Dietary protein is a nutrient of special importance since it is the source of key building blocks (essential amino acids/EAAs) for muscle maintenance, growth and repair. Good muscle mass is fundamental for the promotion of health because reductions in mass are associated with muscle weakness, frailty, adverse health outcomes, morbidity and mortality. Nevertheless, nutritional deficiencies are often reported in older people, resulting in undernutrition and malnutrition characterised mainly by 1) low protein and 2) low energy intake.
A growing body of evidence also suggests that muscle losses and weakness can be accelerated by physical inactivity; we know that physical activity levels decline with age, with levels lowest in the very old (National Statistics, 2017). In UK alone, approximately 39% of adults do not meet physical activity recommendations of at least 150 minutes of moderate intensity activity and resistance training twice a week (BHF, 2017). Resistance exercise training is highly recommended for those who wish to optimise muscle growth and function. Recently, in a pooled analysis of 11 population cohorts with all-cause, cancer, and cardiovascular mortality endpoints featuring 80,306 participants (Stamatakis et al. 2017), strength promoting exercise was associated with significant reductions in cancer mortality risk. These exciting data further emphasise the need for meeting resistance exercise recommendations.
Undoubtedly, sufficient protein intake and resistance exercise are crucial for optimisation of muscle growth and strength in both the young and the old. How much dietary protein is sufficient? Are current recommendations of 0.8 g per kg of body mass (BM) (g.kg.-1BM-1) adequate?
Findings of the most recent and comprehensive systematic review (Morton et al. 2017), which included 49 studies and 1863 participants, on the effects of protein supplementation during resistance exercise training, suggest that protein supplementation may further enhance gains in muscle mass. Specifically, protein supplementation augments muscle growth when dietary protein intakes are below 1.6 g.kg.-1BM-1 in the young. In contrast, the impact of protein supplementation seems to reduce with advancing age. The latter is not surprising since it is known for a while that older people have higher needs for dietary protein (~0.4 g.kg.-1BM-1 of protein per meal), and are less able to use dietary protein for muscle synthesis (Cuthbertson et al., 2006). Therefore, current recommendations of 0.8 g.kg.-1BM-1 are considered far below recent research recommendations to consume 1.2-1.6 g.kg.-1BM-1.
Given that muscle weakness is exacerbated in inactive older people and in those with dietary deficiencies, attractive solutions include prevention and management strategies involving exercise/physical activity as well as nutritional interventions singularly or combined. However, simply asking older people to eat food of high nutritional value creates certain challenges. Adding dietary protein, which is crucial for maintenance of muscle mass, typically enhances satiety resulting in decreased food consumption.
Therefore, nutritional interventions (i.e. involving EAAs supplements) that do not suppress the appetite (Ispoglou et al. 2017) in elderly groups may be used to treat their age-related muscle weakness. A number of other studies, including our recently completed pilot study (Ispoglou et al. 2016), showed that a standard composition of EAAs may not be optimum for the elderly, thus it is necessary that EAAs nutritional prototypes targeting the elderly also need to be of different composition than a standard mixture of EAAs.
We have demonstrated that novel nutritional prototypes can effectively address energy and protein deficiencies in older individuals (Ispoglou et al 2017). These prototypes are palatable, easily absorbed, and overcome issues associated with chewing and swallowing. This makes them especially appropriate for treating or preventing muscle weakness in clinical and non-clinical populations.