Does Air Conditioning Make Me Fat?

Identifying Hidden Causes of Obesity

Could central air be inflating spare tires across the country? In a study published last summer in the International Journal of Obesity, UAB biostatistician David Allison, Ph.D., and colleagues suggest that air conditioning and other factors may play surprisingly important roles in America’s weight crisis.

For years, health providers and the media have focused on the “Big Two”—food marketing and institutionally driven reductions in physical activity—as the major causes of our society’s soaring obesity problem, to the near-exclusion of other important influences, says Allison. He notes that while these factors have quite plausibly contributed to the decades-long rise in obesity, much of the evidence is circumstantial. And while he does not discount the importance of the Big Two, Allison makes a strong case for considering 10 additional causes.

One of the most intriguing factors listed in the study is the “reduction in variability of ambient temperature.” The widespread use of central heating and air conditioning means that most homes and offices are now kept at a relatively constant temperature year-round. Allison’s group found evidence that this causes the body to expend less energy, because it does not have to work to warm up or cool down, potentially leading to increased fat stores. In the South, where obesity rates are the highest in the nation, homes with central air increased from 37 to 70 percent between 1978 and 1990.

Another contributor to obesity may be lack of sleep. Intuitively, it seems that more sleep would lead to less energy expenditure and increased weight gain. But Allison’s team found that the hours of sleep a person gets each night is actually inversely related to Body Mass Index (BMI), a measure of obesity. Restricting sleep may actually produce increased hunger and appetite, contributing to obesity.

Two other culprits identified by the UAB study—smoking cessation and certain medications—are less of a surprise. The Centers for Disease Control and Prevention estimates that between 1978 and 1990, smoking cessation was responsible for about 25 percent of the increase in the number of overweight men in the United States and 16 percent of the increase in the number of overweight women. And medical professionals have long been aware that some medications, including antidepressants, contraceptives, and antihistamines—most of which were introduced or saw a dramatic increase in use over the past three decades—can lead to weight gain.

Allison says other explanations for obesity include an increase of endocrine disrupters such as DDT and PCBs in the environment, which may affect hormones in the body and contribute to increased body fat.

America’s changing demographics may hold another key to weight changes, as some age and ethnic groups have a higher prevalence of obesity than others. Marriage and family matters also play a role. The probability that two individuals will mate may be linked to a similarity in weight levels, which can increase the number of obese offspring. Women are waiting longer to have children, and the older a woman is at childbirth, the higher may be the risk that her child will become obese. Separate research reveals that maternal obesity may promote the same problem in subsequent generations.

“We are not discounting the potential effects of food marketing and reductions in physical activity,” Allison says. “But we believe addressing these additional factors could have implications for both treating and preventing obesity.”

— Robert D. Phillips, M.D., M.P.H.

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