We all agree—and despair—that obesity is on the rise. America has been getting fatter for the past century, and the problem has worsened over the past 35 years. We also all know the obvious explanations. Who would discount the role of new food-marketing practices, like super-sizing or pushing soda sweetened with corn syrup? Or the decrease—even elimination—of physical activity in school and in adult life?
An important new paper, though, cautions us to be skeptical that corn syrup and sitting around are the only factors that matter for understanding the obesity epidemic. The study's lead authors, David Allison and Scott Keith of the University of Alabama at Birmingham, * don't reject these explanations. But they suggest that the obvious reasons for obesity are so popular and widely cited that they have pushed out other equally plausible and well-supported contributing factors. And if we ignore these factors, our proposals for addressing obesity may well fail.
With the help of 20 contributors, Allison and Keith put together a list of 10 alternate explanations for obesity, each of them backed up by good research. In all likelihood, the rise in obesity results from a combination of several of these factors, each making its own contribution and perhaps interacting with other causes in some yet-more-complicated way. Here are five of them:
Inadequate sleep: Average sleep duration has been dropping for children and for adults—80 years ago adults slept an average of 8.77 hours nightly; now the average is 6.85 hours. Sleep-deprived animals eat excessively, and humans subject to sleep deprivation show increased appetite and an increased Body Mass Index, the standard measure of excessive weight. The apparent mechanism for this phenomenon is the effect that sleep deprivation has on at least two hormones that influence appetite: leptin and ghrelin. Sleep deprivation causes a decrease in leptin, which boosts appetite and produces obesity, and increases ghrelin, a potent stimulator of hunger and appetite. A study led by J.P. Chaput and published in the International Journal of Obesity this spring found that children who slept an average of 10.5 to 11.5 hours a night were more at risk for obesity than children who slept between 12 and 13 hours a night. Kids who slept only eight to 10 hours a night were at still greater risk. The study had methodological weaknesses (small sample size, data mostly by parental report, absence of correction for age). Still, the trend is striking and suggests that sleep deprivation is associated with obesity in children as well as adults.
Chemical contamination: The water, soil, and food to which we are exposed increasingly are contaminated with chemicals—used in plastics, power transmission, and even aircraft de-icing—that accumulate in the body and mimic or interfere with hormones that regulate body functions. Some mimic female hormones. Others block male hormone activity. Both properties lead to increased fat accumulation.
Heating and air-conditioning: Living in an environment that is excessively cold or warm forces the body to expend calories to maintain a normal body temperature and thus may keep weight down. In addition, high ambient temperatures seem to kill appetite. So, turning up the air conditioning in the summer may pad on extra pounds.
Smoking cessation: Smoking also kills appetite, and it may be that the (otherwise fortunate) decline in tobacco use has been contributing to population weight gain. (Read a Slate piece about this.)
Medications: Most of the medications frequently prescribed to moderate moods and depression or treat other mental illness, like Prozac (33 million doses in the United States in 2002), Seroquel, or Risperdal, promote weight gain. So do hormone medications, like birth-control pills, anti-diabetic drugs, and blood-pressure medications. The total number of patients taking such medications is unknown but clearly huge.
The lesson of Allison and Keith's study is clear: Until we have better research that demonstrates which factors relating to the obesity rise are the significant ones, we shouldn't put all our money into interventions that target only food marketing and sedentary lifestyle. That conclusion is further warranted by surprising additional recent research. In one large study of more than 1,500 children, attempts to increase physical activity and healthy eating had absolutely no effect on obesity. Another study of 7,000 children found that how close a child lives to the nearest fast-food restaurant has no effect on obesity. Proximity to a playground also doesn't matter.
Despite these findings, almost all doctors believe in our hearts that eating and physical activity do affect obesity rates. But sometimes you can know a cause, go after it, and make no difference at all. For instance, it seems obvious that increasing energy output—by walking to school, for instance, or starting an intense gym program—will help decrease obesity. Unfortunately, another study points out that what's obvious isn't necessarily true.