The debate about whether the chicken or the egg came first has had some steep competition in recent years—what’s to blame for the obesity epidemic in America? Some experts say it’s genetics, some say environmental chemicals are to blame, others say it’s our slothful lifestyle filled with hours of television and video games. But Australian researchers say the rise in obesity is due almost entirely to overeating, and they have the numbers to prove it.
For their study, Professor Boyd Swinburn of Deakin University in Victoria, Australia and colleagues calculated how much adults need to eat in order to maintain a stable weight and how much children must eat to maintain a normal growth curve under normal living conditions. Then, using national food supply data from the 1970s and the early 2000s, they figured out how much Americans were actually eating. From this information, the researchers could predict how much weight Americans would be expected to gain over the 30-year period had food intake been the only influence. Next they used NHANES data on population weight during that time period to determine how much weight was actually gained.
According to the researchers, the predicted and actual weight increase in children matched exactly, indicating that the increase in caloric intake alone could explain the additional pounds. Adults gained less than the data predicted—18.9 pounds versus 23.8 pounds—which “suggests that excess food intake still explains the weight gain, but there may have been increases in physical activity over the 30 years that have blunted what would otherwise have been a higher weight gain,” Swinburn noted.
“There have been a lot of assumptions that both reduced physical activity and increased energy intake have been major drivers of the obesity epidemic. Until now, nobody has proposed how to quantify their relative contributions to the rise in obesity since the 1970s,” said Swinburn. “This study demonstrates that the weight gain in the American population seems to be virtually all explained by eating more calories. It appears that changes in physical activity played a minimal role.” He added that the findings would “probably be similar” for other developed countries as well.
Swinburn said that for the U.S. population to revert to the leaner 1970s levels, children would have to cut their daily caloric intake by about 350 calories—the equivalent of a can of fizzy drink and a small portion of French fries, and adults by about 500 calories—about the same as a Big Mac burger. Alternatively children would have to walk an extra two-and-a-half hours a day, and adults for nearly two hours. “Getting everybody to walk an extra two hours a day is not really a feasible option for countering the epidemic,” Swinburn said. “We need to limit our expectations of what an increase in physical activity can achieve.” In short, Americans must eat less, he said.
However, Swinburn emphasized that the findings in no way seek to negate the value of physical activity. “We absolutely need to continue to promote increased physical activity and a healthy diet because they are both obviously beneficial factors in terms of obesity,” he said. “But when it comes to placing priorities, I think it needs to be on reducing energy intake. It’s particularly important for policymakers to focus on the energy intake side of the equation.”
Other experts agree with the study’s conclusions. “We have long suspected that the decrease in physical activity seen during the past 30 years is playing a minor role in the change in body weight,” said Dr. Robert Lustig, an obesity researcher at the University of California San Francisco. “This was inferred by the fact that virtually all studies of increased exercise in obesity did not translate into weight loss.” He added that exercise plays an essential role in the obesity epidemic, not because it reduces excess weight, but because it improves health. For example, twenty minutes of jogging burns the equivalent of a chocolate chip cookie, he said. Therefore, Americans “need a complete overhaul of our diet if we are to solve the obesity epidemic,” Dr. Lustig said.
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