Whey protein (and other protein powders)
Summary
Protein supplements added to resistance training give small extra gains in strength and fat-free mass, with little added benefit once total intake passes about 1.6 g/kg/day. Soy and animal proteins performed similarly in long-term trials, and timing around workouts mattered less than total daily protein. In adults over 60, results were mixed.
Key facts
- Whey (most protein powders), casein (slower-digesting milk protein), soy and rice protein, and blends[1]
- Soy versus animal protein (including whey): no difference in strength or lean-mass gains in a meta-analysis of 9 trials (B)[5]
- Strength and fat-free mass with resistance training ↑ (A); timing around workouts: no added effect (no-effect); older adults: lower-body strength ↑ with training, other measures unchanged (B)[2][3][6][7]
- Servings of about 19–40 g protein in trials; gains plateaued at total protein intakes near 1.6 g/kg/day[2][8][9][1]
- RDA 0.8 g/kg/day for healthy adults; ODS cites 1.2–2.0 g/kg/day for athletes. No UL has been set.[1]
- 6 graded human outcomes on this page
- October 8, 2026
How it works
EstablishedExercise raises muscle protein breakdown, after which muscle protein synthesis increases for up to a day or two. Eating protein raises blood amino acids that muscle takes up; all nine essential amino acids are needed to stimulate synthesis, and about 10 g of essential amino acids (roughly 25 g of complete protein) appears to be enough after exercise (established). Casein is digested more slowly than whey, so its amino acids reach the blood more slowly.[1]
What the human research says
Human trials only, one row per outcome. The form column shows what each trial actually tested. Grades follow our evidence-grading policy.
At a glance: strongest evidence per outcome
| Outcome | Grade | Effect | Strongest evidence | Rows |
|---|---|---|---|---|
| Performance | Grade A | Improved | Meta-analysis · n = 1,863 · 2018 | 5 rows |
| Healthy aging | Grade B | Mixed | Meta-analysis · n = 2,105 · 2024 | 1 row |
Every graded row
| Outcome | Effect | Grade | Best evidence | PMID |
|---|---|---|---|---|
| Strength and fat-free mass gains during resistance training | Improved | Grade A | Meta-analysis · n = 1,863 · 2018 Healthy adults in 49 resistance-training trials Form: protein supplements (types not broken down in the abstract) · Dose: Varied; added gains in fat-free mass stopped beyond total protein intakes of about 1.62 g/kg/day · Duration: Prolonged resistance training programs Protein supplements added about 2.5 kg to one-rep-max strength gains and 0.3 kg to fat-free mass gains. The effect on fat-free mass shrank with age and was larger in resistance-trained people. | 28698222 (opens PubMed)Source [2] |
| Strength, physical performance and body composition in adults 60+ | Mixed | Grade B | Meta-analysis · n = 2,105 · 2024 Adults aged 60 and older in 30 randomized trials Form: whey protein · Dose: Varied across trials · Duration: Varied across trials Whey did not improve handgrip strength, walking tests, lean body mass or fat mass overall. With resistance training, it modestly improved lower-body strength (SMD 0.25). | 39303495 (opens PubMed)Source [7] |
| Soy versus animal protein (including whey): strength and lean mass gains | No effect | Grade B | Meta-analysis · n = 266 · 2018 Adults doing resistance training in 9 trials (5 compared whey with soy) Form: soy protein vs whey, beef, milk or dairy protein · Dose: Varied across trials · Duration: 6 weeks or longer Strength rose with both soy and whey, with no difference between them; lean mass gains also did not differ between soy and other proteins. | 29722584 (opens PubMed)Source [5] |
| Lean mass and strength in resistance-trained people (whey alone or in multi-ingredient products) | Improved | Grade C | Meta-analysis · n = 192 · 2016 Resistance-trained individuals in 9 randomized trials Form: whey protein, alone or in multi-ingredient formulas · Dose: Varied across trials · Duration: Up to 12 weeks (no trial was longer) Whey alone or in multi-ingredient products gave small extra gains in fat-free or lean mass (g = 0.301) and in upper- and lower-body strength. For whey alone, the subgroup results did not reach significance. | 26403469 (opens PubMed)Source [4] |
| Protein before sleep during a 12-week training program | Improved | Grade D | RCT · n = 44 · 2015 Healthy young men doing progressive resistance training Form: protein drink (casein-based, per ODS) vs noncaloric placebo · Dose: 27.5 g protein plus 15 g carbohydrate nightly · Duration: 12 weeks Strength gains (+164 vs +130 kg on one-rep-max testing) and quadriceps cross-sectional area gains (+8.4 vs +4.8 cm²) were larger with protein than placebo. | 25926415 (opens PubMed)Source [8] |
| Protein timed around workouts vs other times | No effect | No effect | Meta-analysis · n = 525 · 2013 Participants in 23 randomized trials (hypertrophy analysis; 478 in the strength analysis) Form: protein (types varied) · Dose: Varied across trials · Duration: Chronic training studies After adjusting for covariates, protein taken in and around training did not change strength or muscle-size gains versus control; total protein intake was the strongest predictor of hypertrophy. | 24299050 (opens PubMed)Source [6] |
Forms
| Form | Elemental % | Absorption (human data) | GI tolerance | Studied for |
|---|---|---|---|---|
| Whey protein | Varies[1] | Complete protein isolated from milk; digested faster than casein (ODS). Matched for leucine, whey and soy gave similar gains over 12 weeks.[1][9] | A systematic review linked chronic, excessive use to kidney and liver effects, acne and microbiota changes.[10] | Strength and lean mass with resistance training; older adults |
| Casein (milk protein) | Varies[1] | The main complete protein in milk; digested more slowly than whey, so amino acids reach the blood more slowly (ODS).[1] | No form-specific tolerance data in our sources.[1] | Pre-sleep protein (27.5 or 40 g in ODS-cited studies) |
| Soy protein | Varies[1] | Lacks some methionine and may lose cysteine and lysine in processing (ODS); strength and lean-mass gains matched animal proteins in long-term trials.[1][5] | No form-specific tolerance data in our sources.[1] | Head-to-head comparisons with whey |
| Rice protein and blends | Varies[1] | Rice protein lacks isoleucine; many products combine sources to supply all essential amino acids (ODS). No head-to-head human data in our sources.[1] | No form-specific tolerance data in our sources.[1] | Not studied in the trials graded here |
Studied doses
Doses studied in human trials ranged from about 19 to 40 g of protein per serving, alongside resistance training for 6 to 12 weeks or longer. In a large meta-analysis, extra gains stopped once total daily protein passed about 1.6 g/kg of body weight.[1][2][5][8][9]
Intake reference: Protein RDA: 0.8 g/kg/day for healthy adults and 0.85 g/kg/day for adolescents. ODS notes newer data suggesting athletes need 1.2–2.0 g/kg/day. No UL has been set.[1] The Food and Nutrition Board has not set a UL for protein but advises caution with high intakes from foods plus supplements because data on adverse effects are limited. NIH ODS fact sheet (updated April 1, 2024).
Interactions
Published interactions only: each row cites a source. No row means none was found in our sources, not that a combination is safe.
None found in our sources.
Who should be careful
- High intakes
- No UL exists for protein, but the Food and Nutrition Board advises caution with high intakes from foods and supplements combined. In healthy adults, diets of two to three times the RDA did not appear to harm kidney function or bone health (ODS). A systematic review linked chronic, excessive whey use to kidney and liver effects.[1][10]
- Milk-derived products
- Whey and casein are proteins isolated from milk (ODS); check the label if you avoid milk products.[1]
- Before you start
- Talk to your pharmacist or clinician before adding this supplement, especially if you take prescription medicines, are pregnant or breastfeeding, or have a chronic health condition.[1]
Talk to your pharmacist or clinician before starting a supplement, especially if you take prescription medicines.
Articles about Whey protein (and other protein powders)
Plain-English answers to common questions, built on the evidence above.
Frequently asked questions
Does protein powder build more muscle?
Added to resistance training, protein supplements gave small extra gains: about 2.5 kg more one-rep-max strength and 0.3 kg more fat-free mass in a meta-analysis of 49 trials. The extra gain was larger in trained people and smaller with age.[2]
How much protein is enough?
In that meta-analysis, extra gains in fat-free mass stopped once total daily protein passed about 1.6 g/kg. ODS cites 1.2–2.0 g/kg/day for athletes, versus the 0.8 g/kg RDA for healthy adults.[2][1]
Is whey better than plant protein?
In long-term trials, soy and animal proteins (including whey) produced similar strength and lean-mass gains. Plant proteins can be low in some essential amino acids, which is why many products blend sources.[5][9][1]
Does it matter when I have my protein shake?
A meta-analysis of 23 trials found no added effect of timing protein around workouts once total protein intake was accounted for. ODS describes a window of up to 24 hours after exercise when protein supports muscle repair.[6][1]
Does protein before bed help?
One 12-week trial in 44 young men found larger strength and muscle-size gains with 27.5 g protein nightly than with a calorie-free placebo. It is a single small trial, so the evidence is limited.[8]
Does whey help older adults?
In a 2024 meta-analysis of adults 60 and older, whey improved lower-body strength when combined with resistance training but did not change grip strength, walking tests or lean mass overall.[7]
Sources
- [1]Dietary Supplements for Exercise and Athletic Performance: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements, 2024. fact-sheet
- [2]A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults.. Br J Sports Med, 2018. meta-analysis
- [3]Protein supplementation augments the adaptive response of skeletal muscle to resistance-type exercise training: a meta-analysis.. Am J Clin Nutr, 2012. meta-analysis
- [4]Effects of Whey Protein Alone or as Part of a Multi-ingredient Formulation on Strength, Fat-Free Mass, or Lean Body Mass in Resistance-Trained Individuals: A Meta-analysis.. Sports Med, 2016. meta-analysis
- [5]No Difference Between the Effects of Supplementing With Soy Protein Versus Animal Protein on Gains in Muscle Mass and Strength in Response to Resistance Exercise.. Int J Sport Nutr Exerc Metab, 2018. meta-analysis
- [6]The effect of protein timing on muscle strength and hypertrophy: a meta-analysis.. J Int Soc Sports Nutr, 2013. meta-analysis
- [7]Effectiveness of whey protein supplementation on muscle strength and physical performance of older adults: A systematic review and meta-analysis of randomized clinical trials.. Clin Nutr, 2024. meta-analysis
- [8]Protein Ingestion before Sleep Increases Muscle Mass and Strength Gains during Prolonged Resistance-Type Exercise Training in Healthy Young Men.. J Nutr, 2015. RCT
- [9]No Significant Differences in Muscle Growth and Strength Development When Consuming Soy and Whey Protein Supplements Matched for Leucine Following a 12 Week Resistance Training Program in Men and Women: A Randomized Trial.. Int J Environ Res Public Health, 2020. RCT
- [10]Whey protein supplementation and its potentially adverse effects on health: a systematic review.. Appl Physiol Nutr Metab, 2021. systematic-review
What changed
- First draft. Every evidence-row PMID checked with NCBI E-utilities (esummary + efetch abstract) and re-read in an independent adversarial pass.