Objective

To determine the relationship between persistence or change in leisure‐time physical activity habits and waist gain among young adults. Methods

Population‐based cohort study among 3383 Finnish twin individuals (1578 men) from five birth cohorts (1975‐1979), who answered questionnaires at mean ages of 24.4 y (SD 0.9) and 33.9 y (SD 1.2), with reported self‐measured waist circumference. Persistence or change in leisure‐time physical activity habits was defined based on thirds of activity metabolic equivalent h/day during follow‐up (mean 9.5 y; SD 0.7). Results

Decreased activity was linked to greater waist gain compared to increased activity (3.6 cm, P < 0.001 for men; 3.1 cm, P < 0.001 for women). Among same‐sex activity discordant twin pairs, twins who decreased activity gained an average 2.8 cm (95%CI 0.4 to 5.1, P = 0.009) more waist than their co‐twins who increased activity (n = 85 pairs); among MZ twin pairs (n = 43), the difference was 4.2 cm (95%CI 1.2 to 7.2, P = 0.008). Conclusions

Among young adults, an increase in leisure‐time physical activity or staying active during a decade of follow‐up was associated with less waist gain, but any decrease in activity level, regardless baseline activity, led to waist gain that was similar to that associated with being persistently inactive.