Understanding the role of physical activity, physical performance and dietary protein intake on muscle mass and insulin resistance in seniors

The decrease in physical performance (PP) with aging is in part mediated through body composition changes. With aging there is a gain and central redistribution of fat depots and a loss of lean tissue, mainly skeletal muscle. The loss of muscle mass has been implicated in the risk of developing insulin resistance. Physical activity could act as a predictor and an outcome of PP and is a modulator of insulin resistance. Age, sex, energy intake and chronic diseases can also affect PP and insulin resistance. Dietary protein intake is an easy and inexpensive modality to combat loss of muscle mass. Contrary to plant source of protein intake, animal source of dietary protein may confer an increased risk of insulin resistance and diabetes. Determining insulin resistant subjects in epidemiological studies is challenging because of the lack of cut-off for scores used to assess insulin sensitivity. Analyses of effects of interrelationships are complex and leads to challenging interpretations. Our first objective was to explore the complex interrelationships of body composition, physical performance, physical activity, protein intakes and insulin resistance. Our second objective was to determine subjects who were insulin resistant subjects over a 3-year period and compare them to insulin sensitive subjects in regard to body composition measures and other baseline characteristics. A sample of elderly men and women, non-diabetic, community-dwellers participants of the Quebec Longitudinal Study on Nutrition and Successful Aging (NuAge Study) were analysed. Tests employed to assess PP were analyzed by principal component analysis and generated two indices, one related to strength and the other to mobility. Muscle mass index (MMI; kg/height in m2) and % body fat were derived from dual X-ray absorptiometry and bioimpedance analysis. Physical activity was assessed by the Physical Activity Scale for the Elderly, energy intakes and protein intakes were calculated from three non-consecutive 24h-food recalls. Insulin resistance was estimated based on the Homeostasis Model Assessment score. Proposed models associating these variables were tested for validity with the NuAge data using path analysis and employed trajectory analyses to established incidence of insulin resistance. MMI and % body fat were both negatively associated with mobility score, however, muscle mass was positively associated with strength independently of other variables. Direct positive associations were observed for HOMA-IR score with MMI and % body fat. There was a significant, direct negative association for plant protein intake with MMI, whereas there was no association with HOMA-IR. There were significant, positive indirect associations between animal protein intake and HOMA-IR score and significant negative indirect associations between plant protein intake and HOMA-IR mediated through MMI and % body fat. In the longitudinal analyses, 7 group-based trajectories were identified with good posterior probabilities. An inspection of the curves allowed for determination of insulin sensitive subjects and classification of insulin resistance subjects. The logistic regression with the most parsimonious model provided only 3 significant predictors of insulin resistance: higher MMI, higher body fat% and male sex. Muscle mass was associated with strength but positively associated with HOMA score. This relationship is counterintuitive since it suggests muscle mass with aging is positively associated with insulin resistance. There were significant, positive indirect associations between animal protein intake and HOMA-IR score and significant negative indirect associations between plant protein intake and HOMA-IR. These indirect associations were mediated through MMI and % body fat. Our longitudinal analyses showed that a higher muscle mass, % body fat and male sex contribute to a higher odd of insulin resistance with aging.

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