from animals, and inversely in a third diet. Nitrogen balance was not modified in this study, but whole body protein breakdown was not inhibited to the same extent by the meal when the protein source was from vegetables in comparison with meat [60]. This study showed that intake of high‐quality proteins may be an important issue in older people, suggesting also that aging may be associated with more specific amino acid requirements especially for the EAAs. The quality of dietary proteins and its impact on muscle protein synthesis may be less important when protein intake is sufficient. In a previous study, we compared milk and soy proteins were compared for their potential impact on muscle protein synthesis using a classical steady‐state approach [61]. No differences could be found between the different sources of dietary proteins despite changes in leg amino acid uptakes. However, there is not enough data in the field of aging regarding the issue of EAAs especially at low protein intakes.
Several studies have evaluated the effect of consuming plant‐based proteins on muscle protein metabolism in young, adult, and old rats, pigs, and humans, compared to animal proteins, i.e., meat, milk, and its constitutive proteins (casein and whey proteins) [62]. A few of these studies have focused on the impact of plant‐based foods [60], soy protein [63, 64], or wheat protein [65] ingestion on protein synthesis at the whole body or skeletal muscle level in older individuals. The majority of these studies have reported that good‐quality animal proteins have a greater ability to enhance muscle protein synthesis rate and to support muscle mass than plant‐based proteins. Therefore, although plant‐based protein sources that are rich in fiber and micronutrients may be valuable to improve nutritional density of diet, they have lower anabolic potential than animal‐based proteins. Strategies to improve these properties by increasing protein quality (i.e., their amino acid composition and digestibility) including selective breeding, fortifying plant‐based proteins with specific EAAs, mixing several plant proteins, and mixing plant‐ with animal‐based protein sources can support a healthier life, notably to prevent chronic disease, such as sarcopenia, in aging.
Optimizing protein digestion rate to improve amino acid availability?
By analogy with carbohydrates, the concept of “fast” and “slow” proteins was established according to the speed at which protein are digested and absorbed from the gut [18]. The two main milk proteins, i.e. casein and whey protein, have different metabolic fates in the intestinal tract. Whey protein, a soluble protein, is considered as a “fast” protein since the plasma appearance of its constituting amino acids after its digestion and absorption is high, fast, but transient. On the contrary, casein clots in the stomach, which delays its gastric emptying and therefore results in a slower, lower but prolonged release and absorption of amino acids. This concept was applied to study the response of postprandial protein metabolism during aging [66]. In this population, the duration and magnitude of elevated plasma amino acids are key factors to counteract the decrease in muscle sensitivity to amino acids. Accordingly, whole body protein gain was higher after a meal containing “fast” protein, i.e. whey protein, than “slow” protein, i.e. casein, in older persons, when considering either isonitrogenous or isoleucine (as leucine is a well‐known anabolic factor) meals. In addition, postprandial protein utilization by the body was significantly higher with the fast protein than with the slow one [66]. We also demonstrated that postprandial protein retention due to an increase in protein was greater in older men when a fraction of the dietary proteins is provided by soluble milk proteins, i.e. whey proteins, rather than caseins [67]. Precisely, in this study, a daily administration of a drinkable dairy product containing large amount of proteins was able to promote an increase in protein intake and postprandial protein anabolism in older men. However, protein retention was better achieved when soluble milk proteins were provided rather than caseins. Consequently, improving the quality of protein by combining digestion rate and leucine content may be a more attractive option than simply increasing the quantity of protein intake (whatever their biological value) to improve protein retention in older persons. A report from Paddon‐Jones et al. showed that whey proteins were able to stimulate muscle protein synthesis rate in a group of healthy older individuals as well as the free amino acid mixture but the EAA content of the two diets was not similar [68]. We also measured the postprandial muscle protein synthesis in response to a bolus of either whey protein, casein or casein hydrolysate in healthy older persons [69]. It was clearly demonstrated that not only the speed of protein absorption, but also the leucine content were key factors for the stimulation of muscle protein anabolism. In another study, we reported that whey proteins improved postprandial muscle protein synthesis, especially mitochondrial muscle proteins and myosin, in older persons [70]. As myosin is an important protein of the contractile apparatus and mitochondrial muscle proteins are essential for energy production in order to achieve muscle contraction, these results may ultimately have positive consequences on muscle function in older men. Thus, fast‐digesting proteins administrated as a bolus could limit muscle protein loss and speed up muscle recovery after a disease‐related immobilization in older humans. Hence, supplementing breakfast with a vitamin D and leucine‐enriched whey protein medical nutrition drink stimulated postprandial muscle protein synthesis and increased muscle mass after six weeks of intervention in healthy older adults and may therefore be a way to support muscle preservation in older people [71].
Whey protein is efficient not only on muscle mass but also on muscle function, notably after physical exercise. Aged rats supplemented with rapidly digestible soluble milk proteins for two months in association with moderate physical activity displayed an increase in spontaneous locomotor activity and improved dynamic and static gait parameters without increase in muscle mass [72]. In 60‐year‐old participants, significant increases in muscle mass and strength and a reduction in muscle fatigue have been observed following a four‐month training coupled with the daily consumption of 10 g of soluble milk proteins [73]. In another study by Chale et al., whey protein concentrate supplementation for six months did not increase the beneficial effect of resistance training on muscle mass strength and power, and physical function in older persons [74].
These data clearly suggest that a dietary protein mixture which can be quickly digested and absorbed might be more efficient to limit protein loss during aging than a mixture yielding slower kinetics. Age‐related anabolic resistance could be overcome by a better availability of dietary amino acids especially EAAs. For instance, muscle function is improved in older persons during bed rest after EAAs supplementation [45]. Another way to optimize amino acid availability from solid foods is to improve chewing capacity since it may influence postprandial amino acid kinetics. Indeed, it has been demonstrated that chewing alterations induced by dental prosthesis could modulate postprandial AA kinetics following meat ingestion [75].
Is there a specific daily protein feeding pattern?
A “pulse” protein feeding pattern that combines meals rich and low in proteins during the day may improve protein retention in older persons [24, 76]. A “spread” diet composed of four meals, spreading daily protein intake over 12 hours was compared with a pulse diet providing 80% of daily protein intake concentrated at midday. The pulse protein pattern was more efficient at improving nitrogen balances and whole‐body protein retention in aged people after 15 days. Concerning the potential explanation, it can be argued that the pulse protein diet is characterized by a couple of advantages: (i) the midday protein pulse meal may stimulate whole body synthesis by highly increasing amino acid concentration, (ii) high carbohydrate and low protein meals, i.e. at dinner, are known to limit protein loss by reducing protein breakdown rate via postprandial hyperinsulinemia, and (iii) the midday meal is combined with the daily physical activity associated with everyday life. Interestingly, the beneficial effect of the pulse protein pattern on protein accretion still persisted several days after the end of the diet [76]. The pulse protein diet also restored a significant anabolic response of skeletal muscle protein synthesis to feeding without affecting protein breakdown in old rats [77]. These studies suggest that the use of a “pulse” protein pattern increases body protein retention, in particular in skeletal muscle. This concept has been applied to malnourished and at‐risk hospitalized old patients [78] and clearly the protein pulse feeding strategy improves lean mass and skeletal muscle mass in this population.
