Nature is a factor: Obesity includes a genetic element
There is an increasing amount of evidence supporting the genetic predisposition for weight gain. Twin and adoption studies have demonstrated that genetic factors do play a role in determining which individuals are more susceptible to becoming obese in response to a particular environment. A 2007 study published in Science identifies the specific gene that carries higher obesity risk but also finds a link to increased incidence of diabetes. The study involved over 38,750 participants and focused on the FTO gene region on chromosome 16; it showed that the presence of a certain variant was highly correlated with an increased BMI from childhood to old age. In particular, 16% of adults who carried two copies of the gene variant weighed approximately 3 kg more and had a 67% increased risk of type 2 diabetes.
A 2008 study published in the New England Journal of Medicine involving 2,726 children ages 4-10 showed similar weight gain and BMI increase in children with the gene variant. Results found there was no significant difference between the two groups in the rate of metabolism, energy expenditure, or weight of the food ingested by the children. This led to the conclusion that the gene variant likely leads to consumption of high-calorie, energy-dense foods.
These findings coincide with the NIDDK’s research of the Pima Indians and the “thrifty gene”. The concept was proposed in the 1960s to explain why the Pima has such a high incidence of obesity. According to the theory, for thousands of years in southern Arizona and Mexico, the Pima relied on farming, hunting, and fishing for food. This “thrifty gene” would have evolved to enable them to survive the alternating periods of feast and amine. The body would store energy as fat at a higher rate when food was plentiful in order to help the Pima survive periods of starvation. However, now that the Pima have adjusted to the high-fat, Western diet, the trait has become detrimental to their health. The study found that 95% of the Arizona Pima were obese and approximately 50% had diabetes. In contrast, the Mexican Pima, who still live the traditional lifestyle of their ancestors, showed no incidence of obesity and only 9% had diabetes.