Muscle is central to amino acid homeostasis
The following is from a paper, The underappreciated role of muscle in health and disease
Muscle plays a central role in whole-body protein metabolism by serving as the principal reservoir for amino acids to maintain protein synthesis in vital tissues and organs in the absence of amino acid absorption from the gut and by providing hepatic gluconeogenic precursors. Furthermore, altered muscle metabolism plays a key role in the genesis, and therefore the prevention, of many common pathologic conditions and chronic diseases. Nonetheless, the maintenance of adequate muscle mass, strength, and metabolic function has rarely, if ever, been targeted as a relevant endpoint of recommendations for dietary intake. It is therefore imperative that factors directly related to muscle mass, strength, and metabolic function be included in future studies designed to demonstrate optimal lifestyle behaviors throughout the life span, including physical activity and diet.
Muscle “breathes” amino acids – it builds itself up after feeding, and breaks down during fasting to maintain homeostatic levels of amino acids in the blood. Without doing that, you would soon be dead. It follows that having more muscle is better for overall health than having less muscle (perhaps within limits) but this fact is definitely “underappreciated”.
Sarcopenia is a pathological state of muscle wasting
Sarcopenia is the condition in later life or illness when there is not enough muscle. Besides being conducive to disability and falls (with subsequent bone breakage), sarcopenia also means that amino acid homeostasis is harder to maintain. Oxidative stress, seen at much higher levels in older people, is the result.
So should everyone lift weights? I think so. It’s been shown to be of benefit to people as old as in their 90s – of course older people will need special help and supervision – and it’s also being advocated for children.
One interesting item from the above paper is that it’s possible that the RDA for protein is too low.
The optimal intake of protein is uncertain, but one can derive estimates from acute metabolic studies of muscle metabolism. Thus, the maximal response of muscle protein synthesis can be attained with intake of ≈15 g essential amino acids (EAAs) (91), which is approximately equal to the amount of EAAs in the EAR for a 55-kg woman (0.66 g protein · kg−1 · d−1 × 55 kg × 0.42 EAA/g protein = 15.2 g protein). The response to a single dose of amino acids can potentially be achieved multiple times per day, with additive effects, with repeated meal ingestion (91, 92). Consequently, it would not be unreasonable to expect beneficial effects stemming from increased myofibrillar and mitochondrial protein synthesis to be achieved with the ingestion of 15 g EAAs at each meal rather than at only one meal per day. This would translate to a protein intake as high as 1.8 g protein · kg−1 · d−1. Although this amount may seem extreme in the context of the current recommendations, it is in line with the amount of protein in the average American diet, which was reported in the DRIs to be 1.5 g · kg−1 · d−1 for adults (62). Furthermore, detrimental effects of protein intakes ≥2.0 g · kg−1 · d−1 have not been documented (62).
If this line of reasoning is correct, virtually everyone should be eating protein in amounts closer to that of bodybuilders.