Is fish oil good for you? Depends on your DNA

These days, lots of foods come fortified with omega-3 fatty acids because of their supposed health benefits. But not everyone may reap them equally, according to a new study of native Greenlanders. For people lacking the DNA profile of these hardy people, that extra omega-3 might not do much good at all. “This paper represents a significant contribution to our understanding of human environmental adaptation,” says Toomas Kivisild, an evolutionary geneticist at the University of Cambridge in the United Kingdom, who was not involved with the study. The Inuit of Greenland have a tough life: It’s -20°C in the winter, and the usual foods are scarce. Most of their traditional diet consists of fish and marine mammals, which have a very high fat content. “They occupy one of the most extreme environments of all humans,” says Iain Mathieson, a human geneticist at Harvard University. But there is an upside to this diet: Native Greenlanders have a lower risk of heart disease than many other people. To figure out why, Rasmus Nielsen, a population geneticist at the University of California, Berkeley, and his colleagues compared the DNA of 191 Inuits with that of 60 Europeans and 44 Han Chinese, two populations that were part of previous study of genetic variation. The DNA that was most different was on chromosome 11, specifically among genes that control the processing of dietary fatty acids into some of the body’s building blocks, the researchers report online today in Science. One DNA “letter” or base was the same in almost all the Inuits, but most Europeans lacked this base and only 15% of the Han Chinese had it. “It’s rare that we see this extreme,” in the distribution of a particular DNA base, says Nielsen, who adds that the high prevalence of this one base suggests it was very advantageous for the Inuits. The researchers then looked at medical records of the Inuits to see how this one base affects fatty acid composition. The Inuits’ DNA appears to slow the body’s own modification of fatty acids, they found. It’s not clear how this difference leads to less heart disease in Inuits, though it could have to do with an altered mix of fatty acids. (The body takes in, modifies, and uses a range of fatty acids for different purposes.) This difference in fatty acid composition may also explain why Inuits tend to be about 2 centimetres shorter than Europeans, as fatty acids affect the growth hormone that governs height, Nielsen notes. The results imply that people lacking Inuit DNA may not be getting the same protective effects from this substance, Nielsen says. More work clarifying the connection between high omega-3 and the Inuits’ heart health is needed, but this new work is a good start, adds geneticist Joshua Akey of the University of Washington, Seattle: “You can count on your fingers where we have a pretty good idea of what the agent of change was,” he says. In this case, the connection to diet seems clear. But Ulf Gyllensten, a geneticist at Uppsala University in Sweden, is not so sure about Nielsen’s conclusions. “When they looked at the Greenlanders, they hadn’t understood the whole story,” he says. In 2012, his team reported that humans had a different form of these fatty acid genes than did chimps or other ancient human species, one that made them more efficient at processing the fatty acids from plants. Most people in the world today have those more efficient versions. But he thinks that because the Inuit diet was always so rich in fatty acids, Greenlanders didn’t need this efficient version and instead retained the older version. And both Nielsen and Gyllensten envision the day when diets will be determined not by the latest fads and research findings, but through personalised genetic profiles. “We realise now that different human populations have adapted to different diets, so what’s healthy for one person might not be healthy for other people,” Nielsen says. “We need to have personalised diet choices based on genetics.”

Science, 17 September 2015 ; ;