Objectives: 

The objective of the study was to examine how reported previous physical activity affects body composition of Finnish young men during military service, which is associated with marked changes in diet and physical activity.

Methods: 

Altogether 1022 men (19 yrs) were followed throughout their military service (6–12 months). Height, weight, waist circumference and waist-to-hip (WHR) ratio were recorded. Body composition was measured by bioelectrical impedance assessments (BIA) in the beginning and end of the service. The measured parameters were fat mass (FM), fat percent, fat free mass (FFM) visceral fat area (VFA), lean body mass and muscle mass. Previous physical activity was assessed in the beginning of the service by a questionnaire.

Results: 

On average military training decreased weight 0.6%, reduced WHR 1.4%, FM 10.0%, fat percent 6.0 %, and VFA 43.3%. FFM increased by 1.4%, lean body mass 1.3% and muscle mass by 1.6%. The group of under- and normal weight men gained weight, FM and FFM, whereas overweight and obese men lost weight, FM and gained FFM. Fat tissue was reduced most in the groups of overweight (20%) and obese (25%) men. The amount of visceral fat was reduced in all BMI groups (38–44%). Low physical activity before military service was reported most often in obese subjects (32%), and high physical activity most frequently in normal weight men (61%). Among overweight men who reported being inactive previous to the military service weight, WHR, BMI, FM, fat% and VFA was reduced more compared with those who reported more physical activity.

Conclusions: 

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