Protein powder: what it adds to weight training, where the benefit levels off and what it does not do

Protein powder was the subject of a Guardian question-and-answer piece on October 12, in which four dietitians took on what the paper called its most pressing protein questions. One of them was whether a person can have too much. The Guardian also noted that US regulators don’t verify marketing claims promising improved muscle growth or recovery.

Those claims have been tested, though, many times over. The largest pooling of the trials found that protein supplements do add muscle and strength in people who lift weights. The addition is small next to what the lifting does, it stops at a certain daily intake, and it was close to nothing in older adults at the doses they were given. This explainer covers what protein powder is, what the trials show, where the benefit levels off and what is still unsettled.

What protein powder is

Protein powder is protein that has been extracted from a food and dried. Whey, a popular kind, comes from milk. Others are made from soy, peas or casein, another milk protein. The powders are sold as dietary supplements, which matters for how they are regulated, as described below.

People take them mainly alongside resistance exercise training (RET), which means working muscles against a load such as weights, machines or bands. The idea is that muscle is built from protein, so more protein might mean more muscle. The 2017 analysis at the center of this article describes the pairing as a common practice, in athletes and recreational exercisers alike.

The powders are a convenient way to raise protein intake, and the trials measured what happened when total intake went up.

What the trials show protein powder adds to training

The analysis appeared in the British Journal of Sports Medicine in 2017. It is a meta-analysis, which means it pools the results of many separate trials to get a steadier estimate than any one could give. The team searched for randomized trials in which healthy adults did resistance training and some were given extra protein.

The entry rules were specific. Trials had to be at least six weeks in duration, participants had to be performing RET at least twice per week, and the supplement could not be mixed with other muscle-building agents such as creatine. In every trial, the comparator group received an identical RET intervention but was non-supplemented or placebo-supplemented. A placebo is a look-alike product without the active ingredient.

A total of 49 studies from 17 countries met the inclusion criteria. Together they had 1,863 participants with an average age of 35. The training programs ran from 6 to 52 weeks and averaged 13.

The team looked at two main outcomes. One was the one-repetition maximum, the heaviest weight a person can lift once. The other was fat-free mass (FFM), which is body weight minus fat and is the usual stand-in for muscle.

Training by itself did most of the work. Without any supplement, participants raised their one-repetition maximum by an average of 27 kilograms (kg) and gained 1.1 kg of fat-free mass.

The supplement added to both. Extra protein raised the strength gain by 2.49 kg, with a range from 0.64 to 4.33 kg. It raised the gain in fat-free mass by 0.30 kg, with a range from 0.09 to 0.52 kg. Put as shares, that is about 9% more strength and 27% more lean mass than training gave alone. Fat mass fell by 0.41 kg more with the supplement, and body weight did not change.

The authors drew the comparison themselves. They wrote that the practice of RET is a far more potent stimulus for increasing muscle strength than the addition of dietary protein supplementation.

An earlier and smaller pooling by a different group had pointed the same way. Based on 22 trials with 680 participants, it concluded that protein supplementation increases muscle mass and strength gains during prolonged resistance-type exercise training in both younger and older subjects.

A later one, covering 74 trials, came from a team that shared a senior author with the 2017 paper. Its summary used the same scale of language. Increasing daily protein ingestion results in small additional gains in LBM and lower body muscle strength gains in healthy adults enrolled in resistance exercise training. LBM is lean body mass, a close relative of fat-free mass.

Where the benefit of protein powder levels off

The 2017 team asked a second question: is there a daily intake above which more protein adds nothing? To answer it they plotted the lean mass gained in each trial group against that group’s total protein intake from food and supplements.

The gains rose with intake and then went flat. In the paper’s words, protein supplementation beyond total protein intakes of 1.62 g/kg/day resulted in no further RET-induced gains in FFM. The unit is grams of protein per kilogram of body weight per day. For a person weighing 70 kg, 1.62 works out to about 113 grams of protein a day from all sources.

The figure is softer than it looks. Its range ran from 1.03 to 2.20, and the paper reports that a line that rises and then goes flat explained the data better than a straight one but did not pass the usual statistical test, so the bend could be a product of chance. Because of the wide range, the authors suggested that people seeking the largest possible gains might aim nearer 2.2.

Two details put the number in context. The US and Canadian recommended dietary allowance for adults is 0.8 grams per kilogram a day. And the people in these trials were already eating well above that before any supplement. Their average intake at the start was about 1.4 grams per kilogram, and the average supplement added about 35 grams of protein a day.

Who gains more and less from protein powder

Two things changed how much the supplement added to lean mass.

The first was age. When the trials were split at 45, younger participants gained an extra 0.55 kg of fat-free mass with the supplement, with a range from 0.30 to 0.81 kg. Older participants gained an extra 0.06 kg, with a range from a loss of 0.14 to a gain of 0.26 kg, which is no clear effect.

The paper offers a reason that has less to do with age than with dose. The average supplemental daily protein dose given to older participants was surprisingly low, at about 20 grams against 42 grams in the younger groups. The older participants also ate less protein to begin with.

A 2022 review of reviews looked only at older adults and found a split result. Drawing on five meta-analyses, it reported that protein combined with training brought greater increases in lean mass than training alone, but no differences were observed between the interventions on muscle strength increases. The authors graded the evidence cautiously. The quality of evidence ranged from moderate to very low.

The second factor was training history. People who already lifted regularly got more from the supplement, about 0.75 kg of extra fat-free mass. The 2017 paper’s conclusion puts both factors in one line. Protein supplementation is more effective at improving FFM in young or resistance-trained individuals than in older or untrained individuals.

On sex, the paper could say little. It saw no obvious difference between men and women, on limited data, and acknowledged that far less work has been done in women than men.

Timing and type of protein powder

Much of the marketing of protein powder concerns details: whey against plant protein, and a shake straight after training. The pooled trials give these little support.

In the 2017 analysis, the size of the dose taken after exercise made no difference to lean mass gains. The authors concluded that the specifics of protein supplementation (eg, timing, postexercise protein dose or protein source) play a minor, if any, role in determining RET-induced gains in FFM and strength over a period of weeks.

An earlier meta-analysis had looked at timing alone. Three researchers, all later co-authors of the 2017 paper, pooled trials that compared protein taken around a workout with protein taken at other times. Once total protein intake was taken into account, timing had no effect. These results refute the commonly held belief that the timing of protein intake in and around a training session is critical to muscular adaptations, they wrote.

On type, a 2025 analysis compared whey and soy in adults aged 18 to 30. A total of 12 studies with 261 participants were included. Neither protein clearly increased lean mass in that age group, in a pool far smaller than the 1,863 participants of the 2017 analysis. Whey was linked to better bench press and squat results, in a small set of trials.

The Guardian’s dietitians made a related point about labels. Kacie Vavrek, a dietitian at the Ohio State University’s Wexner medical center, said organic protein powders don’t necessarily provide more protein or better muscle-building benefits than non-organic whey.

Protein powder safety: how much is too much

The Guardian’s headline question was whether a person can get too much. Nancy Rodriguez, a professor of nutritional sciences at the University of Connecticut, answered with an example. A person taking five scoops of protein powder a day, Rodriguez said, is “not going to assimilate all of those amino acids in your body and live happily ever after.” Amino acids are the building blocks of protein. The surplus, Rodriguez explained, has to be cleared by the liver and kidneys.

That raises the question of whether high intakes strain healthy kidneys. A 2018 meta-analysis, from a group that overlaps with the 2017 authors, pooled 28 trials with 1,358 participants who ate higher-protein or lower-protein diets. It measured glomerular filtration rate (GFR), which is how fast the kidneys filter blood. The change in GFR did not differ between interventions. Only trials in adults without kidney disease were included, so the result says nothing about people whose kidneys are already impaired. The authors also flagged a weakness in their material. The main limitation of the current analysis is the unclear risk of selection bias of the included trials.

A separate concern is contamination. The Guardian noted that the US Food and Drug Administration doesn’t test the safety of protein powders, including whether they contain heavy metals such as lead. Heavy metals are elements such as lead, cadmium, arsenic and mercury that can be toxic.

Independent testing has found them. A 2020 paper in Toxicology Reports recounts that the Clean Label Project tested 133 protein powder supplements, and reported measurable lead in 70% and measurable cadmium in 74%. The paper’s authors then asked whether those amounts were enough to matter, assuming one or three servings a day. Their answer was no. They concluded that the typical intake of dietary supplements would not result in adverse health effects due to heavy metals. The highest estimated exposures were in powders sold for mass gain and the lowest in whey. The assessment rested on test results published by others, and for most products it had no brand names or serving sizes.

Certification schemes exist that test products independently. Nyree Dardarian, an assistant clinical professor at Drexel University, was measured about what they offer. “Third-party testing is a good sign, but it doesn’t make a product bulletproof,” Dardarian said.

What the protein powder evidence cannot show

Powder without training. Every trial in the main analysis involved resistance training at least twice a week. It does not show what a supplement does for a person who is not training.

Long-term results. The trials averaged 13 weeks. Whether a small edge at three months grows, holds or fades over years was not measured.

Older adults. Only 13 of the 49 trials enrolled people over 45, and most gave low doses. The weak result in that group may reflect the doses as much as age.

The exact threshold. The 1.6 figure has a range from 1.0 to 2.2, and the analysis behind it did not pass the usual statistical test.

Weight loss. Only trials in people who were healthy and not energy-restricted were accepted. Energy-restricted means dieting. The analysis does not cover people trying to lose weight while keeping muscle.

Who did the work. A correction published in 2020 added a declaration for one of the 2017 paper’s eleven authors. It states that Brad Schoenfeld served on the advisory board for Dymatize Nutrition, a manufacturer of sports supplements, at the time this paper was being written. The correction did not change any result. Several of the supporting analyses cited here share authors with the 2017 paper.

Whether a protein supplement makes sense for a particular person, including someone with kidney disease, is assessed by their doctor.

Protein powder, on this evidence, is a modest addition to weight training whose effect levels off, as best the trials can tell, at a total protein intake about twice the standard daily allowance.