The Effects Of Age On Body Composition Differ Between Male And Female Crossfit Exercisers

Abstract

Advancing age predicts physiological deterioration. Resistance training helps preserve body composition across the lifespan. Males and females who participate regularly may experience a different time course of decline. PURPOSE: To evaluate the effects of age and sex on body composition among weight lifters. METHODS: We assessed 542 men and women on the InBody 770 body composition analyzer. All subjects were active CrossFit members and were evaluated for body weight, body fat mass, body fat percent, skeletal muscle mass, lean leg mass, and arm circumference. Multivariate analyses compared these variables across categorical age groups (10-19, 20-29, 30-39, 40-49, 50-59, 60-69). Linear regression models estimated the effect of age on body composition holding sex constant. RESULTS: Subjects were 32.2 ± 11.1 years of age, weighed 185.6 ± 46.8 lb, had 51.0 ± 33.1 lb of fat mass, 26.5 ± 12.0% body fat, 76.3 ± 19.1 lb of skeletal muscle mass, 39.3 ± 9.6 lb of lean leg mass, and had an arm circumference of 14.1 ± 3.4 in. Significant differences were found by age group in body weight (p = 0.001), body fat mass (p < 0.001), body fat percent (p < 0.001), skeletal muscle mass (p = 0.008), and lean leg mass (p = 0.022). Arm circumference (p = 0.098) was not significant. Interaction effects were found with sex in body weight (p < 0.001), body fat percent (p < 0.001), skeletal muscle mass (p < 0.001), and lean leg mass (p < 0.001). Both sexes increased in body fat percent throughout the lifespan, with body weight reflecting this in men but not women, for whom it remained steady until age 60. Among men, skeletal muscle mass was maintained until age 60 whereas women experienced gradual decline beginning in their third decade. Interaction effects did not reach significant with body fat mass (p = 0.080) or arm circumference (p = 0.068). Holding sex constant, each additional year of age predicted a 0.3 percentage point increase in body fat percent (p < 0.001; 95% CI: 0.238 to 0.388) and a 0.1 lb decrease in leg lean mass (p < 0.001; 95% CI = -0.134 to -0.036), but did not significantly predict change in arm circumference (p = 0.086). CONCLUSIONS: Men and women who engage in regular resistance training may experience age-related decline differently. Both sexes are likely to increase in body fat percentage, but men may preserve skeletal muscle mass better under the age of 60.