Cited by 87Muscle metabolism and nutritionNutrition and Health in AgingMuscle Physiology and Disorders

The Role of Protein Intake and its Timing on Body Composition and Muscle Function in Healthy Adults: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

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Wirth, Janine, Hillesheim, Elaine, Brennan, Lorraine·2020·The Journal of Nutrition

Abstract

Background Increased protein intake has been suggested to improve gains in muscle mass and strength in adults. Furthermore, the timing of protein intake has been discussed as a margin of opportunity for improved prevention measures.Objective This systematic review investigated the effect of protein supplementation on body composition and muscle function (strength and synthesis) in healthy adults, with an emphasis on the timing of protein intake.Methods Randomized controlled trials were identified using PubMed, Web of Science, CINAHL, and Embase, up to March 2019. For meta-analyses, data on lean body mass (LBM), handgrip strength, and leg press strength were pooled by age group (mean age 18-55 or >55 y) and timing of protein intake. The quality of evidence was assessed using the Grading of Recommendations, Assessment, Development and Evaluations approach.Results Data from 65 studies with 2907 participants (1514 men and 1380 women, 13 unknown sex) were included in the review. Twenty-six, 8, and 24 studies were used for meta-analysis on LBM, handgrip strength, and leg press strength, respectively. The protein supplementation was effective in improving (mean difference; 95% CI) LBM in adults (0.62 kg; 0.36, 0.88) and older adults (0.46 kg; 0.23, 0.70), but not handgrip strength (older adults: 0.26 kg; -0.51, 1.04) and leg press strength (adults: 5.80 kg; -0.33, 11.93; older adults: 1.97 kg; -2.78, 6.72). Sensitivity analyses removing studies without exercise training had no impact on the outcomes. Data regarding muscle synthesis were scarce and inconclusive. Subgroup analyses showed no beneficial effect of a specific timing of protein intake on LBM, handgrip strength, and leg press strength.Conclusion Overall, the results support the positive impact of protein supplementation on LBM of adults and older adults, independently of intake timing. Effects on muscle strength and synthesis are less clear and need further investigation. This systematic review was registered on PROSPERO as CRD42019126742.

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  • Most meta-analyses have reported consistently positive results regarding the effect of additional protein ingestion on RE training-induced increases in LBM.

    “Most meta‐analyses have reported consistently positive results regarding the effect of additional protein ingestion on RE training‐induced increases in LBM. 13 , 14 , 21 , 23”

    From Systematic review and meta-analysis of protein intake to support muscle mass and function in healthy adults (2022) · cites this paper as [21]

    The Discussion reports that three previous meta-analyses found a positive effect of protein supplementation on LBM, and this paper’s own meta-analysis found improved LBM in adults and older adults. But the Discussion explicitly calls previous results inconsistent, citing a meta-analysis with no benefit in nonfrail older adults; it does not establish that the positive findings were consistently specific to resistance-exercise-induced gains. (LLM, conf=0.85)

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  • Protein supplementation rarely substantially affects strength outcomes, which highlights that the extra LBM gain stimulated by protein supplementation may not be muscle, or at least not sufficient muscle that is contributing to increases in strength.

    “This question is relevant as protein supplementation rarely substantially affects strength outcomes, 14 , 21 , 22 , 23 which highlights that the extra LBM gain stimulated by protein supplementation may not be muscle, 111 or at least not sufficient muscle that is contributing to increases in strength.”

    From Systematic review and meta-analysis of protein intake to support muscle mass and function in healthy adults (2022) · cites this paper as [21]

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  • The insertion of clinical trials testing multi-ingredient supplements, including energy-restricted weight-loss diets, or using different cut-off points for age sub-groups likely explains differences in main effects and conclusions when comparing studies.

    “The insertion of clinical trials testing multi‐ingredient supplements, 106 including energy‐restricted weight‐loss diets, 13 or using different cut‐off points for age sub‐groups likely explain the differences in main effects and conclusions when comparing studies. 14 , 21 , 22 , 23”

    From Systematic review and meta-analysis of protein intake to support muscle mass and function in healthy adults (2022) · cites this paper as [21]

    The Discussion says a different review’s positive muscle-strength finding could be attributed to its inclusion of energy restriction and supplements not predominantly composed of protein. The Overview of study characteristics defines this review’s older-adult group as having a mean age over 55, but our copy does not identify differing age cut-offs between reviews as an explanation for their differing conclusions. (LLM, conf=0.86)

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  • Wirth et al. 21 and Haaf et al. 22 found no effect of additional protein ingestion and exercise on lower-body strength.

    “In contrast, Wirth et al. 21 and Haaf et al. 22 found no effect of additional protein ingestion and exercise on lower‐body strength.”

    From Systematic review and meta-analysis of protein intake to support muscle mass and function in healthy adults (2022) · cites this paper as [21]

    The Leg press strength results report no statistically significant improvement with protein supplementation, including in analyses grouped by timing around exercise. But the Other strength tests section says there was no clear consensus for other measures, including squats and leg or knee extension and flexion; the finding therefore does not establish no effect across lower-body strength measures or of exercise itself. (LLM, conf=0.82)

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  • A meta-analysis showed protein supplementation can help preserve lean body mass in adults and older adults, although effects on muscle strength and synthesis were less clear.

    “A meta-analysis showed that protein supplementation can help preserve lean body mass in adults and older adults, although the effects on muscle strength and synthesis were less clear.”

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Authors
Wirth, J., Hillesheim, E., & Brennan, L.
Affiliations
University College Dublin (IE)
Corresponding author
Brennan, Lorraine
Venue
The Journal of Nutrition
Publisher
Elsevier BV
Published
2020-06
Volume
150
Issue
6
Pages
1443-1460
ISSN
0022-3166 (ISSN-L)
OpenAlex
W3013097860
Cited by
87
References
83
Citations by year
2026: 11 · 2025: 13 · 2024: 22 · 2023: 16 · 2022: 18 · 2021: 4 · 2020: 3
FWCI
1.18
Type
journal article
Language
en
Open access
Locations
  • Journal of Nutrition (publishedVersion)
  • PubMed (publishedVersion)
  • Arrow@dit (Dublin Institute of Technology) (submittedVersion)
  • Research Repository UCD (University College Dublin) (submittedVersion, cc-by-nc-nd)
Funders
  • European Union's Horizon 2020
  • Enterprise Ireland (713654)
  • European Research Council (647783)
  • Brazilian Federal Agency
  • Evaluation of Graduate Education
  • CAPES (88881.174061/2018-01)
Topics
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Cell Biology › Muscle metabolism and nutrition
  • Health Sciences › Medicine › Physiology › Nutrition and Health in Aging
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Muscle Physiology and Disorders
Keywords
protein supplementation, body composition, lean body mass, muscle strength, grip strength, leg press strength, older adults, healthy adults
MeSH terms
Adolescent, Adult, Body Composition, Dietary Proteins, Female, Humans, Male, Middle Aged, Randomized Controlled Trials as Topic, Muscle, Skeletal, Resistance Training, Young Adult

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