Muscle atrophy is an unfortunate consequence of ageing and disease and can compromise physical function and impair vital metabolic processes. Interventions to improve the recovery of muscle strength and function include physical exercise and appropriate nutrition (protein intake) because both have a positive impact on protein anabolism( Reference Deutz, Bauer and Barazzoni 1 ). Beelen et al.( Reference Beelen, de Roos and de Groot 2 ) recently assessed the effect of protein enrichment on physical function in older adults with adequate protein intakes in the first 6 months after hospital discharge. The findings suggest that a higher protein intake (1·5 g of protein/kg body weight per d) may be useless in improving the physical performance of older patients after hospitalisation. There might be several reasons. A major criticism is that, in most randomised controlled trials, it is hypothesised that supplementation, not the nutrient status itself, is protective. Protein enrichment may not be effective in enhancing physical recovery in these older adults because protein intake was already adequate. In fact, most nutrients have a non-linear, inverted U-shaped association with optimum physiological function. For individuals who are already at a nutrient level for optimum functioning, further protein supplementation would provide no additional benefit.
The authors believe rightly that limited physical activity may be one of the reasons for the absence of treatment effects on physical performance. Indeed, Burd et al.( Reference Burd, Gorissen and van Loon 3 ) highlighted the hypothesis that physical inactivity is a key factor responsible for the proposed anabolic resistance of skeletal muscle protein synthesis with ageing. However, it is important to distinguish between physical activity and physical exercise. Physical activities, for example performed as part of daily living, are of substantial benefit in increasing energy expenditure in elderly sedentary subjects, but may not improve the functional capacity of the main organs (lung, heart and skeletal muscles), and hence physical performance. On the other hand, regular exercise (endurance and/or resistance) training with a sufficiently high intensity increases a variety of physiological parameters, and all of them may contribute to the training-induced increase in physical performance. Resistance training is considered to be the best exercise method for increasing muscle mass, strength and functional performance in the older population, without restriction( Reference Churchward-Venne, Tieland and Verdijk 4 ), and this may be additionally enhanced by the intake of a protein-rich diet( Reference Rondanelli, Klersy and Terracol 5 ).
Although associated with greater muscle strength, simply consuming more protein without appropriate contractile manipulation may not be a suitable approach to improving physical recovery, as there is little effect of nutritional support by itself on functional capacity and physical performance in older adults( Reference Farsijani, Payette and Morais 6 ). Similarly, the results of a recent systematic review of randomised clinical trials demonstrates that, although nutritional therapy increases daily energetic and protein intake, there is little effect of nutritional support on clinical and functional outcomes in malnourished medical inpatients( Reference Bally, Blaser Yildirim and Bounoure 7 ). Perhaps just increasing protein might not be sufficient unless you train these patients, because low skeletal muscle area may play a significant role. Thus, older adults should build up a functional reserve early enough through an individually tailored exercise programme based on objective assessments of muscular fitness and physical activity, which is the most important and almost the only effective intervention to avoid or to break the circulus vitiosus of deadaptation of the musculoskeletal and cardiorespiratory systems, for example due to ageing, disease and physical inactivity.
In the study by Beelen et al.( Reference Beelen, de Roos and de Groot 2 ), a large number of the subjects were chronic obstructive pulmonary disease (COPD) patients, commonly characterised by rapid muscle wasting, impaired exercise performance and reported to have low plasma levels of leucine, which is a nutrient ‘trigger’ for muscle anabolism and may protect skeletal muscle during periods of disuse. Likewise, tryptophan is a promising candidate since this essential amino acid is linked to inflammation and immune activation via the so-called kynurenine pathway, which is often systemically up-regulated when the immune response is activated( Reference Strasser 8 ). Recently, the NUTRAIN-trial( Reference van de Bool, Rutten and van Helvoort 9 ) showed that skeletal muscle mass increased after 4 months of high-intensity exercise training, but this response was not augmented by protein supplementation, supporting the notion that the training component per se is of primary importance when it comes to improving muscle strength and function in COPD patients( Reference Constantin, Menon and Houchen-Wolloff 10 ).
All in all, it might be concluded that the effect of protein enrichment on physical recovery is minimal in older adults when habitual protein intake is already adequate and physical activity is limited. Nutritional intervention is probably effective in undernourished patients, but most effective when combined with an exercise programme. This simple strategy may help to preserve muscle mass and function in the older population and, as such, support healthy ageing.
Acknowledgement
The author has no funding to report. The author declares that there are no conflicts of interest.
