Genetics & environment
The expanding North American waistline is due to a complex relationship between environment and genetics. Research Watch explores the obesity epidemic.
Excessive body fat accumulates when we take in more calories than we expend. But the expanding North American waistline is caused by a complex relationship between environment and genes. This month Research Watch explores the causes of the obesity epidemic.
In the United States the obese population is approaching 30 percent. While the “fat American” may be a common stereotype, the US has plenty of company at the table. Nearly one-quarter of Canadians over 18 years of age are obese, with a body mass index (BMI) over 30.
In all regions of the world except southern and eastern Asia, more than 60 percent of men and 50 percent of women are overweight, with a BMI over 25.
Blame the Genes
Genes do play a role in the development of obesity. The “thrifty gene” theory postulates that our early human ancestors developed very efficient fat-storage capabilities that helped them to survive periods of famine.
The genetics of obesity are proving to be startlingly complex. The Human Obesity Gene Map, published every year, outlines more than 400 genes or markers that appear to influence some aspects of an individual’s relationship with food. Taste, appetite, fat storage, and caloric expenditure–all probably have some genetic root.
Research shows that our chance of being obese increases by 30 to 70 percent depending on heritability. Twins have the highest heritability, but the risk of obesity is two to three times higher for persons with family histories of obesity and correlates directly to the severity of the family’s obesity.
But even taking family history and genetic predisposition into account, only about 1 percent of the obese have an identifiable genetic defect that causes excessive weight gain or abnormal eating behaviours. The rest must control caloric intake and thus, weight. This does take more work for some than for others.
Tip the Scales
If the “thrifty gene” theory is correct, people who store fat easily would be most likely to survive periods of famine, as would their offspring. In our age of plenty, this ability has created an obesity epidemic within the last decade.
In 1990 the prevalence of obesity was less than 15 percent across the US, and in 10 states it was less than 10 percent. By 2007 only Colorado had an obesity rate of less than 20 percent, and in Alabama, Mississippi, and Tennessee, the rate was more than 30 percent.
Obesity rates in Canada, though lower than the US, are also troublesome. The highest obesity rates were in Newfoundland (33 percent men and 35 percent women) in 2004. The greatest gender difference in obesity is in Saskatchewan, the province with the second-highest obesity rate, at 33 percent female and 29 percent male. The lowest obesity rates were found in British Columbia–at 18 percent of men and 20 percent of women.
More worrying are the rates of childhood obesity, which have nearly tripled in the past 25 years. The rates for First Nations children are estimated to be two to three times higher than the Canadian average.
It seems that unprecedented plenty coupled with increasingly sedentary lifestyles and overconsumption have coalesced to create an obesity epidemic.
Reach Out and Fatten Up Someone
Recent studies also suggest that our families and friends can influence whether we become obese. The closer the social connection, the greater the influence. Even living apart from family or friends doesn’t negate this influence.
According to researchers, as one person gains weight, those in the same social network may also gain weight, resulting in a “spread of obesity.”
In close same-sex friendships, if one friend becomes obese, the other has a 71 percent chance of becoming obese, too. This result was not seen in opposite-sex friendships.
In married couples, if one spouse becomes obese, the likelihood of the other spouse becoming obese increases by 37 percent. The implications for parent-to-child influences are disturbingly obvious.
Move It to Lose It
Genes do not make most people fat. Obesity is a condition that results from ingesting too many calories and performing too little exercise. However, by predisposing some people to store fat or burn calories less efficiently, genetics can make us fatter and can make it harder to take the fat off.
Preventing obesity is the best course for those predisposed to weight gain. For the already obese, weight management requires a combination of diet control and increased physical activity. Luckily, body weight is one of those rare instances where positive behavioural changes can counterbalance genetics.