Why we're fatter.

Health and medicine explained.
July 13 2006 12:46 PM

Why We're Fatter

Five reasons you haven't thought of.

Illustration by Robert Neubecker. Click image to expand.

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.

T.J. Wilkin and his colleagues at the Peninsula Medical School in Devon, England, looked at three groups of English and Scottish children. They measured physical activity using accelerometers, devices that record duration and intensity of movement 600 times a minute. The idea was to determine whether total daily activity level is essentially invariant for any one child—"in her nature," as my mother would have said—or whether it can be increased if a child goes to gym, walks to school, or doesn't watch television. Wilkin and his team also tracked whether children's daily activity level varied between weekdays, presumably spent sedately in school, and weekends, presumably spent going wild. If the total amount of expended energy remained constant despite the typically different structure of a weekday vs. a weekend, then it would suggest that daily total activity level is determined by something internal and specific to each child. (Do adults have an internally regulated daily activity level, too? I wish I could tell you, but we have no idea of the answer yet.)

The results of Wilkin's study would have brought despair to the many gym teachers who made my childhood miserable. Their efforts, it turns out, were for naught: Sluggard I was, and sluggard I was doomed to be. Wilkins and his team found that every child has his or her own very consistent daily level of activity. It remains the same on weekends and weekdays; it's not affected by school physical education, or by whether the child walks or drives to school, or how much time he spends awake or in front of a television. We don't know what determines this intrinsic level of activity. But engineering the environment to make available or even to require more activity will apparently have little impact on children whose nature is to be inactive.

All this makes me feel a bit vindicated. The medical students and residents I teach are chronically frustrated (and not a little angry) that my contribution to their education about managing obesity is often unhelpful and discouraging. Having read these new studies, I'm more depressed than ever.

Before we write off obesity as a hopeless problem, though, one more thought: As worried as I am about many of my heavy patients, I often do see heartening improvement, especially in later adolescence (even if I am skeptical that I played a role). The critical question is why some young people thin down while others do not.

I don't have an answer, but I do have an impression. It's that adolescents who lose weight are more likely to have acquired a positive sense of themselves, because they've had some academic or athletic success, or some other notable accomplishment. Sometimes they have embarked on a successful romantic relationship. And often parents and other adults in their life focus on their strengths rather than harping on weight and appearance.

I feel particularly moved to say this now because of a troublesome development on the horizon. The Centers for Disease Control and Prevention, the American Academy of Pediatrics, and the American Medical Association are talking about renaming degrees of fatness in children. Up to now, the heaviest children have been called "overweight," and the next heaviest group classified as "at risk of overweight." The tough-love talk being bandied about is that we should tell it like it is, and call obese obese. I don't think so. The impulse to rename reflects doctors' frustration at their inability to help heavy children: If we can't help them, then it's time to blame them. If I'm right, however, calling heavy children "obese" is likely to do the opposite of what we want, by making them feel worse about themselves.

Correction, July 14: The original sentence stated that David Allison and Scott Keith work at the University of Alabama. In fact, they work at the University of Alabama at Birmingham, a separate university. Click here to return to the corrected sentence.

Sydney Spiesel is a pediatrician in Woodbridge, Conn., and clinical professor of pediatrics at Yale University's School of Medicine.