labels an "essential fatty acid deficiency." This is actually the protected state. As soon as breastfeeding stops or formula with added PUFA is introduced, the brain begins assimilating linoleic acid, forming cholesterol esters that progressively block metabolism into adulthood.
Linoleic acid intake during pregnancy can render male offspring infertile. Animal studies show that high-PUFA feeding during pregnancy produces infertile male offspring, and that giving pure polyunsaturated fats to healthy males can render them infertile, with prolonged consumption potentially causing structural and partially irreversible damage to the gonads. The natural infertility agent gossypol in cottonseed oil has been studied as a male contraceptive pill.
Vitamin E intake should scale with linoleic acid intake. The functional RDA is roughly 2 milligrams of vitamin E per gram of linoleic acid consumed. At 50 grams of PUFA per day, that is around 100 milligrams of mixed tocopherols. Most commercial vitamin E products are dl-alpha-tocopherol acetate, which delivers only about 25 percent of the activity of the natural d-isomer non-esterified form. The L-isomer can also displace the active D-isomer from tissues, which can leave you worse off than not supplementing.