2.1. Daily Quantity
The current international Recommended Dietary Allowance (RDA) for protein is 0.8 g per kg of body weight (bw), regardless of age [28
]. In the UK, the Reference Nutrient Intake (RNI) is 0.75 g/kg/bw [30
]. These recommendations are derived as a minimum amount to maintain nitrogen balance and are not optimised for physical activity level (PAL). Individuals with low PAL have decreased rates of nitrogen retention and therefore in order to maintain muscle tissue have increased protein requirements in comparison to those who are active [31
]. Considering that physical activity decreases with age [32
], this is an important factor when protein needs are evaluated. Furthermore, the body of an ageing adult undergoes multiple physiological changes which alter protein utilisation, and thus requirements, i.e., anabolic resistance, insulin resistance, impaired digestion, inflammation, and decreased IGF-1 levels [3
]. The adequacy of current protein recommendations has also been challenged because of potential methodological pitfalls. First, the nitrogen-balance method used in the majority of pooled studies may not be accurate, possibly due to unaccounted routes of nitrogen input and output [3
]. A second limitation is that nitrogen-balance studies must be carried out in a controlled, clinical environment, hence the protein requirement assessment is relatively short-term [33
]. Data regarding long-term evaluations of protein needs in ageing adults, with a use of novel, more accurate assessment techniques, is scarce and is identified as an academic research priority [34
Acknowledging all these factors, and supported by a large body of new evidence, the International PROT-AGE Study Group [3
] and European Society for Clinical Nutrition and Metabolism (ESPEN) [11
] concluded that daily protein requirement of healthy individuals over 65 years is 1.0–1.2 g protein/kg/bw. A further increase is recommended for individuals with acute or chronic illnesses (1.2–1.5 g protein/kg/bw) and severe illnesses, injuries, or malnutrition (2.0 protein g/kg/bw) [3
]. Although these new recommendations have been formulated for adults >65 years, this is only an agreed conceptual cut-off point. Since it has been estimated that 0.5–1% of muscle mass is lost annually from the age of 50 [36
], an increased dietary protein intake may be required earlier in life in order to mitigate the muscle ageing process. One of the longest (14-week) interventional studies to date revealed that in adults aged 55–77, ingestion of 0.8 g protein/kg/bw was associated with decreased mid-thigh muscle area and decreased urinary nitrogen excretion (when compared to the second week of the intervention), suggesting that the current RDA might be below the actual requirements of an ageing adult [9
]. The link between protein consumption at the RDA level and adverse health outcomes was also confirmed in longitudinal observational studies. As discovered by Houston and others [37
], older adults (70–79 years) whose daily protein intake was 1.1 ± 0.4 g/kg/bw had lost 40% less lean body mass over the course of three years than those who consumed 0.8 ± 0.3 g/kg/bw.
In summary, protein requirements vary on an individual basis and depend on various factors, such as age, health status and PAL. These factors are not reflected in current recommendations for the general population. Therefore, an increase in intake of dietary protein beginning around midlife merits further research
2.2. Daily Distribution
Apart from the total daily intake, per-meal protein quantity and daily frequency of protein ingestion have also been shown to play an important role in preserving muscle mass and function. It is estimated that consumption of two to three meals a day, each containing ~25–30 g of high-quality protein, is optimal for the stimulation of 24-h muscle protein synthesis (MPS) in healthy adults [38
]. This approximate quantity is thought to be sufficient, both for younger and healthy older adults [39
Interestingly, ‘the more, the better’ approach is not necessarily optimal. Moore and colleagues [43
] investigated the per-meal threshold in relation to body weight and age. In this study, protein utilisation plateaued after the ingestion of 0.24 g of whey and egg protein/kg/bw in young men (~22 years) and after 0.40 g/kg/bw in older men (~71 years) [43
]. Similar findings were reported in an acute feeding study by Symons and colleagues [39
] who compared the effects of 30 g of lean-beef protein/meal to 90 g/meal ingestion on MPS in young (35 ± 3 years) and older adults (68 ± 2 years) and found there was no additional benefit of increased protein consumption in either age category. The estimated per-meal threshold after consuming a plant protein-rich meal is still unknown, particularly in ageing adults [44
Some studies argue that consuming a higher dose of protein on one daily occasion (pulse feeding) can stimulate a higher anabolic response than smaller doses across multiple meals [45
]. Arnal and colleagues [45
] reported that during the trial, women (mean age 68 ± 1 years) who consumed 79% of daily protein at noon, 7% in the morning and 14%% in the evening had improved nitrogen balance, when compared to women who consumed their protein spread over four meals (21.5 ± 0.5, 31.2 ± 0.2, 19.1 ± 0.5, 28.3 ± 0.5% of daily protein intake). However, none of these four meals contained the required bolus of 25–30 g protein per serving, which could be a potential cause why the spread pattern treatment was less effective. Reports by Bouillanne and colleagues [46
] were in line with Arnal’s findings, suggesting that pulse feeding was more effective in improving lean mass index in older adults. However, since participants in these studies were recruited from very old (mean age 84.1 years) hospitalised patients at risk of malnutrition the results should be cautiously interpreted and cannot be generalised to younger, healthy adults.
In most Western societies, the daily pattern of dietary protein ingestion is skewed regardless of age or sex, with the lowest amount of protein being consumed in the morning and the greatest in the evening meal [48
]. As shown in a British cohort study that has followed the dietary intake of adults aged 36 years for 17 years, the protein content of meals has gradually shifted toward the evening [48
]. Although these results refer to years 1982–1999, evidence suggests, that this trend has been sustained because it is mirrored in data collected more recently from other countries. Data from U.S. National Health and Nutrition Examination Survey (NHANES) [49
] indicate that in 2006 the average protein intake (g/meal) among women aged 51–71 year age group was 11.9 ± 0.4 (breakfast), 17.9 ± 0.5 (lunch) and 30.4 ± 0.7 (dinner) with snacks constituting 7.4 ± 0.3 [49
]. The intake (g/meal) in men was higher and accounted for 15.8 ± 0.5, 23.2 ± 0.8, 43.5 ± 1.0 and 10.5 ± 0.5, respectively [49
]. Results from this study have also confirmed that the same pattern was observed in a ≥71 years group. However, the amount of protein consumed in each meal was lower in both sexes, in comparison to the younger age group [49
]. Regarding the population of the very old community-dwelling adults, the pattern of daily protein distribution appears to peak at mid-day. The Newcastle 85+ study [52
] revealed that the highest amount of protein in this British cohort was consumed at lunch time, accounting for ~35% (around 20 g) of daily protein intake, followed by dinner ~21% (12 g), and in the period between lunch and dinner ~17% (10 g). The remaining protein were consumed at two consecutive morning occasions ~22% (13 g, combined), and late evening meal.
The commonly observed uneven pattern of protein ingestion in older adults suggests a potential risk of insufficient stimulation of MPS, even when RDA on a daily basis is being met. Meaning, the stimulation with a meal containing ~25–30 g of protein occurs only once a day, during the main meal (lunch or dinner). As suggested by Bollwein and others [13
], the protein distribution at older age is of higher importance than the total daily amount per se. In this study, the recommendation of 0.8 g/kg/bw was exceeded by all participants (>75 years), even those from the lowest quartile of protein intake. No differences were observed between frailty status and daily protein intake. However, those with a more uneven distribution were more likely to be classified as frail, characterised by lower walking speed and higher exhaustion [13
]. Ingestion of a high-protein meal before sleep has been shown to increase overnight MPS [53
], therefore this dietary habit should be sustained. In addition, to stimulate 24-h MPS, enriching the content of remaining meals with high-quality protein should be strongly encouraged, to ensure a sufficient dose of protein in each meal.
Although discussed recommendations regarding optimal protein quantity and distribution seem to be well supported, it has to be noted that most studies have analysed outcomes related solely to animal proteins and muscle health, omitting the accompanying effects of protein ingestion on appetite. Placing these findings in this context would provide valuable insight and guidance for adults who also need to limit or increase their daily energy intake in order to optimise nutritional status.