New research has found a gene carried by half of Samoans is partly behind the island nation's world-leading obesity rates.
Figures from 2010 show 80 percent of Samoan men and 91 percent of women are overweight, placing them sixth out of 196 countries.
Scientists have now found a small part of the epidemic can be blamed on a gene. Rare globally but present in half the Samoan population, its role in fat storage had gone unnoticed until now.
"A previously unknown genetic variant in an understudied gene is strongly associated with body-mass index (BMI) levels and other adiposity (being severe or morbid overweight) measures in Samoan men and women we studied in 2010," said Stephen McGarvey of the United States' Brown University School of Public Health.
Having the gene is associated with a 35 percent higher risk of being obese, "much greater than any other known common BMI risk" according to the study, published in Nature Genetics.
"When people with this form of the gene eat food, they will deposit more of it as fat tissue than people who don't have this version of the gene," says Peter Shepherd, professor of cell signalling at the University of Auckland.
Prof McGarvey says it probably dates back to Samoans' ancient roots in sailing the Pacific and the difficulties they faced in always having enough food to eat. Over the generations, natural selection favoured those carrying it as they were better able to survive.
But on its own, it only accounts for about 2 percent of Samoans' overall weight gain over the centuries, failing on its own to explain Samoans' current position as the world's fattest nation.
"Samoans weren't obese 200 years ago," says Prof McGarvey. "The gene hasn't changed that rapidly - it's the nutritional environment that changed that rapidly."
Dr Lisa Te Morenga of the Department of Human Nutrition at University of Otago says with so many Samoans overweight, whether they have the gene or not, the focus needs to be on fixing the "obesogenic environment to reduce the availability of highly palatable, cheap, energy dense, nutrient poor processed foods".
"Although genetic studies give us clues as to those who may be more or less at risk of obesity, this becomes rather irrelevant when more than half of the population is obese," she says.
Prof McGarvey agrees that keeping a healthy diet and staying active are all that's needed to offset the gene's effects.
"Don't take this as, 'You are Samoan, you are fated to be obese.' We don't think that's true. We don't have any evidence that that's the case."
Health and genetics experts are calling for more research to be done, particularly on the closely related Māori population.
"The finding that Samoan people have a higher incidence of this gene variant than Europeans shows that different groups in our society are facing different challenges in their efforts to reduce the impact of the modern environment," says Prof Shepherd.
"This knowledge is important as it means we now need to think of strategies that take these differences into account, and to develop targeted strategies rather than trying to use a one-size-fits-all strategy to tackle the epidemic of obesity and diabetes that is facing our society."