The percentage of women reporting no physical activity jumped from 19 percent to 52 percent between 1988 and 2010; the percentage of inactive men rose from 11 percent to 43 percent over the same period.

Obesity also increased, climbing from 25 to 35 percent in women and from 20 to 35 percent in men.

Surprisingly, however, the number of calories consumed per day did not change significantly. Nonetheless, diet remains a proven and important component of health, and participants may have been tempted to under-report how much they ate, Ladabaum said.

He added that, although the reported average caloric intake did not change substantially during those periods, it didn’t mean that the number of calories consumed were optimal. “We simply did not detect a substantial increase over time,” he said, noting that caloric intake and physical activity are both important determinants of weight.

Both obesity and abdominal girth, which the team analyzed independently, contribute to a variety of well-documented conditions, such as cancer and cardiovascular disease, as well as increased mortality.

In 2010, 61 percent of women and 42 percent of men had too much belly fat, up from 46 percent and 29 percent in 1988. In addition, the waists of even normal-weight women swelled between 1988 and 2010, the study showed.

Ladabaum noted that the study did not follow one group of participants over that 22-year span; instead, the data came from different samples in each survey cycle. But the samples are constructed to be representative of the population.

Clarion call

In an accompanying editorial, the journal’s managing editor, Pamela Powers Hannley, MPH, called the study “a clarion call.”

Obesity is a complex, multifaceted problem linked to a variety of societal factors, Hannley said in an interview. “There are societal and economic forces at work that we must address,” she said. “Take, for example, the struggle of single mothers who are trying to balance work and child care. They may lack the time or resources to exercise. We shouldn’t assume that people are just lazy. Their lives might be overwhelming to them.”

Recommendations to exercise 30 minutes a day aren’t enough, Hannley added.

“It’s going to take widespread change,” she said. “We shouldn’t just tell patients they need to work out. We need to work with communities, employers and local governments to enable healthy lifestyles by ensuring that there are safe spaces to exercise that are cheap or free.”

Other Stanford co-authors of the study are Ajitha Mannalithara, PhD, social science research associate; Parvathi Myer, MD, a former postdoctoral scholar who is now at Kaiser Permanente, and Gurkirpal Singh, MD, adjunct professor of gastroenterology.

The study was funded by the National Institutes of Health (grant T32DK007056).

Stanford’s Department of Medicine also supported the research.