Examining the effects of different types and timings of protein on muscle mass and strength Original paper

In this network meta-analysis of randomized controlled trials, adults undergoing resistance training gained muscle mass and strength when adding protein from dairy (e.g., milk, whey protein), meat, or mixed sources and when adding protein after exercise and/or at night.

The effects of protein type and timing for enhancing muscle mass, muscle strength, and physical performance in the context of resistance training.

A total of 4,711 adults (average age of 46; 64% men and 36% women) without apparent health conditions.

A network meta-analysis of 116 randomized controlled trials was conducted. The duration of the interventions ranged from 4 to 74 weeks, with a median duration of 12 weeks. Muscle mass was assessed in 100 studies, muscle strength in 78 studies, and physical performance in 23 studies.

Protein timing included protein supplementation before exercise (preexercise), after exercise (postexercise), during the day, and at night (before sleep), as well as various combinations of these time points. Protein supplements from various sources were assessed in the studies, including plant proteins (soy and rice) and animal proteins from meat (fish, chicken, and red meat) and milk (whey, casein, yogurt, and whole milk). Moreover, some studies used collagen, insect protein, and bovine colostrum (i.e., the milk that comes from cows in the first few days after giving birth).

Muscle mass was assessed using fat-free mass, lean body mass, skeletal muscle mass, and appendicular lean mass (i.e., lean mass of the arms and legs). Muscle strength was analyzed either indirectly via handgrip strength (an indicator of overall muscle strength) or directly via one-repetition maximum (1RM) levels of the squat, leg press, bench press, chest press machine, and leg extension machine. Physical performance was assessed using gait speed (i.e., the time one takes to walk a certain distance), the time up-to-go test (i.e., the time it takes for standing up, walking a certain distance, turning, and sitting down), and the 6-minute walking test (i.e., the distance an individual can walk over a total of six minutes). All outcomes were compared to a placebo control or another timing or type of protein supplementation.

Consuming protein of any type after exercise increased fat-free mass (+0.54 kg) and skeletal muscle mass (+0.34 kg), whereas taking protein before and after exercise enhanced fat-free mass (+1.87 kg). Protein intake before sleep was most effective for enhancing handgrip strength (+2.85 kg) and leg press 1RM (+12.1 kg). Moreover, consuming protein after exercise and during the day enhanced leg press 1RM (+14.1 kg), whereas protein after exercise and before sleep enhanced leg extension 1RM (+15.0 kg).

Supplementing with whey protein was most effective for improving squat 1RM (+9.2 kg), casein for bench press 1RM (+15.5 kg), and yogurt and red meat for leg extension 1RM (+15.0 kg and +18.0 kg, respectively). Mixed protein sources were most effective for enhancing fat-free mass (+1.12 kg) and handgrip strength (+2.65 kg), whereas milk and red meat improved appendicular lean mass (+1.06 kg and +1.03 kg, respectively). Moreover, supplementing with collagen improved fat-free mass (+1.5 kg), and consuming bovine colostrum (milk produced by cows for the first days following birth) improved bench press 1RM (+15.5 kg).

Protein type and timing had no clear effects on physical performance outcomes.

Effective protein types and times

1 = high-certainty evidence, 2 = moderate-certainty evidence, 3 = low-certainty evidence, 4 = very-low-certainty evidence, N/A = no specific type/time identified as statistically significant.

In the dynamic field of sports nutrition, understanding the role of dietary protein in enhancing physical performance, muscle mass, and strength is crucial. This is not just an academic pursuit because it has practical implications for athletes, fitness enthusiasts, and health care professionals. The summarized study delves into the intricate relationship between protein type, timing, and muscle development, shedding light on a subject that is as complex as it is relevant.

The study suggests a specific approach for optimizing muscle and strength gains: prioritize animal protein, especially whey protein and red meat, and consume it before or after exercise and before sleep for optimal outcomes.

However, integrating this narrative with previous research complicates the picture. For instance, earlier meta-analyses^[1][2] found that protein timing around exercise is less critical, provided total protein intake is adequate. Additionally, some studies^[3] suggest that plant protein is as effective as animal protein for muscle synthesis. In light of these conflicting findings, how much can we make sense of the summarized study’s findings?

First, let’s look at about protein type, for which the results are easier to interpret. Although some studies found animal and plant protein to be equally effective, the majority of the evidence has shown that animal protein is superior for muscle development compared to protein from plant-based sources (though this difference is less apparent in older people).^[4] This result can be attributed to the richer amino acid profile of animal protein, especially the content of essential amino acids like leucine, which are key for supporting muscle growth and stimulating muscle protein synthesis. Protein from animal sources is also easier to digest than protein from whole plant-based sources, leading to more efficient absorption, although plant protein isolates do approach the digestibility of animal proteins.^[5] Hence, choosing the right type of protein is a crucial consideration for optimal dietary strategies for muscle gain, but the choice should equally be influenced by personal preferences and ethical considerations.

Essential amino acid content of various protein sources (percentage of total protein)

Note that this graph does not reflect differences in total protein content of the included foods. Adapted from Pinckaers PJM, et al., 2021, Sports Med.^[5]

Regarding protein timing, the debate becomes more complex and technical. The fact that prior meta-analyses^[1][2] didn’t find a significant impact of protein timing on muscle development challenges the summarized study’s findings. How can these seemingly contradictory study results be reconciled?

One key difference that could explain these discrepancies lies in the study design. Although the two previous studies were meta-analyses^[1][2], the summarized study is a network meta-analysis. Although both methods are valid statistical approaches used to synthesize research findings, they differ in methodology and scope. A traditional meta-analysis combines results from multiple studies that have a common comparator, such as a placebo or standard treatment, providing a direct comparison of interventions. It's most reliable when the studies are similar in their designs, populations, and interventions. However, the findings can be limited when direct comparisons are not available for all relevant interventions.

On the other hand, a network meta-analysis extends the principles of a meta-analysis by incorporating both direct and indirect comparisons across a network of studies. This approach allows for the comparison of multiple interventions, even if they have not been directly compared in a head-to-head study. As such, network meta-analyses are particularly useful in fields where numerous treatments exist but direct comparisons are scarce, as is in the case of protein type and timing. The validity of the findings relies heavily on the assumption of transitivity, meaning that the studies forming the network are sufficiently similar in terms of patient populations, settings, and other key variables.

So, which study design should we trust more? The trustworthiness of both approaches depends on several factors. If both analyses are well conducted and everything else is equal, the network meta-analysis might be more informative because it leverages a broader range of data, including indirect comparisons. However, the validity of its results can be compromised if the assumption of transitivity is violated. Conversely, the meta-analysis, although it is more limited in scope, might offer more reliable results for the specific comparisons it evaluates, assuming high quality and homogeneity among the included studies.

Another limitation of these earlier meta-analyses is that they only examined protein intake before and after exercise, whereas the summarized study also considers different times of day, including protein intake before sleep. The summarized study suggests that consuming protein at night is effective for building muscle strength. This finding aligns well with previous research^[6], which showed that protein intake before sleep has the potential for increasing muscle protein synthesis and aiding postexercise recovery overnight. The summarized study further extends general understanding of this topic, indicating that protein intake at night not only enhances recovery, but also translates to more potent gains in muscle strength.

Finally, there is one major limitation that strongly reduces confidence in the summarized study’s findings. The majority of the included studies examining protein timing effects did not rigorously equate total daily protein intake between the control and intervention groups. Hence, the protein timing groups had an inherent advantage of a larger daily protein intake. This is important because previous meta-analyses showed that if the results are adjusted for total protein intake, the benefits of protein timing on hypertrophy vanished.^[1][2] The studies concluded that total daily protein intake is more important than the time of consumption.^[1][2] Thus, the differences in muscle mass observed in the summarized study could equally well be explained by discrepancies in total daily protein intake.

Bottom line: Total daily protein intake is most likely the most important factor for optimizing muscle mass and strength in adults who are undergoing resistance training. Protein type also matters, with animal protein tending to be more effective than plant protein, given its richer amino acid profile and more efficient digestion and adsorption. Protein timing, on the other hand, remains a controversial topic. Although the summarized study showed favorable effects of protein intake before and after exercise, previous research has found no benefits of these practices. Protein intake before sleep might also be beneficial, according to the findings from the summarized study, but more high-quality research with direct head-to-head comparisons is needed to confirm this beneficial effect.

Despite its novel insights into the topic of protein timing, the summarized study also has noteworthy limitations:

• The study focused only on adults undergoing resistance training, not on addressing the effects of protein supplementation timing or type in adults engaged in endurance exercise or not engaged in exercise training.
• Although the network meta-analysis design can be a strength, the reliance on indirect comparisons in the absence of robust head-to-head studies introduces a degree of uncertainty to the conclusions.
• Many of the included studies on protein timing did not equate total protein intake between control and experimental groups, potentially biasing the results toward those with higher protein consumption. As such, the summarized study’s emphasis on protein timing over total protein intake may overlook the broader, potentially more impactful aspect of total protein consumption throughout the day.