Objective: To determine whether habitual leisure time physical activity and body mass change of working-aged men and women are associated.

Design: Prospective 10 y follow-up study.

Subjects: A regionally representative cohort of 19-63 y old men (n = 2564) and women (n = 2695) in three municipalities in Finland.

Measurements: The main outcome measures were average body mass change during the 10 y of follow-up and the clinically significant body mass gain defined as a body mass gain of 5 kg or more during the follow-up and a body mass index of 26 or higher at the end of the follow-up. Leisure time physical activity was determined from self-administered questionnaires.

Results: After adjustment for the potential confounders, the logistic regression analysis showed that the men and women with no regular weekly activity at the end of the follow-up had an odds ratio of 2.59 (95% confidence interval, 1.69-3.97) and 2.67 (1.65-4.31), respectively for clinically significant body mass gain in comparison with the most active groups. The men who had decreased their activity during the 10 y follow-up had an odds ratio of 1.96 (1.39-2.75), and the men who were physically inactive all the time had an odds ratio of 1.62 (1.18-2.20) for clinically significant body mass gain when compared with the subjects who were active all the time. Among the women the odds ratios for clinically significant body mass gain after adjustment for potential confounders were 2.49 (1.72-3.60) and 1.61 (1.17-2.21), respectively.

Conclusion: These findings are consistent with the hypothesis that regular physical activity prevents body mass gain and physical inactivity is a risk factor for body mass gain and obesity among adults.