Most Americans believe obesity adheres to the First Law of Thermodynamics. If too many calories are ingested and there is insufficient activity to burn them off, those calories will be converted into fat. But while this is the proximal cause of obesity, the ultimate cause remains elusive.
There are multiple theories. On the diet side, these include the rise of fast food and snack foods, an abundance of high fructose corn syrup and processed foods, portion sizes, the rise of supermarkets or even the lack of supermarkets (and thus, the rise of food deserts).
Theories on the exercise side include too much television, increased car use with less walking, decreasing work in manual labor and an increase in computer use.
But what contributes the most to obesity? The answer may surprise you. It comes from a recent review written by health care economists Roland Sturm and Ruopeng An and a review of studies regarding the heritability of obesity. Although race and economic status have long been associated with obesity, examination of the longitudinal data reveals that Americans are becoming obese at the same rate. This is also true of location. Coloradans may have a lower body mass index overall, but they are becoming obese at the same rate as people from Mississippi. Whatever is causing the obesity epidemic, it is affecting all of us.
The economists examined whether decreased leisure time or decreased exercise could have led to increasing obesity. Surprisingly, their data indicates that Americans have more leisure time and are exercising more than in the 1960s. They also looked at whether fruit and vegetable consumption has decreased and found that the availability (and likely consumption) of fruits and vegetables has either remained stable or increased slightly in the past few decades.
The answer, it turns out, is remarkably simple. We are a victim of our own success. As Sturm and An wrote in their study:
"Americans now have the cheapest food in history when measured as a fraction of disposable income. In the 1930s, Americans spent one-quarter of their disposable income on food, dropping to one-fifth in the 1950s...the share is now under one-tenth of disposable income. The decline of food expenditures relative to income becomes even more dramatic when one factors in 'quality' improvements, including greater convenience, reduced time costs for preparation, variety and ubiquitous availability of food."
The laws of economics are just as immutable as the First Law of Thermodynamics. If you decrease the price, people will consume more. This theory has a simplicity and an elegant power, but it is not the whole story. Why do some individuals suffer disproportionately to others? Why do the most unfortunate weigh hundreds of pounds while others couldn't gain that much if they tried?
That answer also turns out to be elegantly simple and devilishly complex at the same time. Is it our genes or the environment we're raised in that determines whether we will be obese? The best scientific way to answer this question is with twin studies. Researchers look at both identical and non-identical twins that are reared together and apart and examine whether they are obese. Many of these studies were performed, and they all overwhelmingly demonstrate that obesity is an inherited trait. They estimate 30% to 80% of obesity is inherited, with most of the larger studies putting that number well past 50%.
A picture of the obesity epidemic emerges from the synthesis of these two theories. The epidemic is a result of genetically susceptible individuals in an obesogenic environment due to decreasing relative food prices. This was a radical reframing of the issue for me as a physician, and it is important to consider these fact-based findings when determining solutions to the very real problem of obesity.
Joel Hartl is a pediatrics physician at Children's Hospital of Wisconsin.
Studies cited in this op-ed
■ "Obesity and economic environments," by R. Sturm and R. An (2014), A Cancer Journal for Clinicians.
■ "The response to long-term overfeeding in identical twins," by A. Bouchard, et. al, New England Journal of Medicine (1990).
■ "The body-mass index of twins who have been reared apart," by A.J. Stunkard, N.L. Pedersen and G.E. McClearn (1990), New England Journal of Medicine.