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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].