Omega-3s, omega-6s and fatty acids: what a blood test measures and what to do about it

Circulating fatty acids reflect the fats consumed over the previous weeks. A nuclear magnetic resonance (NMR) blood test quantifies total, saturated, monounsaturated and polyunsaturated fatty acids, including omega-3s (with DHA) and omega-6s (with linoleic acid). The literature supports a diet rich in fish and unsaturated vegetable oils far more than supplements.

What are the families of fatty acids, and what are they for?

Fatty acids are the building blocks of dietary fats and of our cell membranes. They are classified according to their chemical structure: saturated, monounsaturated and polyunsaturated, the latter being divided into omega-6s and omega-3s according to the position of their first double bond. Two of them, linoleic acid (omega-6) and alpha-linolenic acid (omega-3), are called essential: the body cannot make them, so they must come from the diet.

The benchmarks of ANSES, the French Agency for Food, Environmental and Occupational Health & Safety, set the share of fats at 35 to 40% of energy intake; they do not define a target omega-6/omega-3 ratio, but separate reference intakes for each family. The World Health Organization recommends limiting total fat to 30% of energy or less, saturated fat to less than 10% and trans fat to less than 1%, and favoring unsaturated fats, found in fish, avocado and nuts, and in sunflower, soybean, canola and olive oils (“Healthy diet” fact sheet, January 2026).

Why these families matter: excess saturated fats raise LDL cholesterol, polyunsaturated fats lower it when they replace them, and French National Health Insurance (Assurance Maladie) notes that omega-3s help reduce triglycerides and blood pressure in people with hypertension.

What is the omega-3 index, and how useful is it as a marker?

The omega-3 index is the proportion of EPA and DHA among the fatty acids in red blood cell membranes, expressed as a percentage. Proposed in 2004 by Harris and von Schacky in Preventive Medicine as a “new risk factor for death from coronary heart disease,” it reflects intake over the previous two to three months. The authors proposed two zones: an index above 8% associated with the lowest risk, below 4% with the highest risk.

An analysis pooling 17 prospective cohorts and 42,466 people (15,720 deaths during follow-up), published in Nature Communications in 2021, showed that people in the highest fifth of blood EPA and DHA levels had a risk of all-cause death about 13% lower than those in the lowest fifth (hazard ratio 0.87), with inverse associations for cardiovascular, cancer and other-cause deaths; plant-derived alpha-linolenic acid was not associated with mortality (Harris et al.). These data are observational: a high level goes along with better health, without proving that a pill would produce it.

The omega-3 index is rarely measured routinely in France. It is not included as such in NMR panels, which measure fatty acids in plasma rather than in red blood cells: the two are correlated but not interchangeable.

What does NMR metabolomics measure among blood fatty acids?

Nightingale Health’s NMR metabolomics panel quantifies, from a single sample, total fatty acids and their distribution: saturated, monounsaturated, polyunsaturated, omega-3s, omega-6s, docosahexaenoic acid (DHA) and linoleic acid, as absolute concentrations and as percentages of total fatty acids, as well as the average degree of unsaturation (Julkunen et al., Nature Communications, 2023). These measurements cover all plasma or serum lipids, that is, the fatty acids carried by the lipoproteins described in our article on cholesterol and lipoproteins.

What NMR does not do: it does not distinguish EPA from DHA (it measures DHA and total omega-3s) and does not measure the red blood cell omega-3 index. What it provides: a reading of your recent diet, comparable from one sample to the next. We explain the technique in our article on NMR metabolomics and the place of fatty acids among other blood biomarkers.

What does the literature say about omega-3s, omega-6s and cardiovascular risk?

The literature converges on three points: replacing saturated fats with polyunsaturated fats reduces cardiovascular events; eating fish once or twice a week is associated with lower risk; omega-3 supplements have not demonstrated a benefit in primary prevention in the general population.

• Replacing saturated fats. The 2017 American Heart Association presidential advisory (Circulation) concludes that, in randomized trials, replacing saturated fats with polyunsaturated vegetable oils reduced cardiovascular disease by about 30%, whereas replacing them with refined carbohydrates and sugars provides no benefit. This advisory does not find an adverse effect of omega-6s: the linoleic acid in vegetable oils is among the fats with a documented benefit.
• Fatty acids measured in the blood. In three population cohorts (13,441 people, 1,741 events), NMR profiling identified higher levels of omega-6s and DHA associated with lower cardiovascular risk, and higher levels of monounsaturated fatty acids and phenylalanine associated with higher risk (Würtz et al., Circulation, 2015).
• Fish. The 2018 American Heart Association science advisory recommends one to two servings of seafood per week to reduce the risk of heart failure, coronary heart disease, ischemic stroke and sudden cardiac death, “especially when seafood replaces the intake of less healthy foods” (Rimm et al.). ANSES, cited by French National Health Insurance, recommends two servings of fish per week, including one rich in EPA and DHA (salmon, sardines, mackerel, herring, smoked trout), varying species and origins to limit exposure to methylmercury and PCBs.

These effects fit within the overall risk described by the SCORE2 score and within comprehensive cardiovascular prevention; fatty acids are a dietary lever, not a treatment.

Diet or supplements: what is a reasonable position in 2026?

Our position, as doctors, is that of the medical societies: diet first, supplements for specific indications decided with a doctor. The 2017 American Heart Association science advisory on fish oil supplements notes that their effect on the primary prevention of cardiovascular events in the general population had not been studied in dedicated trials, and reserves reasonable supplementation for certain patients who already have coronary heart disease or heart failure, on medical advice (Siscovick et al.). More recent trials in high-risk people, using high doses of purified EPA, have produced conflicting results.

Why fish probably does better than a capsule: it replaces other foods and is part of a dietary pattern, Mediterranean or similar, whose benefit is established. Our practical guidelines are in diet and nutrition in prevention.

The question of supplements legitimately arises, with the doctor, for a person who never eats fish, in cases of very high triglycerides, and during pregnancy and breastfeeding, when DHA has a documented role for the baby.

What are the limitations of a fatty acid test?

A fatty acid test is a partial and recent reflection of diet, without established clinical thresholds, whose main value is monitoring over time. Five limitations to be aware of.

1. It reflects the last few weeks. A fish meal the day before changes the result; take samples under comparable conditions from one time to the next.
2. It does not measure diet alone. The share of monounsaturated fats depends on fat synthesis by the liver, itself linked to sugars, alcohol and insulin resistance; the gap with dietary intake is information in itself, to be read alongside blood glucose and HbA1c.
3. No consensus normal values. NMR percentages are interpreted by comparison with cohort distributions and with your own previous results, not against a standard.
4. Association is not causation. The low DHA seen in at-risk cohorts may be a marker of lifestyle rather than an actor; this explains why supplements do not reproduce the observed benefits.
5. It decides nothing on its own. A fatty acid profile is read together with lipoproteins, low-grade inflammation, blood glucose and reported habits; it guides dietary advice, never a treatment.

That is how we use it, following the logic of personalized health: a marker for monitoring what the person has chosen to change.

Key takeaways

• Saturated, monounsaturated, omega-6 and omega-3 polyunsaturated: ANSES sets separate benchmarks for each family (EPA + DHA: 500 mg per day) and no target omega-6/omega-3 ratio.
• The omega-3 index (EPA + DHA in red blood cells) is a marker of exposure; high blood levels are associated with lower mortality in 17 cohorts (Harris et al., 2021), without proof of causation.
• NMR measures total, saturated, monounsaturated and polyunsaturated fatty acids, omega-3s, omega-6s, DHA and linoleic acid, as concentrations and percentages, without clinical thresholds.
• Replacing saturated fats with unsaturated oils reduces cardiovascular events by about 30% in trials (AHA 2017); one to two servings of fish per week are recommended.
• Omega-3 supplements have not demonstrated a benefit in primary prevention; whether they are indicated is discussed with a doctor.

What Sokrate lets you do

The Sokrate service prepares your prevention check-up online with an adaptive self-assessment questionnaire that documents your eating habits, then a doctor writes and signs your Personalized Prevention Plan, which is sent to your regular doctor (médecin traitant) unless you object. As an option, an NMR metabolomic analysis of 249 biomarkers, performed on the Nightingale Health platform, includes the full fatty acid profile, interpreted by a doctor alongside your lipoproteins, blood glucose and GlycA, then monitored over time. A coordination nurse supports you. Join the waitlist or find out how the pathway works.