Role of muscle loss in the age-associated reduction in V̇(O₂ max)

A progressive decline in maximal O₂ consumption (V̇(O₂ max)) expressed traditionally as per kilogram body weight generally occurs with advancing age. To investigate the extent to which this decline could be attributable to the age-associated loss of metabolically active tissue, i.e., muscle, we measured 24-h urinary creatinine excretion, an index of muscle mass, in 184 healthy nonobese volunteers, ages 22-87 yr, from the Baltimore Longitudinal Study of Aging who had achieved a true V̇(O₂ max) during graded treadmill exercise. A positive correlation was found between V̇(O₂ max) and creatinine excretion in both men (r = 0.64, P < 0.001) and women (r = 0.47, P < 0.001). As anticipated, V̇(O₂ max) showed a strong negative linear relationship with age in both men and women. Creatinine excretion also declined with age in men and women. When V̇(O₂ max) was normalized for creatinine excretion, the variance in the V̇(O₂ max) decline attributable to age declined from 60 to 14% in men and from 50 to 8% in women. Thus comparing the standard age regression of V̇(O₂ max) per kilogram body weight with that in which V̇(O₂ max) is normalized per milligram creatinine excretion, the decline in V̇(O₂ max) between a hypothetical 30 yr old and a 70 yr old was reduced from 39 to 18% in men and from 30 to 14% in women. We conclude that in both sexes, a large portion of the age-associated decline in V̇(O₂ max) in non-endurance-trained individuals is explicable by the loss of muscle mass, which is observed with advancing age.