Commentaries on viewpoint: the curious case of anabolic resistance: old wives' tales or new fables?

TO THE EDITOR: We appreciate Burd et al. (1) providing an interesting perspective on whether anabolic resistance contributes to age-associated muscle wasting. In our opinion this perspective illustrates the need to consider alternative assessments of muscle protein turnover. The authors state that, “Such a gradual decline in muscle mass would likely be too small to detect using contemporary stable isotope methodology ” It must be asked then, why do these methods continue to be the norm? Perhaps a better approach is to initiate an intervention and assess outcomes over a longer period of time so that “other things” that happen during the day are accounted for, and it can be determined whether an intervention adds up to meaningful differences. In this regard there is much to be learned from exercise intervention studies where increasing energy expenditure by purposeful activity decreases other energy expenditure throughout the day, resulting in no net increase in expenditure (2). We (3) and others (4) have used the approach of measuring protein synthesis over weeks using deuterium oxide. We found that feeding protein after each bout of aerobic exercise does not increase cumulative mixed muscle protein synthesis over 6 wk in older individuals, which is counter to what would be predicted from acute studies (3). However, V̇O2 max, an important predictor of morbidity and mortality, did improve more in those who consumed protein after exercise, indicating mitochondrial adaptation. Ultimately the goal of an intervention is to improve functional outcomes. Given current methodological limitations, perhaps other approaches should be considered.