World Population

Modeling How Obesity Moves Through the Population

Questions of how and why the pandemic of obesity has progressed through the whole earth’s population defies simple answers. Even more vexing is the question of how to change these dynamics. In Obesity, Keisuke Ejima, Diana Thomas, and David Allison have published new modeling that offers clues for answering these questions.

Genetic Susceptibility

Obesity moves through the population because of the genes we inherit. It happens when circumstances activate those genes. The supposition has long been that we have more obesity because our environment has grown more obesogenic. Our genes have not changed. The circumstances that activate them have become more common.

Ejima et al build their model from data on births, deaths, and the development of obesity. Risk factors for activating a person’s susceptibility included maternal influences, spontaneous weight gain, and social transmission. Their model suggests that obesity prevalence will be 41% in the U.S. and 27% in the U.K. by 2030.

Social Transmission

The idea that social transmission plays a role in obesity has developed rapidly since it was documented in a landmark NEJM publication ten years ago. More recently, Cain Clark went so far as to propose that obesity is really a communicable disease. Accounting for family and social networks must be a part of any efforts to treat and prevent the disease, he said.

The current model suggests he might be right. Ejima et al found that shifts in maternal and spontaneous risk factors had little impact on obesity trends. A much larger effect came from changing social risks for obesity. But changing those risks might not be so easy, said co-author Thomas:

I think it will very challenging to change the social risk for obesity. Certainly at a place like West Point, where I teach, the social pressure for fitness is overwhelming. But I would suppose the threshold for an effect must be very high.

A Need to Refocus?

Much of the energy that goes into addressing obesity focuses on choosing good foods and promoting exercise. And yet, experts have told us that it might be some different dynamics that are driving obesity trends. Kevin Hall recently pointed to “normative eating behaviors” as an key factor.  Timothy Church and Corby Martin told us that routine work and daily activities – not personal recreation – account for our lives becoming less active.

In other words, social norms and defaults deserve more attention. Focusing on individual behaviors and personal virtues might not move the needle. But we’re barely scratching the surface on shaping social and community norms for less obesity.

Click here for the paper by Ejima et al and here for more on obesity as a communicable disease.

World Population, photograph © Anders Sandberg / flickr

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April 8, 2018