Seed Oils: Toxic or Heart-Healthy? Your FADS Genes Are the Missing Variable

Few food debates are as heated as seed oils. To critics, soybean, corn and sunflower oil are inflammatory industrial products behind modern chronic disease. To most cardiologists, they are among the better fats you can eat. In 2026 both sides got new ammunition: the new Dietary Guidelines for Americans stopped recommending oils rich in linoleic acid and pointed to olive oil, butter or beef tallow instead, while a review published in May 2026 concluded that the totality of the evidence supports the safety of seed oils. One thing both camps usually leave out is that people do not process linoleic acid the same way, and a gene cluster called FADS is a big part of the reason.

What is actually in seed oils

The main fat in soybean, corn, sunflower and safflower oil is linoleic acid, an omega-6 fatty acid your body cannot make. Some of it is converted, step by step, into arachidonic acid, the raw material for signalling molecules involved in inflammation and blood clotting. The worry about seed oils rests on that conversion: more linoleic acid, the argument goes, means more arachidonic acid and more inflammation.

What the evidence says on average

• Large cohorts: in a pooled analysis of 30 cohort studies with 68,659 people, those with the highest blood levels of linoleic acid had a lower risk of cardiovascular disease and death. Higher arachidonic acid levels were not linked to higher risk (Marklund et al., Circulation, 2019).
• Old trials: critics point to recovered data from two trials of the 1960s and 1970s, the Sydney Diet Heart Study and the Minnesota Coronary Experiment. Replacing saturated fat with linoleic-rich oil lowered cholesterol but did not lower deaths, and in Sydney deaths rose (Ramsden et al., BMJ, 2013 and 2016). Defenders of seed oils note the limits of those trials, including a margarine used in Sydney that likely contained trans fats.
• The 2026 review: published in Critical Reviews in Food Science and Nutrition, it concluded that seed oils are safe and benefit heart health when they replace saturated fat. It was partly funded by the soy industry, which is worth knowing when you read it.

On average, then, linoleic acid looks neutral to beneficial for the heart when it replaces saturated fat, mostly on the strength of observational data. But "on average" is exactly where genetics comes in.

FADS1 and FADS2: your conversion speed

The enzymes that turn linoleic acid into arachidonic acid are coded by FADS1 and FADS2. Common variants in this cluster have some of the strongest effects of any gene on blood fatty acids. In one genome-wide study, a single FADS1 variant, rs174537, explained about 18% of the differences in arachidonic acid levels between people (Tanaka et al., PLoS Genetics, 2009).

These variants differ greatly between populations. A fast-converting version of the cluster is very common in people of African ancestry and is thought to have spread as diets shifted toward plants (Mathias et al., PLoS One, 2012). Greenland Inuit, whose ancestors lived on a marine diet for thousands of years, carry versions that convert less (Fumagalli et al., Science, 2015).

The experiment that tested it directly

In Finland, researchers recruited men with two copies of one or the other version of a FADS1 marker, rs174550, which is inherited together with rs174537 in most people of European ancestry. For four weeks, the men added linoleic-rich sunflower oil to their diet (Lankinen et al., American Journal of Clinical Nutrition, 2019). The same oil had different effects depending on genotype: the response of plasma fatty acids, fasting glucose and the inflammation marker hsCRP all depended on it. In men with the TT genotype, signalling molecules made from arachidonic acid rose and fell with hsCRP; in men with CC, they did not.

The authors are clear that this was a small, short study in men only, and that dietary advice based on FADS1 needs larger trials. But it is the most direct evidence so far that "seed oils are inflammatory" may be closer to true for some people than for others.

What to do with your FADS result

• High-converting genotype: you turn linoleic acid into arachidonic acid efficiently. You do not need to fear seed oils, but you have the most reason to keep them moderate, to make olive oil your everyday fat, and to eat oily fish for omega-3.
• Low-converting genotype: linoleic acid is less likely to raise your arachidonic acid, but you also convert plant omega-3 (ALA) poorly, so oily fish or algae oil matter more for your EPA and DHA. Our FADS and omega-3 guide explains why.
• Everyone: the strongest evidence is about what seed oils replace. Cooking with sunflower oil instead of butter is not the same as adding fried snacks to your diet. In Western diets, much of the linoleic acid comes from fried and ultra-processed foods, which have other problems of their own.

Read your FADS genes

Your raw data file from 23andMe, AncestryDNA or MyHeritage contains the main FADS1 and FADS2 markers. The Complete report reads five of them and explains how efficiently you convert these fatty acids, with the matching advice on oils and omega-3.

Educational information, not medical advice. If you have heart disease or high cholesterol, discuss changes to your fats with your doctor or dietitian.

References

1. US Department of Agriculture, US Department of Health and Human Services. Dietary Guidelines for Americans, 2025-2030. January 2026.
2. Soy Nutrition Institute Global. Totality of evidence supports safety and benefits of seed oils, new review finds (review in Critical Reviews in Food Science and Nutrition). Press release, May 2026.
3. Marklund M, Wu JHY, Imamura F, et al. Biomarkers of dietary omega-6 fatty acids and incident cardiovascular disease and mortality: an individual-level pooled analysis of 30 cohort studies. Circulation. 2019;139(21):2422-2436.
4. Ramsden CE, Zamora D, Leelarthaepin B, et al. Use of dietary linoleic acid for secondary prevention of coronary heart disease and death: evaluation of recovered data from the Sydney Diet Heart Study and updated meta-analysis. BMJ. 2013;346:e8707.
5. Ramsden CE, Zamora D, Majchrzak-Hong S, et al. Re-evaluation of the traditional diet-heart hypothesis: analysis of recovered data from Minnesota Coronary Experiment (1968-73). BMJ. 2016;353:i1246.
6. Tanaka T, Shen J, Abecasis GR, et al. Genome-wide association study of plasma polyunsaturated fatty acids in the InCHIANTI Study. PLoS Genet. 2009;5(1):e1000338.
7. Mathias RA, Fu W, Akey JM, et al. Adaptive evolution of the FADS gene cluster within Africa. PLoS One. 2012;7(9):e44926.
8. Fumagalli M, Moltke I, Grarup N, et al. Greenlandic Inuit show genetic signatures of diet and climate adaptation. Science. 2015;349(6254):1343-1347.
9. Lankinen MA, Fauland A, Shimizu BI, et al. Inflammatory response to dietary linoleic acid depends on FADS1 genotype. Am J Clin Nutr. 2019;109(1):165-175.