Does creatine make you gain weight? Yes, about 1 to 2% of body mass

Creatine does make you gain weight, and the size of it is known: a review of the creatine literature in athletes put the increase at 1.0% to 2.3% of body mass, which for someone weighing 80 kg works out at roughly 0.8 to 1.8 kg [8]. That review attributed the gain to fat-free mass, meaning everything in the body that is not fat, and more specifically to skeletal muscle [8].

Whether that is water or muscle is the part the evidence has not settled. Two reviews conclude it is most likely water pulled into the muscle, with some new muscle protein possible [11][9]. The review reporting the 1.0% to 2.3% range argues water retention is unlikely to account for all of it, while also noting that a rise in muscle protein synthesis, the process of building new muscle protein, has never actually been measured after creatine supplementation [8].

How quickly the scale moves depends on how much is taken. Eighteen men given 0.3 grams of creatine per kilogram of body weight a day for seven days, the kind of short high intake trials use as a loading phase, gained body weight over that week [3]. Twenty young men and women given roughly a tenth of that amount for six weeks showed no measurable change in body mass, fat-free mass, fat mass or total body water, which is the total amount of water held in the body [7]. Their muscles still became 7% to 11% more resistant to fatigue [7].

Training changes the picture again. A pooled analysis of 357 adults averaging about 64 years old, doing roughly 12 weeks of resistance training with weights or machines, found that adding creatine increased total body mass and fat-free mass more than training alone did, with no effect on fat mass [5]. Not everyone gains, either: muscle creatine rises most in people whose starting levels were lowest, such as vegetarians, and in some people it does not rise at all [9].

In women the figure is smaller and much less certain. A pooled analysis of 29 trials that tracked side effects in 951 female participants found women taking creatine carried a little over a kilogram more body mass than the comparison groups, but the spread across those studies was wide enough to include no difference at all [4]. A review of creatine in women's health reports positive effects on body composition mainly where creatine was combined with resistance training [13].

Creatine is not a foreign substance. About 2 grams of it turn over in the body each day, and that is met roughly half by food and half by what the liver, kidneys and pancreas make themselves [9]. Most of the body's supply sits in skeletal muscle, where around 40% is free creatine and 60% is creatine phosphate, a storage form that donates a phosphate group to rebuild the body's immediate energy molecules during short, hard effort [9].

Taking more raises the amount held inside muscle cells, and water follows it in. Dissolved substances pull water towards themselves, so a muscle cell holding more creatine also holds more water, which is why reviews describe the early part of the gain as water retention inside muscle rather than added fat [11].

Beyond the water, the mechanism is not established. The gain maps onto skeletal muscle rather than fat, and water alone is thought unlikely to explain all of it, but the building of new muscle protein has never been observed to speed up after creatine, and the evidence for it is indirect, taken from measurements of gene activity [8].

The evidence sits mostly in men who train, and the measurement is coarser than the numbers suggest.

• One systematic search identified 656 creatine trials, of which 58 - about 9% - studied women only, and the pooled body-mass figure from the female trials was wide enough to include no difference at all [4].
• Nothing here covers pregnancy or perimenopause, the years leading up to the end of monthly periods; a review of creatine in women's health describes the perimenopausal data as limited [13].
• The 1.0% to 2.3% range summarises a varied literature in athletes, not one trial measured one way [8].
• The trials are short. The older-adult analysis averaged about 12 weeks of training [5] and the low-intake trial ran six weeks [7], so what body mass does over years is largely unmeasured.
• Reviewers of this literature state that creatine should not be used by people with existing kidney disease, and that the risk should be assessed before and during any supplementation [8].

None of this makes creatine a way to gain or lose weight deliberately. The trials measured body mass as an outcome of supplementation, and a number that moves in the first week is not a measure of fat gained [5][8].

Someone who starts creatine at a loading-sized intake and weighs themselves a week later will probably see the number go up: that is what the seven-day trial at 0.3 grams per kilogram found [3], and the reviewed range puts it at roughly one to two kilograms for a typical adult [8]. Someone taking a small daily amount instead may see nothing move for six weeks and still get the fatigue resistance the trials measured [7]. The people for whom that kilogram has practical consequences are weight-class athletes: the International Society of Sports Nutrition's position stand on combat sports treats body mass as something fighters manage deliberately against a division limit, and lists creatine among the supplements used during preparation phases [1].