Replacing sedentary time for physical activity: Does intensity matter for body composition in oldest-old adults?

To assess the independent and combined relationships among objectively measured sedentary time (ST), light intensity PA (LPA), and moderate-to-vigorous intensity PA (MVPA) with muscle mass and fat mass (FM) and how theoretical displacement of these inter-dependent behaviours relates to body composition in oldest-old men. A total of 1046 men participating in the year 14 visit of the prospective Osteoporotic Fractures in Men (MrOS) cohort study with complete data for accelerometry, dual x-ray absorptiometry, and deuterated creatine dilution (D3Cr) muscle mass were included in the analysis (84.0 ± 3.8 yrs.). Single, partition, and isotemporal substitution models were used to assess the interrelationships between PA intensities and ST with body composition measures, while controlling for relevant confounders. Replacing 30-min of ST with 30-min of MVPA was associated with lower FM (β =-0.17, p  < 0.001) and higher D3Cr muscle mass, although this was of borderline significance (β = 0.07, p  = 0.05). Replacing 30-min of ST for LPA was associated with lower FM (β =-0.15, p  < 0.001), but there was no effect on D3Cr muscle mass ( p  > 0.05). Exchanging ST with any intensity of PA is associated with benefits for FM in oldest-old adult men, although substitution with MVPA may be more beneficial than LPA for maintaining/improving skeletal muscle mass.