Lack of Exercise, Not Overeating, Contributes to Obesity
A new Stanford University study suggests the rising number of obese Americans may be due to increasingly sedentary lifestyles, as opposed to excessive calorie consumption.
Uri Ladabaum, MD, associate professor of medicine at Stanford University School of Medicine, and colleagues analyzed data from the U.S. National Health and Nutrition Examination Survey (NHANES) over the past 20 years, studying trends in obesity, waistline obesity, physical activity, and calorie intake among American adults up to the year 2010. The researchers found a sharp decline in levels of leisure-time physical activity among Americans during that time—especially among young women—as well as an increase in average body mass index (BMI), while calorie consumption remained somewhat steady over the same span.
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The study “needs to be taken with caution. It explores associations, but cannot address cause and effect relationships. It also needs to be viewed in the context of everything else that is known on the subject,” says Ladabaum, noting that “we did not find substantial changes in caloric intake over time.”
In contrast, continues Ladabaum, “we did find dramatic changes in the proportion of person self-reporting no leisure time physical activity. The trends in obesity were associated with the changes in physical activity.”
Indeed, the authors found that more than half (51.7 percent) of female adults in the United States reported no leisure-time physical activity in 2010. In 1994, just 19.1 percent of adult American women reported doing no exercise. While the proportion of men who reported no leisure-time physical activity in 2010 (43.5 percent) was lower than women, this is nearly four times the 11.4 percent of men who said they did not exercise in 1994.
When the researchers analyzed the data by subgroups, they found women, and black and Mexican-American women in particular, showed the greatest decreases in reported exercise. In the period the investigators studied, the U.S. saw average body mass index rise by an average of 0.37 percent per year, with the most dramatic increase seen in young women. The researchers found that average waist size went up by 0.3 percent per year for women, and 0.27 percent for men, while abdominal obesity increased in both normal-weight and overweight women. For men, that rate only went up among the overweight.
It’s important to note that “the fact that reported average caloric intake did not change substantially does not mean that caloric intake has been optimal at the population level or at the level of individuals,” says Ladabaum. “We simply did not detect a substantial increase over time. Clearly, caloric intake and physical activity are both important determinants of weight.”
The results “do not mean caloric intake has not been important in the rise in obesity prevalence,” concludes Ladabaum. “But the findings do suggest that physical inactivity may be an important factor that should not be overlooked, both in public health initiatives as well as in the care of individual patients. This is consistent with the Institute of Medicine’s report on obesity.”
—Mark McGraw
Reference
Ladabaum U, Mannalithara A, et al. Obesity, abdominal obesity, physical activity, and caloric intake in US adults: 1988-2010. AJM. 2014.