New research suggests that Americans aren't obese because we eat too much. It's because we exercise too little. And by too little I mean, not at all.
And whatever exercise we get peaks before the age of 10 — and perhaps as early as 2 — and is in steady decline after that, according to another study.
Researchers at Stanford University Medical Center examined 22 years of data from the National Health and Nutrition Examination Survey, a long-term project of the Centers for Disease Control and Prevention. It included results from surveys and physical exams.
And they were shocked to discover that while rates of obesity went up during that period, the calorie intake of Americans did not. However, the amount of physical activity plummeted, especially for women between the ages of 18 and 39.
Over those two decades, the number of women who reported no leisure time physical activity jumped from 19 percent to almost 52 percent. Among black and Mexican-American women ages 40 to 64, more than 70 percent reported no exercise.
During that same time, the rates of obesity increased from 25 to 35 percent in women and from 20 to 35 percent in men.
Medicine has adopted a second obesity calculator besides the well-known body mass index or BMI, and that is belly fat or waist size. Those numbers, too, are swelling.
Sixty-one percent of women and 42 percent of men — up from 46 percent and 29 percent — have too much belly fat. Even among people with normal BMIs, waist size can signal obesity. (34 inches or less is good for women, 40 inches for men.)
All of this while the daily calorie intake did not increase appreciably.
The researchers admit that people might be underreporting their food intake. We all think we eat less than we actually do. But that would not have accounted for the increase in obesity because, presumably, we have always underreported how much we eat.
And the researchers cautioned that they did not consider the caloric intake reported to be optimal. In other words, doctors still think we are eating too much of the wrong things.
What explains this drop in leisure time exercise?
It is likely that the 18 to 39 female age group, which would include single mothers and working mothers with young children, can't find time to exercise. Men and women are all working longer hours, often while sitting at computers. Unless you run the stairs at lunch time, exercise isn't easy to cram into the work/life mix.
"We shouldn't assume that people are just lazy," said Pamela Powers Hannley in an editorial that accompanied the report in the Journal. "Their lives might be overwhelming to them."
Indeed, the researchers found that more than 50 percent of workforce-age adults in eight demographic subgroups reported no leisure time physical activity.
(The group with the best numbers? White, non-Hispanic men, ages 18 to 39. Only 29 percent report no physical activity. Apparently, the guys find time for the gym or sports. How much is enough exercise? 150 minutes a week of moderate exercise or 75 minutes of vigorous exercise, plus muscle strengthening activities at least twice a week, according to the CDC.)
Things are even worse among our kids.
Another study from the CDC reports that less than a third of young people ages 12 to 18 achieve the recommended levels of physical activity for their age group, which is one hour a day. That's because children spend eight to 10 hours a day in front of a television or computer screen — numbers that only increase during the summer.
Researchers asked children ages 12 to 15 to come to a mobile lab for some physical testing. They found that only 42 percent were as fit as they should be, given their ages, and the numbers were worse for girls. And the ethnicity and income level of the family didn't matter.
The inescapable conclusion is that kids who are sedentary grow up to be sedentary adults. And what we have is a country of sedentary families.
But this actually makes the problem easier to solve, suggests Gretchen Reynolds, writing in the New York Times.
Make physical fitness — a walk, a bike ride, a trip to the pool — a family activity. Get everybody moving.
Susan Reimer's column appears on Mondays and Thursdays. She can be reached at email@example.com and @SusanReimer on Twitter.com.To respond to this commentary, send an email to firstname.lastname@example.org. Please include your name and contact information.