The evidence is pretty overwhelming: Americans' waistlines have gotten considerably bigger in the past couple decades. Is that the product of over-eating, reduced amounts of exercise, or both? As ScienceDaily reports, researchers led by the director of the WHO's Collaborating Centre for Obesity Prevention, Boyd Swinburn, lay the blame almost squarely on increased calorie intake. Here's how they reached their conclusion:
The scientists started by testing 1,399 adults and 963 children to determine how many calories their bodies burn in total under free-living conditions. The test is the most accurate measure of total calorie burning in real-life situations. . . . Once they had determined each person's calorie burning rate, Swinburn and his colleagues were able to calculate how much adults needed to eat in order to maintain a stable weight and how much children needed to eat in order to maintain a normal growth curve.
They then worked out how much Americans were actually eating, using national food supply data (the amount of food produced and imported, minus the amount exported, thrown away and used for animals or other non-human uses) from the 1970s and the early 2000s. . . . The researchers used their findings to predict how much weight they would expect Americans to have gained over the 30-year period studied if food intake were the only influence. They used data from a nationally representative survey (NHANES) that recorded the weight of Americans in the 1970s and early 2000s to determine the actual weight gain over that period.
The results? For children, the prediction and actual data were an "exact match." (In other words, increased food intake alone explained the weight gain.) For adults, Swinburn's team predicted a 24-lb average gain, but real-world data only showed a 19-lb increase. That suggests adults are actually exercising more now than they were in the 70s.
ScienceDaily quotes Swinburn as saying, "changes in physical activity played a minimal role [in the obesity epidemic]." The researcher also notes that reverting to 1970s-era waistlines would involve reducing energy intake by just 350 calories for children and 500 calories for adults—that's the equivalent of a large hamburger for adults and one soda can plus a small portion of fries for kids.
Another option: get children to walk an extra 150 minutes a day and adults to walk an extra 110 minutes. Swinburn nevertheless cautions, "Although a combination of both is needed, the focus would have to be on reducing calorie intake."