By studying the genomes of more than 5000 Samoans, researchers from the University of Pittsburgh have uncovered a single gene that boosts a person’s obesity risk by upwards of 40 per cent. Remarkably, this gene — which appears in a quarter of all Samoans — may have arisen in the population as they colonised the South Pacific.
As described in the latest edition of Nature Genetics, this “thrifty” genetic variant, called CREBRF, is associated with a 1.5 per cent increase in Body Mass Index (BMI). So, for a person of average height weighing around 82kg, this gene corresponds to an extra 5kg. As noted by the researchers in their study, CREBRF promotes more efficient storage of fat and features “an effect size much larger than that of any other known common BMI risk variant”.
The University of Pittsburgh researchers, led by Stephen McGarvey, made the discovery while scanning the genomes of thousands of Samoans. This populations has some of the highest obesity rates in the world, a fact that prompted the scientists to conduct a genetic investigation. Around a quarter of all Samoans involved in the study had the genetic variant, which was associated with 30 to 40 per cent increased odds of being obese compared to those who don’t have the gene. At the same time, this gene is virtually non-existent in European and African populations and occurs at very low frequencies among East Asians.
“Although we have found a genetic variant with a reasonable biological mechanism, this genetic variant is just one part of the many reasons for the high levels of BMI and obesity among Samoans,” noted McGarvey in a press statement.
Other factors include diet and physical activity. Indeed, the globe’s shift to kilojoule-rich processed foods and more sedentary lifestyles has contributed significantly to the elevated rates of obesity among Samoans. But as this new study points out, their genetics are also working against them.
This gene appears to work by causing cells to store more fat and release less energy. As Alice Klein pointed out in New Scientist, it’s as “if [cells] are trying to conserve as much fuel as possible”. And indeed, there may be a very good reason why this gene appears at elevated levels among Samoans. It has to do with their history of colonising the South Pacific Islands.
Starting around 3500 years ago, ancestors of Samoans began the arduous task of settling the 24 major island groups of Polynesia. This colonisation process — one of the most extreme examples in all of human history — took possibly thousands of years to complete. “They had to endure voyages between islands and subsequently survive on those islands,” study co-author Ryan Minster told New Scientist.
As Darwin pointed out many years ago, evolution requires long timescales. But in some instances, when environmental conditions are particularly severe and attritional, selectional processes accelerate the process — an evolutionary phenomenon dubbed “punctuated equilibrium” by the late evolutionary biologist Stephen Jay Gould.
The problem, however, is that Samoans no longer require this gene. This would explain why upwards of 80 per cent of men and women in Samoa are now overweight. “Samoans weren’t obese 200 years ago,” noted McGarvey. “The gene hasn’t changed that rapidly — it’s the nutritional environment that changed that rapidly.”