The Creatine Illusion: When Lean Mass Isn’t New Muscle

A DXA scanner never sees muscle. It divides the body into fat, bone mineral and everything else, and reports the remainder as lean mass: contractile protein, but also water, glycogen, organs and gut contents. Anything that changes how much water muscle holds can move that number without any new tissue being built.

Creatine is the obvious test case. A 2025 randomized trial gave untrained adults creatine for a week before they started lifting ^[1]. The creatine group came out about half a kilogram ahead on lean mass before anyone had trained; over the next 12 weeks of training, the two groups gained similar amounts on average.

That is not proof that creatine’s lean-mass benefit is water. It is a reason to keep lean mass, muscle, body weight and performance apart.

The question behind the trial

Meta-analyses agree that creatine plus resistance training adds lean mass beyond training alone: 1.14 kg (95% CI 0.69 to 1.59) across 12 studies in adults under 50, in a 2024 analysis by largely the same research group ^[2], and 1.10 kg (95% CI 0.56 to 1.65) in the resistance-training subgroup of a 2022 meta-analysis of 35 randomized trials ^[3].

Pooled estimates cannot say when the extra mass appears. In a conventional trial, supplement and training start together, so any quick, supplement-only change is credited to the combination. The 2025 authors note that none of the trials in the 2022 meta-analysis ^[3] used a wash-in period, and propose that this timing may partly inflate pooled gains ^[1]: a hypothesis, not an established finding. Their question: does 5 g of creatine a day change DXA lean mass before training starts, and does it add anything once training begins?

How the study was run

The single-blind randomized trial (outcome assessors and analysts were masked; participants were not) was part of a larger, prospectively registered study of the gut microbiome and training-induced muscle growth ^{[1, 4]}. Participants were healthy, untrained adults aged 18 to 50 with a BMI of 30 or below and no resistance training in the previous year. Sixty-three were included in the wash-in analysis (34 women, 29 men; mean age 31): 30 on creatine and 33 controls ^[1].

The creatine group took 5 g of creatine monohydrate a day for all 13 weeks, with no loading phase and no supervision of intake. Controls took nothing, not even a placebo. Both groups were asked to avoid exercise for the first seven days, then completed the same supervised program: three full-body sessions a week for 12 weeks, four sets of 6–12 repetitions per exercise ^[1].

Whole-body and regional lean and fat mass were measured by DXA (GE Lunar iDXA) at baseline, after the no-exercise week and after training: early in the morning, after an 8-hour fast, with no exercise for 12 hours and the bladder emptied. Test–retest reliability was excellent (ICC 0.996), with a standard error of measurement of 0.59 kg for lean mass. Water intake and hydration were neither controlled nor recorded, and neither body water nor muscle creatine was measured ^[1].

Each phase was analyzed separately, adjusted for lean mass at its start. The comparison by sex was post hoc, and there was no advance power calculation for the creatine question. Fifty-four people completed training (26 on creatine, 28 controls) ^[1].

What the scans showed

After the no-exercise week, the creatine group had gained about 0.5 kg more lean mass than the controls (0.51 vs. 0 kg; p = 0.03), mostly in the trunk (0.32 vs. −0.10 kg; p = 0.01) ^[1]. Arm and leg lean mass, and fat mass, showed no significant differences. Body weight rose by 0.40 kg on creatine and was unchanged on average in controls; no test of that difference was reported.

In the reported means, most of the 0.74 kg whole-study gap had already opened during the no-exercise week ^[1].

A post hoc split found the first-week difference (0.55 vs. −0.03 kg; p = 0.04) and a significant whole-study advantage (2.60 vs. 1.40 kg; p = 0.01) in women, but not in men ^[1]. With roughly 12 to 18 people per sex in each arm (the paper’s withdrawal figures do not fully reconcile), no tracking of the menstrual cycle, and a 2022 meta-analysis that found the opposite pattern ^[3], this is a lead, not a finding to act on.

The authors conclude that creatine’s apparent extra benefit was likely driven by short-term effects of supplementation on the lean-mass measurement, and offer trunk fluid or stomach contents as plausible explanations ^[1]. Neither was measured. Their suggestion that more than 5 g a day may be needed to add to training gains is untested; only one dose was studied ^[1].

How far the result goes

Causation, and what it covers

Random allocation lets the first-week difference be attributed to being assigned creatine. Without a placebo, that may include expectancy effects or behavior that came with taking the powder, such as drinking more water. Randomization also says nothing about what the extra half-kilogram was ^[1].

An effect inside the error band

The first-week difference (0.51 kg) is smaller than the scanner’s standard error of measurement (0.59 kg). The authors acknowledge this and argue that the error applies to both groups alike ^[1]. That is fair for a group comparison, but one small trial at p = 0.03 is suggestive, not definitive.

What p = 0.71 does not mean

The training-phase result is not evidence that creatine added nothing once lifting began. The paper reports no confidence interval, but working back from its reported difference (0.13 kg) and p-value gives a 95% interval of roughly −0.6 to +0.8 kg (our approximate calculation; because the paper’s p-value comes from an adjusted model, the true interval may be wider) ^[1]. That is compatible with no added benefit, and with a training-phase gain of 0.8 kg or more: a large share of the roughly 1 kg pooled estimates, which also include any pre-training shift ^{[2, 3]}.

What the study could not see

The trial did not image muscle, so it cannot show that the extra lean mass was not muscle, only that it appeared before training. It reported no strength data, although strength testing is a registered secondary outcome of the parent trial ^[4]. Participants were untrained, so the results may not transfer to experienced lifters, and the dose was 5 g a day without loading. The International Society of Sports Nutrition (ISSN) notes that lower daily doses raise muscle creatine gradually, over about 28 days at 3 g a day ^[7], so stores were probably only partly raised after one week. Some figures differ between the abstract, flow diagram and tables (63 vs. 64 randomized, for example). The study had no external funding; the creatine came through a supplement company’s sponsorship of the corresponding author ^[1].

The wider evidence

Lean mass: a consistent signal

A GRADE-assessed meta-analysis of 143 randomized trials found that creatine increased body mass by 0.86 kg (95% CI 0.76 to 0.96) and fat-free mass by 0.82 kg (95% CI 0.57 to 1.06; significant only with resistance or combined training), both with high certainty ^[8]. Its authors note that some of the gain may be water, which too few trials measured to tell apart. Without exercise, the 2022 meta-analysis found no significant effect (0.03 kg), but its 95% CI of −0.65 to 0.70 kg ^[3] includes the 2025 wash-in result.

Muscle: smaller and less certain

Restricted to direct imaging, the effect shrinks. Across 10 studies using ultrasound or peripheral quantitative CT, the pooled standardized mean difference was 0.11 (95% credible interval −0.02 to 0.25), which the authors describe as a very small effect favoring creatine; in young adults it was 0.23 (0.01 to 0.44) ^[5]. They suggest the gap with whole-body lean mass may partly reflect extracellular fluid, or growth in tissues the imaging did not cover, such as bone. Unimaged muscle they consider an unlikely explanation, given similar effects across the muscles studied, though trunk muscles have yet to be examined ^[5]. The fair summary is neither “builds no muscle” nor “proven to build muscle.”

Water: real, and clearest early

In 32 resistance-trained men and women, a week of loading at 25 g a day followed by 5 g a day raised muscle creatine, body mass and total body water, measured by dilution methods, with no significant shift in its split between the inside and outside of cells; total body water also rose on placebo, by less ^[9]. Without any training, 10 days of creatine increased fat-free mass, total body water and body weight in 12 young men ^[10]. The ISSN describes short-term fluid retention of about 0.5–1.0 L with loading ^[7]. Most of this evidence involves loading, not 5 g a day.

Acute and long-term effects differ. A 2021 review says water retention appears to be the most common adverse effect in the first several days, when both extracellular and intracellular water have been seen to rise. Findings over longer periods are mixed: several 5–10-week training studies found no rise in total body water, and in one eight-week study in which intracellular water did rise, its ratio to muscle mass held steady ^[11]. A small body of stable-isotope research has not found that short-term creatine raises muscle protein synthesis ^[12], which makes new contractile tissue a less likely explanation for a first-week jump (an inference, not a measurement).

Performance: a separate question

The 2025 trial reported no performance outcomes. A 2024 meta-analysis of 23 studies in adults under 50 found that creatine plus resistance training increased upper-body strength by 4.43 kg and lower-body strength by 11.35 kg more than placebo; the gains were significant in men but not in women, the subject of only two studies ^[13]. After loading, the ISSN reports that high-intensity or repetitive exercise performance generally improves by 10–20%, depending on how much muscle phosphocreatine rises ^[7].

What to do with this

The practical lesson is about timing. A scan taken before creatine and one taken in its first weeks measure muscle holding different amounts of creatine, and possibly water. Treat that change as a new baseline rather than as evidence of new muscle. Without loading, muscle stores rise more slowly ^[7], so the shift may stretch over weeks rather than days; the right wash-in length is still unstudied ^[1].

Standardize scan conditions: the 2025 protocol is a sensible starting point, ideally adding consistent fluid intake, which that trial did not control ^[1]. None of this is a reason to drop creatine: its strength benefits are well supported, at least in men ^{[7, 13]}, its effect on imaged muscle is small but probably positive ^[5], and its water effect is clearest in the first days ^[11]. People with a medical condition should check with a clinician before starting.

The illusion in the title is a narrow one. Creatine’s effects are real; what can mislead is the precision read into a single body-composition number early in supplementation. Whether its remaining lean-mass advantage is muscle, and how much, is still open; trials with body-water and direct muscle measures can answer that.