The new study, published this week in the International Journal of Obesity, looked at 2,680 young adults who were not exercising regularly or dieting. Scientists found that after exercising for several weeks, formerly sedentary study participants were more likely to choose foods like lean meats, fruits and vegetables, while preferences for fried foods, sodas and other unhealthy options decreased.
Participants were instructed not to change their diets in any significant way, but it happened anyway. Although this study did not examine the mechanism at work behind the changes, previous research has found that moderate exercise can reduce a preference for high-fat foods in animals through changes in dopamine levels. Several studies also have shown a relationship between the intensity of exercise and the amount of appetite-regulating hormones in the body.
"The process of becoming physically active can influence dietary behavior," said Molly Bray, corresponding author of the paper and chair of the Nutritional Sciences department at UT Austin and a pediatrics faculty member at Dell Medical School. "One of the reasons that we need to promote exercise is for the healthy habits it can create in other areas. That combination is very powerful."
Bray says what drives food-preference changes when people exercise would probably be consistent across a wide span of ages. The study examined people between the ages of 18 and 35, a period of young adulthood critical for forming healthy habits. Previous studies have found that considerable weight gain occurs during the college years and that being mildly to moderately overweight at age 20-22 increases the risk of obesity later in life.
"Many people in the study didn't know they had this active, healthy person inside them," Bray said. "Some of them thought their size was inevitable. For many of these young people, they are choosing what to eat and when to exercise for the first time in their lives."
The participants in the study were students at the University of Houston and the University of Alabama at Birmingham. Participants who said they exercised less than 30 minutes a week at the beginning of the study started 30-minute aerobic workouts three times a week for 15 weeks, with instructions not to change their diet in any significant way. The exercise sessions consisted of 30 minutes of aerobic exercise at 65-85 percent of the person's age- and gender-specific maximum heart rate, along with a 5-minute warmup and a 5-minute cool down. Participants wore heart-rate monitors and could choose from a variety of exercise types, such as on stationary bikes, treadmills or elliptical machines.