U.S. adults have been eating steadily fewer calories for almost a decade, despite the continued increase in obesity rates, according to survey data from the Centers for Disease Control and Prevention (CDC).
"It's hard to reconcile what these data show, and what is happening with the prevalence of obesity," co-author Dr. William Dietz, former CDC director of Nutrition, Physical Activity and Obesity, told Reuters Health.
The results, published in the American Journal of Clinical Nutrition, are based on the nine National Health and Nutrition Examination Surveys (NHANES) the CDC has conducted between 1971 and 2010. Several thousand adults aged 20 to 74 were randomly selected every two to four years and asked to recall what they ate over the previous 24 hours.
Dietz and a colleague analyzed trends since the 1970s and found that among adults, average daily energy intake rose by a total of 314 calories from 1971 to 2003, then fell by 74 calories between 2003 and 2010.
"Seventy-four calories is a lot, and as I said before, we would expect to see a measurable impact on obesity," said Dietz.
Nevertheless, about 35 percent of U.S. adult women are obese, and that percentage has held steady since 1999, according to the CDC. For men, obesity has risen from 27 percent to 35 percent over the same time period.
Dietz said he would have expected obesity rates to have leveled-off for both sexes and to be decreasing at this point, if people are consuming fewer calories.
The CDC released similar results last month for children: boys have cut their calorie intake by 150, and girls by 80, since 1999. Obesity rates for boys continue to increase, however, while holding steady for girls.
Experts said it's possible more time is needed to see obesity rates respond to changes in calorie intake. It's also possible that Americans have changed their eating habits but are still not getting enough exercise to burn the calories they do consume. Or, the surveys may simply be wrong.
"If you cut back on calories by 100 calories, you'll plateau 10 pounds lower," but you'll only see about half of that progress over the first year, Dr. Claire Wang, who studies energy intake and expenditure at the Columbia University Mailman School of Public Health in New York, told Reuters Health.
There wouldn't be an immediate reduction in obesity at the population level, said Wang, who was not involved in the study.
She believes the change in calorie consumption could be due to more awareness of sugary drinks and added sugar, and that awareness campaigns such as efforts by the White House to promote healthier eating are working.
But by now, "People should be losing weight," Dietz said. The fact that they are not could be bad news, he said, because it could mean people are burning fewer calories with exercise, and exercise plays an important role reducing the risk of many chronic diseases.
It's also possible that increased awareness of unhealthy foods has caused people to be embarrassed about eating junk foods or drinking sodas, so they may still be eating those foods but are less likely to admit it on a survey, Dietz added.
In general, data from surveys don't line up with actual calorie intakes when measured by a third party, according to nutritionist Marion Nestle of New York University.
The NHANES surveys indicate that men take in an average of 1500 calories per day, and women 1800, for example. When doctors or nutritionists measure calorie intakes, they find an average of 3,000 for men and 2,400 for women, according to Nestle.
While the estimates may not be exactly on point, surveys are the best available way to identify trends in the population, she said.
"NHANES is as good as we have on trend data," she said. "Taken at face value, they are somewhat good news."