Does fish oil lower triglycerides?

What the studies found

Triglycerides are a type of fat in the blood, and high levels may raise the risk of heart disease.^[3] The Cochrane review included trials that lasted at least 12 months and found a fall of about 15%, larger with higher doses.^[1] A 2023 review of 90 trials and 72,598 people found an almost straight line: more omega-3, lower triglycerides.^[4] The link was clearest in people with high blood fats or overweight who took more than 2 g a day.^[4]

Two reviews put a number on this in mg/dL (milligrams per decilitre of blood):

• 1 g a day of omega-3 from food or supplements lowered triglycerides by 5.9 mg/dL in an analysis of 58 trials.^[5]
• 1 g a day of omega-3 from fish oil lowered them by about 9 mg/dL in a review of 64 trials.^[6]

High doses and high starting levels give the largest falls:

• In people with very high triglycerides (500 mg/dL or more), 4 g a day of prescription EPA and DHA lowered them by 30% or more.^[2]
• In people on statins with levels of 200 to 499 mg/dL, purified EPA lowered them by 21.5% at 4 g a day and by 10.1% at 2 g, after 12 weeks.^[7]

Why it may work

The mechanism is not fully known.^[8] Omega-3 fats appear to slow the making of triglycerides in the liver.^[8] They also appear to speed up the breakdown of triglycerides, perhaps through an enzyme in the blood called lipoprotein lipase.^[8]

How studies gave it

The AHA advisory covers prescription products at 4 g a day, which give more than 3 g of EPA and DHA.^[2] A typical 1000 mg capsule holds about 300 mg of EPA and DHA, so that amount would take more than 10 such capsules.^[5] The make-up and testing of prescription products differ from those of supplements, so their effects may not be the same.^[3]

What happens to LDL cholesterol

DHA and EPA act differently on LDL cholesterol. In a 2011 review, DHA raised LDL by 7.23 mg/dL compared with placebo, and EPA did not raise it.^[9] EFSA found that EPA and DHA at 2 to 6 g a day raised LDL by about 3%.^[10] The AHA advisory says EPA and DHA products raise LDL in people with very high triglycerides, but not at levels of 200 to 499 mg/dL.^[2]

Who should be careful

At 4 g a day, trials found more atrial fibrillation.^[5] Changes in diet, weight and exercise are used to lower triglycerides, and some people also need medicine.^[3] People with high triglycerides should ask a doctor which treatment fits them, and tell a doctor or pharmacist about any fish oil they take.^[3]

What is still unknown

A lower triglyceride level does not always mean fewer heart events. In the ORIGIN trial, 12,536 people took 1 g a day for about 6 years. Triglycerides fell, but heart attacks, strokes and cardiovascular deaths did not.^[5]