Physical activity has many proven benefits.
It strengthens bones and muscles, improves mental health and mood, lowers blood pressure, improves cholesterol levels and reduces the risk of cardiovascular disease, diabetes, breast cancer and colon cancer. Exercise is also good for your brain.
It may not be a cure-all for obesity, however.
Though better nutrition coupled with exercise has long been the favored prescription for losing weight and avoiding obesity, a new study suggests diet actually plays the key role.
Researchers from Loyola University Health System and other centers compared African American women in metropolitan Chicago with women in rural Nigeria. On average, the Chicago women weighed 184 pounds and the Nigerian women weighed 127 pounds.
Researchers had expected to find that the slimmer Nigerian women would be more physically active. To their surprise, they found no significant difference between the two groups in the amount of calories burned during physical activity.
"Decreased physical activity may not be the primary driver of the obesity epidemic," said Loyola nutritionist Amy Luke, a member of the study team.
U.S. government guidelines state that each week, adults need at least 2 ½ hours of moderate aerobic activity (such as brisk walking) or 75 minutes of vigorous activity (such as jogging). Adults also should do muscle-strengthening activities, such as weight-lifting or sit ups, at least twice a week.
The benefits to overall health are clear. Exercise has even been shown to improve kids' academic performance.
People burn more calories when they exercise. Thing is, they compensate by eating more, said Richard Cooper, co-author of the study and chairman of the Department of Preventive Medicine and Epidemiology.
"We would love to say that physical activity has a positive effect on weight control, but that does not appear to be the case," Cooper said.
Diet is a more likely explanation than physical activity expenditure for why Chicago women weigh more than Nigerian women, Luke said. She noted the Nigerian diet is high in fiber and carbohydrates and low in fat and animal protein. By contrast, the Chicago diet is 40 percent to 45 percent fat and high in processed foods.
More research seems to be needed, however, as the new finding conflicts with other studies. A study in the September issue of the journal Archives of Internal Medicine found that Old Order Amish people who had a gene related to obesity stayed thin nonetheless because they exercised a lot.
Yet results of the new study are similar to those of a 2007 study of men and women in Jamaica. Researchers from Loyola and other centers found there was no association between weight gain and calories burned during physical activity.
"Evidence is beginning to accumulate that dietary intake may be more important than energy expenditure level," Luke said. "Weight loss is not likely to happen without dietary restraint."
The results, announced in a statement from the university today, were published in the September 2008 issue of the journal Obesity.
Other centers involved in the study include University of Ibadan in Nigeria, Howard University, Johns Hopkins Bloomberg School of Public Health and University of Wisconsin.