The lifestyle changes associated with military service markedly reduce fat tissue and increase the amount of lean tissue. These beneficial changes are prominent among previously inactive subjects with high BMI.

Objective. To examine the association between aerobic performance and body composition changes by body mass index (BMI). Design. 6 -12 months ' follow-up during military service. Setting. Conscripts entering military service in 2005 in Sodankyl ä Jaeger Brigade (Finland). Subjects. 945 men (19 years, SD 1 years). Main outcome measures. Height, weight, waist circumference, BMI, and aerobic performance (Cooper test) were recorded. Body composition was measured by bioelectrical impedance analysis (BIA). The measured parameters were fat mass (FM), fat free mass (FFM), and visceral fat area (VFA). All the measurements were performed at the beginning and end of service. Results. On average, the military training period improved the running distance by 6.8% (169 m, p Ͻ 0.001) and the improvements were more pronounced in overweight (223.9 m/9.5%, p Ͻ 0.001) and obese (273.3 m/13.6 %, p Ͻ 0.001) conscripts. A strong inverse correlation between aerobic performance and body composition changes was observed, especially for weight (r ϭ -0.305, p Ͻ 0.001) and VFA (r ϭ -0.465, p Ͻ 0.001). A signifi cant association between aerobic performance and changes in weight (p Ͻ 0.001), waist circumference ( p Ͻ 0.001), FM ( p Ͻ 0.001), and VFA ( p Ͻ 0.001) by BMI was detected. The associated decrease in weight, waist circumference, FM, and VFA with improved aerobic performance was more substantial between overweight and obese compared with normal-weight subjects. Conclusions. Favourable changes in body composition are associated with improved aerobic performance during a physical training period such as military service. These fi ndings are pronounced among overweight and obese men and can be applied at the population level in reducing obesity and co-morbidities.

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