The omega-3 index: what does it tell an endurance athlete?

Omega-3 testing is increasingly offered to athletes. A finger-prick blood sample gives a single number, the omega-3 index, and a set of thresholds tells the athlete whether that number is "undesirable", "intermediate" or "desirable". At the same time, endurance athletes are told that fish oil lowers heart rate, improves oxygen efficiency, increases maximal oxygen uptake (VO₂max) and improves endurance performance. In the first blog of this series we looked at fish oil and muscle, and in the second at fish oil and injury. In this blog we will explore three questions. What does the omega-3 index measure, and where do its thresholds come from? Is a low index a reason for an athlete to change the diet or take a supplement? And do the effects of fish oil on heart rate and oxygen cost translate into better endurance performance?

What does the omega-3 index measure?

The omega-3 index is the amount of eicosapentaenoic acid (EPA) plus docosahexaenoic acid (DHA) in the membrane of the red blood cell, expressed as a percentage of all fatty acids in that membrane. A drop of blood from a finger prick is blotted onto a card and analysed in a laboratory. Three categories are used: below 4% is considered undesirable, 4 to 8% intermediate and above 8% desirable. These thresholds come from cardiology, where an index of 8% or more was associated with the lowest risk of death from coronary heart disease and an index of 4% or less with the highest.

The omega-3 index responds to fish oil supplementation. In pooled data from 14 intervention trials, an average of about 2 g of EPA plus DHA per day for about 14 weeks increased the index from 4.9% to 8.1% (1). At about 300 mg of EPA and DHA per standard 1 g capsule, 2 g per day is 6 to 7 capsules. So the question for sports nutrition is not whether the index can be measured or changed, but whether an athlete with an index of 4 or 5% is worse off in training, recovery or performance than an athlete at 8%.

Where do athletes sit?

A review of 18 studies with a total of 1,452 athletes found an average omega-3 index of 4.4% (2). Fewer than 1% of athletes had an index above 8%, about a third had an index below 4%, and the differences between sexes and between sports were within 1 percentage point.

"Fewer than 1% of the athletes measured so far have an omega-3 index above 8%."

By the cardiological definition almost every athlete is therefore "suboptimal". Whether that label means anything for athletic performance is a different question, and there are currently no studies that link a particular omega-3 index to training adaptation, recovery or performance. The wider evidence on fish oil in athletes is summarised in our blog on whether athletes need fish oil supplements.

Is a low index a reason to act?

One practical approach for sports nutrition practitioners uses a decision tree (3). If the omega-3 index can be measured, it is used to classify the athlete. If it cannot, the habitual intake of EPA plus DHA is estimated from the diet instead: less than 0.5 g per day is considered high risk, 0.5 to 1 g per day moderate risk and more than 1 g per day low risk. An athlete with an index below 4% would aim for about 2 g of EPA plus DHA per day, for example 1 g from a supplement and the rest from fish. An athlete between 4 and 8% would aim for about 1 g per day, for example 0.5 g from a supplement plus two to three fish meals per week. Above 8% no change is needed.

The food sources of EPA and DHA are described in our blog on omega-3 sources. In food, an 85 g portion of fresh salmon provides about 1.7 g of EPA plus DHA, 85 g of sardines about 1.2 g and 85 g of tuna canned in water only about 0.3 g (3). Two portions of salmon per week provide on average about 0.5 g per day. The decision tree is easy to use, but it is based on the cardiological thresholds, so it tells the practitioner what to do with a number rather than what that number means for an athlete.

If a supplement is used, two things affect how much EPA and DHA ends up in the red blood cell. In fish, omega-3 fatty acids are present as triglycerides, with the fatty acids attached to a glycerol backbone. Many supplements contain ethyl esters instead, in which the fatty acids are attached to ethanol, which allows more omega-3 to be concentrated in a capsule. Ethyl esters are less well absorbed, and in the pooled data mentioned above, triglyceride products increased the index by about 1 percentage point more than the same amount of ethyl ester (for example, from 4% to 6% instead of from 4% to 5%) (1). Absorption is also better when the supplement is taken with a meal that contains fat. Finally, the accuracy of fish oil labels has been debated, and for athletes subject to anti-doping rules a third-party tested product remains the sensible choice (see our blog on protecting athletes from the risks of supplements).

Three proposed mechanisms for endurance performance

The case for fish oil in endurance exercise is based on three tissues. The first is the red blood cell. The hypothesis was that DHA in the membrane would make red blood cells more deformable, so that they pass more easily through the smallest blood vessels and deliver oxygen more effectively. However, when 45 healthy men took 1.5, 3 or 6 g of EPA plus DHA per day as ethyl esters for 12 weeks, red blood cell deformability was not affected in a dose-dependent way (4). At the doses athletes actually take, this mechanism does not appear to operate.

The second is the heart. In 16 well-trained male cyclists who took 8 g of fish oil per day for 8 weeks, heart rate was lower during incremental exercise and during prolonged exercise at 55% of peak workload, and whole-body oxygen consumption was also lower (5). However, it must be noted that 8 g of fish oil is 8 capsules of 1 g a day, which is more than most athletes would take.

The third is skeletal muscle. The hypothesis is that incorporation of EPA and DHA into the muscle and mitochondrial membranes increases insulin sensitivity, and therefore the use of carbohydrate relative to fat. Because the oxidation of carbohydrate requires less oxygen per unit of energy than the oxidation of fat, the oxygen cost of exercise would be reduced. In 26 trained men who took a much lower dose of 560 mg of DHA and 140 mg of EPA per day for 8 weeks, the omega-3 index increased from 4.7% to 6.3% and oxygen consumption during a cycling time trial was lower after supplementation (6). A daily intake of 700 mg is about 4.9 g per week, which is roughly what three portions of salmon provide. Of course, a lower oxygen consumption does not provide information about the mechanism and does not mean that there is also an effect on performance.

The physiology changes, the performance does not

In the same study, average power output during the 5-minute time trial did not change with supplementation: it increased from 253 to 265 W in the fish oil group and from 267 to 278 W in the control group (6). In the cyclists taking 8 g of fish oil per day, peak oxygen uptake and time to fatigue did not change either (5). This is the pattern in most studies: a lower heart rate and a lower oxygen cost at a fixed submaximal workload, but no improvement in VO₂max or in performance, particularly when performance is measured with a time trial.

"Fish oil lowered heart rate and oxygen cost, but power output in a time trial did not change."

Part of the explanation is that performance is determined by many factors, so a small change in heart rate or oxygen cost may not show up in a time trial. Studies are small and differ in dose, duration, the ratio of EPA to DHA, the control oil, the performance test and the training status of the participants, and few have included women. It is possible that a small performance effect exists and has not been detected, but at present the physiological effects have not been shown to improve performance.

Practical takeaways

• The omega-3 index is a well-defined measurement, but its thresholds come from the risk of coronary heart disease, and there is no evidence that a particular index value affects training adaptation, recovery or endurance performance in athletes.

• Athletes have an average index of about 4.4%, and fewer than 1% are above 8%. A value in the intermediate range is normal for an athlete rather than a problem that needs correcting.

• An intake of less than about 0.5 g of EPA plus DHA per day is a reasonable reason to change the diet. Two to three portions of oily fish per week, with or without a supplement, will cover most situations.

• Triglyceride products are better absorbed than ethyl esters, and taking either with a meal that contains fat improves absorption.

• Fish oil lowers heart rate and oxygen cost at a fixed submaximal workload, but the studies are small and none has shown an improvement in VO₂max or time trial performance.

References

1. Walker RE, Jackson KH, Tintle NL, Shearer GC, Bernasconi A, Masson S, Latini R, Heydari B, Kwong RY, Flock M, Kris-Etherton PM, Hedengran A, Carney RM, Skulas-Ray A, Gidding SS, Dewell A, Gardner CD, Grenon SM, Sarter B, Newman JW, Pedersen TL, Larson MK, Harris WS. Predicting the effects of supplemental EPA and DHA on the omega-3 index. Am J Clin Nutr 110(4) 1034-1040, 2019.

2. Heileson JL, Sergi TE, de Souza LC. The omega-3 index in athletes: brief review. J Exerc Nutr 8(1), 2025.

3. Ritz PP, Rockwell MS. Promoting optimal omega-3 fatty acid status in athletes. Sports Sci Exch 34(212) 1-7, 2021.

4. Blonk MC, Bilo HJ, Nauta JJ, Popp-Snijders C, Mulder C, Donker AJ. Dose-response effects of fish-oil supplementation in healthy volunteers. Am J Clin Nutr 52(1) 120-127, 1990.

5. Peoples GE, McLennan PL, Howe PRC, Groeller H. Fish oil reduces heart rate and oxygen consumption during exercise. J Cardiovasc Pharmacol 52(6) 540-547, 2008.

6. Hingley L, Macartney MJ, Brown MA, McLennan PL, Peoples GE. DHA-rich fish oil increases the omega-3 index and lowers the oxygen cost of physiologically stressful cycling in trained individuals. Int J Sport Nutr Exerc Metab 27(4) 335-343, 2017.