For many people, understanding obesity is a pretty straightforward proposition. Eat more you gain weight. Eat less you lose weight. The laws of thermodynamics. Energy in v. Energy out. End of story. Alas, we know it is not that simple. For one thing, we know that obesity is highly inheritable. For another, we know that changes in the environment can produce rapid changes in the prevalence of obesity in the population. But, while finding many genes and their variations (called alleles) have been identified, they account for only about 2% of the total genetic variation in BMI. Skeptics doubt a genetic explanation arguing that the gene pool cannot change as quickly as the current changes in obesity. (These folks have tended to see the obesity epidemic as a 75 year phenomenon. But see the book review on The Changing Body, for how long this change has been going on..around 350 years.)  On the other hand, the environmental approach has raised the question, “If the environment is so prone to creating obesity, why isn’t everyone obese?” This line of doubt is reinforced by high variations in obesity in small geographic areas.

So now Danish researchers have use the Danish Twin Registry to try to explain the interaction of genes and the environment. The Danish Twin Registry contains information on virtually all twins born in Denmark since 1870. Two groups were surveyed. What they found was that the environment may modify the genetic expression related to obesity. This environmental effect grows stronger, but only for some individuals and not for others. They estimate that there is a 33.3% increase in the expression of adiposity-related genes associated with a 10% increase in the prevalence of obesity. They conclude that the genetic architecture of obesity should not be considered independent from the environmental context. PLoS ONE: Increased Genetic Variance of BMI with a Higher Prevalence of Obesity