The study, published in the journal PLOS Genetics, focused on fish oil (and the omega-3 fatty acids it contains) and its effect on triglycerides, a type of fat in the blood and a biomarker for cardiovascular disease.

"We've known for a few decades that a higher level of Omega-3 fatty acids in the blood is associated with a lower risk of heart disease", said researcher Kaixiong Ye from the University of Georgia. "What we found is that fish oil supplementation is not good for everyone. It depends on the genotype. If you have a specific genetic background, then fish oil supplementation will help lower your triglycerides. But if you do not have that right genotype, taking a fish oil supplement actually increases your triglycerides".

For the study, the team examined four blood lipids (fats) high-density lipoprotein, low-density lipoprotein, total cholesterol and triglycerides which are biomarkers for cardiovascular disease. The data for their sample of 70,000 individuals were taken from UK Biobank, a large-scale cohort study collecting genetic and health information from half a million participants.

The team divided the sample into two groups, those taking fish oil supplements (about 11,000) and those not taking fish oil supplements. Then they performed a genome-wide scan for each group, testing for 8 million genetic variants to compare. After running over 64 million tests, their results revealed a significant genetic variant at gene GJB2. Individuals with the AG genotype who took fish oil decreased their triglycerides. Individuals with the AA genotype who took fish oil slightly increased their triglycerides. (A third possible genotype, GG, was not evident in enough study volunteers to draw conclusions.)