Increasing protein intake has been suggested as an effective strategy to ameliorate age-related loss of muscle mass and strength. Current reviews assessing the effect of protein supplementation are strongly influenced by the inclusion of studies with frail older adults. We assessed the effect of protein supplementation on lean body mass, muscle strength, and physical performance in exclusively nonfrail community-dwelling older adults. Moreover, we assessed the superior effects of protein supplementation during concomitant resistance exercise training on muscle characteristics. A systematic literature search was conducted on PubMed, Embase, and Web of Science up to 15 May 2018. We included randomized controlled trials that assessed the effect of protein supplementation on lean body mass, muscle thigh cross-sectional area, muscle strength, gait speed, and chair-rise ability and performed random-effects meta-analyses. Data from 36 studies with 1682 participants showed no significant effects of protein supplementation on changes in lean body mass [standardized mean difference (SMD): 0.11; 95% CI: -0.06, 0.28], handgrip strength (SMD: 0.58; 95% CI: -0.08, 1.24), lower extremity muscle strength (SMD: 0.03; 95% CI: -0.20, 0.27), gait speed (SMD: 0.41; 95% CI: -0.04, 0.85), or chair-rise ability (SMD: 0.10; 95%: CI -0.08, 0.28) compared with a control condition in nonfrail community-dwelling older adults. Moreover, no superior effects of protein supplementation were found during concomitant resistance exercise training on muscle characteristics. Protein supplementation in nonfrail community-dwelling older adults does not lead to increases in lean body mass, muscle cross-sectional area, muscle strength, or physical performance compared with control conditions; nor does it exert superior effects when added to resistance exercise training. Habitual protein intakes of most study participants were already sufficient, and protein interventions differed in terms of type of protein, amount, and timing. Future research should clarify what specific protein supplementation protocol is beneficial for nonfrail community-dwelling older adults with low habitual protein intake.

Background: An inadequate protein intake may offset the muscle protein synthetic response after physical activity, reducing the possible benefits of an active lifestyle for muscle mass. We examined the effects of 12 weeks of daily protein supplementation on lean body mass, muscle strength, and physical performance in physically active older adults with a low habitual protein intake (<1.0 g/kg/day).Methods: A randomized double‐blinded controlled trial was performed among 116 physically active older adults [age 69 (interquartile range: 67–73) years, 82% male] who were training for a 4 day walking event of 30, 40, or 50 km/day. Participants were randomly allocated to either 31 g of milk protein or iso‐caloric placebo supplementation for 12 weeks. Body composition (dual‐energy X‐ray absorptiometry), strength (isometric leg extension and grip strength), quadriceps contractile function, and physical performance [Short Physical Performance Battery, Timed Up‐and‐Go test, and cardiorespiratory fitness (Åstrand–Rhyming submaximal exercise test)] were measured at baseline and after 12 weeks. We assessed vitamin D status and markers of muscle damage and renal function in blood and urine samples before and after intervention.Results: A larger increase in relative lean body mass was observed in the protein vs. placebo group (∆0.93 ± 1.22% vs. ∆0.44 ± 1.40%, P_Interaction = 0.046). Absolute and relative fat mass decreased more in the protein group than in the placebo group (∆−0.90 ± 1.22 kg vs. ∆−0.31 ± 1.28 kg, P_Interaction = 0.013 and ∆−0.92 ± 1.19% vs. ∆−0.39 ± 1.36%, P_Interaction = 0.029, respectively). Strength and contractile function did not change in both groups. Gait speed, chair‐rise ability, Timed Up‐and‐Go, and cardiorespiratory fitness improved in both groups (P < 0.001), but no between‐group differences were observed. Serum urea increased in the protein group, whereas no changes were observed in the placebo group (P_Interaction < 0.001). No between‐group differences were observed for vitamin D status, muscle damage, and renal function markers.Conclusions: In physically active older adults with relatively low habitual dietary protein consumption, an improvement in physical performance, an increase in lean body mass, and a decrease in fat mass were observed after walking exercise training. A larger increase in relative lean body mass and larger reduction in fat mass were observed in participants receiving 12 weeks of daily protein supplementation compared with controls, whereas this was not accompanied by differences in improvements between groups in muscle strength and physical performance.

It is well-known that the postprandial muscle protein synthetic response to protein ingestion is regulated on various levels, including dietary protein digestion and amino acid (AA) absorption, splanchnic AA retention, the availability of dietary protein–derived AA in the circulation, delivery of AA to the muscle, uptake of AA by the muscle, and intramuscular signaling. AA availability after consumption of dairy products is primarily determined by the rate of gastric emptying of milk proteins, which is mainly linked to coagulation of milk proteins in the stomach. Caseins form gastric coagula, which make their gastric emptying and subsequent postprandial aminoacidemia notably slower than that of whey proteins. Only recently, the role of processing, food structure, preservation and matrix on coagulation herein has been getting attention. In this review we describe various processes, that affect gastric coagulation of caseins and therewith control gastric emptying, such as the conversion to caseinate, heat treatment in the presence of whey proteins, conversion to stirred yoghurt and enzymatic hydrolysis. Modulating product characteristics by processing can be very useful to steer the gastric behavior of protein, and the subsequent digestion and AA absorption and muscle anabolic response to maintain or increase muscle mass.

Skeletal muscle plasticity is reflected by a dynamic balance between protein synthesis and breakdown, with basal muscle tissue protein synthesis rates ranging between 0.02 and 0.09%/h. Though it is evident that other musculoskeletal tissues should also express some level of plasticity, data on protein synthesis rates of most of these tissues in vivo in humans is limited. Six otherwise healthy patients (62±3 y), scheduled to undergo unilateral total knee arthroplasty, were subjected to primed continuous intravenous infusions with L-[ring-¹³C₆]-Phenylalanine throughout the surgical procedure. Tissue samples obtained during surgery included muscle, tendon, cruciate ligaments, cartilage, bone, menisci, fat, and synovium. Tissue-specific fractional protein synthesis rates (%/h) were assessed by measuring the incorporation of L-[ring-¹³C₆]-Phenylalanine in tissue protein and were compared with muscle tissue protein synthesis rates using a paired t test. Tendon, bone, cartilage, Hoffa’s fat pad, anterior and posterior cruciate ligament, and menisci tissue protein synthesis rates averaged 0.06±0.01, 0.03±0.01, 0.04±0.01, 0.11±0.03, 0.07±0.02, 0.04±0.01, and 0.04±0.01%/h, respectively, and did not significantly differ from skeletal muscle protein synthesis rates (0.04±0.01%/h; P>0.05). Synovium derived protein (0.13±0.03%/h) and intercondylar notch bone tissue protein synthesis rates (0.03±0.01%/h) were respectively higher and lower compared to skeletal muscle protein synthesis rates (P<0.05 and P<0.01, respectively). Basal protein synthesis rates in various musculoskeletal tissues are within the same range of skeletal muscle protein synthesis rates, with fractional muscle, tendon, bone, cartilage, ligament, menisci, fat, and synovium protein synthesis rates ranging between 0.02 and 0.13% per hour in vivo in humans. Clinical trial registration: NTR5147