Motivation to eat foods can play a large role in dietary intake and therefore body weight. One way to determine a person's motivation to attain food is by measuring the reinforcing value of the food. A higher reinforcing value of food is associated with greater energy intake. While food reinforcement has been studied extensively under various conditions, including food restriction, deprivation, and repletion, the effects of physical activity on food reinforcement have never been investigated. Previous research on the effects of either acute or long term exercise on appetite processes and energy intake are equivocal. The purpose of this thesis was to determine whether a short term exercise intervention could alter the reinforcing value of HED and LED foods in sedentary male and female adults who were either lean or overweight.

Forty-one sedentary male and female adults were randomized to one of two groups, exercise or no exercise. At the first visit all participants had baseline food reinforcement tested for high and low energy density foods, and height and weight measured. All participants reported for their second visit for VO₂ max testing to control for fitness level of groups. The exercise group reported to the lab twice more that week, and three times the next week to engage in 35 minutes of aerobic exercise at 60% of predicted maximum heart rate. Two to three days after the final exercise session, participants in the exercise group returned to the laboratory for the final food reinforcement testing session. Participants in the no exercise group returned for the same session two weeks following the exercise test.

Our results showed that after two weeks of aerobic exercise, the reinforcing value of HED foods decreased. A greater response was seen for LED versus HED foods both at baseline and follow-up, and the exercise group showed significantly higher responses for LED foods than other groups at baseline. For energy consumed we found that the exercise group consumed significantly more energy from HED foods than did the no exercise group, and lean exercisers consumed significantly more total energy than lean individuals in the no exercise group. No changes were seen as a function of exercise in energy consumed. For food liking, at baseline and follow-up, lean individuals decreased their liking of HED foods from pre to post food reinforcement. From baseline to follow-up lean exercisers increased their liking of HED foods while the overweight exercisers decreased their liking of HED foods.

When taken together these data suggest that in sedentary individuals, motivation to obtain unhealthy foods can be changed with a short term exercise intervention. The results support previous knowledge that indicates there is individual variability in the effects of exercise on appetite control processes, while adding information on short term exercise.