Although obesity rates have been rising around the world, the problem is particularly pronounced in Samoa. In 2010, 80 percent of Samoan men and 91 percent of Samoan women had obesity. A new Yale co-authored study may help explain the high rates of obesity among Samoans.
Researchers genotyped thousands of Samoans and found a genetic variant on chromosome five, which has genes that contribute to more efficient fat storage. According to the study, which was a collaborative effort that included researchers from the Yale School of Public Health, this variant is rare globally, but very common among people living in Samoa. Although this variant, coined as the “thrifty” gene, may once have protected Samoans during times of food scarcity, it now appears to play a role in Samoa’s current obesity epidemic, according to the paper, which was published on July 25 in the scientific journal Nature Genetics.
Obesity is associated with a variety of health problems, according to the Centers for Disease Control and Prevention. Obesity-related conditions include heart disease, stroke, Type 2 diabetes and certain cancers. However, even though the “thrifty” variant made fat storage more efficient, it also reduced the risk of Type 2 diabetes for carriers.
“Although this variant is extremely rare in other populations, it is common in Samoans, with an effect size much larger than that of any other known common body-mass-index risk variant,” the researchers wrote in the paper. Nicola Hawley, professor of chronic disease epidemiology at Yale, was a co-first author.
Researchers studied the genomes of 3,072 Samoans across 33 different villages, genotyping more than 600,000 markers on each genome. A variant on chromosome five was significantly associated with obesity — having the mutation increased the odds of having obesity by 35 percent.
The researchers noted that there were three possible genotypes among the participants they studied. They found that 7 percent of subjects had two copies of the variant, 38 percent had one copy, and 55 percent did not have the variant at all.
“We plan to expand our work in humans to see whether and how the metabolism and body composition differs in individuals with each of the three genotypes,” said Ryan Minster, study co-author and human genetics professor at the University of Pittsburgh Graduate School of Public Health. “We’d also like to understand the distribution of the BMI-increasing version of [the gene] across the Pacific.”
The variant is virtually absent from populations outside of Samoa, according to the study paper.
To understand the mechanism behind the variant’s association with greater weight gain, researchers used laboratory cell models of mice fat cells. Cells that included the “thrifty” variant were able to store fat more efficiently than were cells without that variation.
Although the “thrifty” variant can make it easier to gain weight, researchers say that it plays only a small role in obesity. According to the study paper, it only accounts for two percent of BMI variation among Samoans.
“A healthy diet and physical activity are still key to maintaining a healthy weight,” said Stephen McGarvey SPH ’84, study co-author and professor at the Brown University School of Public Health, in a July 25 press release.
The World Health Organization defines obesity as a BMI of 30 or higher.
Correction, Aug. 27: A previous version of this article incorrectly stated that Nicola Hawley led the study; in fact, she was a co-first author.