Protein contributes to growth and maintenance of muscle mass. The effect of protein supplementation when used in conjunction with resistance exercise training (RET) on enhancing muscle mass and strength in athletes has been widely investigated. However various studies have produced conflicting results. Morton et al (2018) performed a meta-analysis to further investigate the effects of protein supplementation during RET in healthy adults.
Summary | The aim of this meta-analysis was to provide evidence-based recommendations on optimal protein intakes for nutritional strategies designed to enhance skeletal muscle adaptation during RET. Morton et al (2018) conclude that whilst RET alone is a more potent stimulus for gains in muscle mass and strength, supplemental dietary protein significantly enhanced changes in muscle strength and size during RET in healthy adults. They conclude that a protein intake of ~1.6 g/kg/day is sufficient in healthy adults to augment strength and muscle mass gains when combined with RET over a period of weeks.
There is a lack of consensus between the previous meta-analyses and systematic reviews that have been performed. This might be attributed to the differing study inclusion criteria such as the age and training status of participants, the duration of RET interventions, as well as differences in energy intake and the amount, type and source of supplemental protein used. This latest systematic review by Morton et al (2018) is the most comprehensive meta-analysis of studies on protein supplementation and RET to date. The authors aim to provide an evidence-based answer to the question ‘Are protein supplements effective in augmenting resistance training induced gains in muscle mass and strength?’
For inclusion in this analysis, participants had to be performing RET at least twice per week with protein supplementation in at least one group for a minimum duration of six weeks. Only participants who were healthy, consuming an energy intake at or above their energy requirements and were not using any other anabolic agents such as creatine, β-HMB, or testosterone enhancing products were included.
A total of 49 studies with 1863 participants with a mean age of 35 years (±20 years) were accepted for this meta-analysis. Further analysis using meta-regression was performed in order to explore the effects of baseline protein intake (g/kg/day), post-exercise protein dose (g), age and training status on the efficacy of protein supplementation. A separate break point analysis was conducted to explore the influence of protein intake (g/kg/day) on the change in fat free mass (FFM).
With respect to the protein source, whey protein in the form of a drink or bar and milk based beverages were most commonly used to supplement protein intake. Soy and pea proteins were used in 6 studies and 7 studies instructed participants to increase protein intake using whole foods which included beef and yoghurt.
The RET interventions lasted from 6 weeks to 52 weeks (13±8 weeks) performing RET between 2 days and 5 days per week (3±1 days/week).
With protein supplementation there was a significant increase in relative daily protein intake from 1.4 to 1.8 g/kg/day and no change in the control group.
Protein supplementation beyond a total daily protein intake of ~1.62 g/kg/day during RET provided no further benefit on gains in muscle mass or strength.
In terms of strength gains RET resulted in an overall increase of 27 kg (mean ±SD; 27±22 kg) in all measures of one-repetition-maximum (1RM) strength. Protein supplementation augmented the increase in 1RM strength by 2.49 kg (9%) suggesting that RET is the greatest stimulus for increasing muscle strength, however this increase is augmented by extra protein. Meta-regression analysis revealed that age, training status, post-exercise protein dose and baseline protein intake did not influence the efficacy of protein supplementation on changes in 1RM strength.
RET alone resulted in a mean increase in fat free mass (FFM) of 1.1 kg (±1.2 kg). Protein supplementation further augmented the increase in FFM by 0.30 kg (27%). Dietary protein supplementation during RET was found to be more effective at increasing changes in FFM in resistance-trained individuals (0.75 kg) and less effective in older participants (−0.01 kg) suggesting that older adults may not respond as well to protein supplementation. This finding may well also be explained by the lower average daily amount of supplemental protein of 20 ±18 g/day given to older participants in the studies analysed compared with an intake of 42 ±32 g/day supplemental protein in younger participants. The authors pointed out that older adults have a tendency to be anabolically resistant and require higher per-meal protein doses compared with younger participants to achieve similar rates of muscle protein synthesis (MPS). However a limitation of this meta-analysis is that there were only 13 studies that met the authors inclusion criteria in older (>45 years) adults.
Meta- regression analysis found that the post-exercise protein dose (24 ±11 g; young: 24 ±12 g; old: 23 ±10g) on training days had no éxtra effect on the efficacy of protein supplementation on RET-induced gains in FFM and strength over a period of weeks. The authors concluded that a baseline protein intake of 1.6 g/kg/day, divided into ~0.25 g/Kg doses, and training status, were the stronger influences on adaptive change in muscle mass.
Morton et al (2018) concludes that whilst RET alone is a more potent stimulus for gains in muscle mass and strength, supplemental dietary protein significantly enhanced changes in muscle strength and size during RET in healthy adults. A protein intake of ~1.6 g/kg/day is sufficient in healthy adults to augment strength and muscle mass gains when combined with RET over a period of weeks.