Differences between young and old in muscle fibres, microvasculature and aminoacidemia after a beef meal

Abstract

Background: Aging is associated with loss of muscle- mass and strength, leading to a reduced quality of life and an increased risk of illness and injury. Such developments may be partly attributable to the phenomenon of anabolic resistance, wherein the muscle protein synthesis response to anabolic stimuli such as protein feeding is blunted. Recent studies have suggested that age-related changes in microvasculature may contribute to the manifestation of anabolic resistance. The present thesis aimed to investigate postprandial changes in plasma BCAA concentrations following a protein-rich meal in healthy old and young individuals and whether any differences could be related to indices of muscle capillarization.

Methods: 16 healthy young (28.4±6 yrs) and 13 healthy older (75±2 yrs) participants underwent a minimum of two test days consuming 100g beef. Blood samples collected throughout the postprandial period provide data on changes in plasma BCAA, glucose, and insulin concentrations. Muscle biopsies were collected and immunohistochemically treated, from which muscle fiber cross-sectional area (type I & II) and capillarization were quantified.

Results: The postprandial rise in plasma BCAA concentration did not differ between the young and old. In the young group, moderate-strong correlations indicated an association between capillarization and postprandial clearance of plasma BCAAs. No notable correlations were observed in the old group. Moreover, type I and type II fiber cross-sectional areas were 34% (p < 0.01) and 66% (p < 0.01) lower in the old compared to the young. The capillary-to-fiber ratio was 32% lower (p < 0.01) and capillary density was 21% higher in the old (p < 0.01) compared to the young.

Conclusion: The postprandial rise in plasma BCAA concentration did not differ between healthy, habitually exercising young and old individuals after a 100g beef meal. Furthermore, a greater number of capillaries per muscle fiber may enhance the postprandial clearance of plasma BCAAs in young. However, this relationship is confounded by the interdependence between capillarization and muscle fiber size. As such, the enhanced clearance of plasma BCAAs may not result from greater capillarization per se. Lastly, the microvasculature of healthy, physically active old individuals does not represent a likely pathway for the manifestation of anabolic resistance.