Samoa, with its string of beautiful islands and coral atolls in the South Pacific, is attracting more than just tourists these days. Scientists are heading there, too. The nation holds the uneviable position of being No. 1 in the world for obesity. Among Samoan men, 80 percent are either overweight or obese, and that figure reaches 91 percent for Samoan women. They are the fattest people in the world, and they are proving revealing subjects for genetics studies.

I learned about obesity in Samoa after seeing a recent report on a genetic analysis of more than 5,000 people on the islands.

The study, led by Ryan Minster of the University of Pittsburgh, Pennsylvania, uncovered a gene variant that boosts obesity risk by 30-40 percent.

It’s worth stressing that this isn’t an “obesity gene.” With complex traits such as this (intelligence is another one), there isn’t likely to be a single gene that determines whether you do or do not have the trait. And, of course, environmental factors such as diet and exercise play a big part. To date, however, no other gene has been found to be so influential in obesity as this particular gene. It goes by the instantly forgettable name of CREBRF.

My first thought was to question what the gene does. My second was about famous former sumo wrestlers Musashimaru and Konishiki. Musashimaru, who reached the top rank of yokozuna (grand champion) in 1999, was born in Samoa, while Konishiki, who made ozeki (the second-highest rank) in 1987, was born in Hawaii of Samoan descent. They are big men and, at their peak weights, were colossal. Musashimaru reached 235 kilograms, while Konishiki is the heaviest sumo wrestler in history, weighing 287 kilograms. What do their genes look like?

The way men put on weight when they are professional sumo wrestlers is very different to how sedentary people get fat on a diet of junk food (we’ll come back to this), but I can’t help thinking about their genetics and, indeed, about the genetics of the sumo population in general.

After all, most sumo wrestlers have a body mass index of 40 or above, which is the category where “morbidly obese” begins. And since there’s only one weight class in sumo, there is an incentive to put on as much weight as possible. People with genes that aid in tremendous weight gain would be at an advantage.

Back to the CREBRF gene. Minster’s team made cells in the lab that carried the CREBRF variant. By measuring how the cells grew, the team found that the gene variant enabled the cells to store more fat. In effect, the cells became thrifty. They preferred to hoard fat than to release it.

Around 25 percent of Samoans carry the gene, Minster says. You can see how having the gene would have been useful in the past. Around 3,500 years ago, people from Australia and the South Pacific started settling the more isolated islands in the region. There is evidence of human settlement dating back to about 3,000 years ago on Samoa. To survive the voyages across the open ocean, and to endure and thrive when they arrived on the islands, the people needed to be thrifty and to conserve energy wherever possible. Any gene that aids this goal will be selected.

The problem now, of course, is that the environment and their way of life has changed radically. Levels of physical exertion are lower than they were even a few decades ago. Fast food is easily available, and, by some measures, 40 percent of children are overweight — even by the time they are 15 months old. In this new environment, you can see how a gene that is intent on hoarding as much fat as possible can contribute to obesity. That’s why I expect many professional sumo wrestlers have the CREBRF gene variant.

I read a piece in the Asia Pacific Journal of Sport and Social Science arguing that obesity is a professional requirement for sumo wrestlers. Surprisingly, sumo wrestlers have the largest fat-free mass of all professional athletes. This means that ignoring the fat, they have large body masses. The fat they put on is different to how the rest of us get fat. Sumo wrestlers lay down subcutaneous fat that doesn’t inhibit muscle development and which can be more easily accessed for energy use. The rest of us tend to put on visceral fat in the abdomen, around the organs, and this is much less healthy.

Sumo wrestlers also eat differently from the rest of us. They consume large amounts of food in two sittings per day, unlike the rest of us who typically have three meals a day. (I once heard that sumo wrestlers have their intestines massaged so they can fit in more food — does anyone know if this is true?) And the food they eat — the famous chanko nabe hot pot — is highly nutritious (not to mention tasty). A sumo wrestler eats about 5,000 calories per day, compared to around 2,200 for the average Japanese.

Interestingly, although obesity is a strong risk factor leading to diabetes, Minster’s team found that people with CREBRF had lower rates of diabetes. If the variant is common in sumo wrestlers, as I suspect it is, this is good news for their health when they retire.

Rowan Hooper is the news editor of New Scientist magazine. The second volume of Natural Selections columns translated into Japanese is published by Shinchosha. The title is “Hito wa Ima mo Shinka Shiteru” (“The Evolving Human”). Follow Rowan on Twitter @rowhoop.

KEYWORDS genetics, weight, Sumo