ZACH Insights / Creatine

A small molecule. A fast reserve of energy.

Muscle can only hold enough ATP for a few seconds of hard work. Creatine, bound to phosphate, refills it almost instantly. This article shows how that reserve works, how much of it comes from food, what supplements change in the muscle and what the trials show.

By Zacharias Razvi · Updated 14 November 2026 · 11 min read

Why muscle needs a reserve

ATP is spent faster than it can be made.

Every muscle contraction is paid for with ATP, adenosine triphosphate. When a myosin head pulls on actin, it splits one ATP into ADP and phosphate. The muscle holds only a small amount of ATP, so it has to be rebuilt as fast as it is used.

Mitochondria rebuild ATP from fat and carbohydrate, but they need oxygen and they are slow to speed up. Glycolysis, the breakdown of glucose without oxygen, is faster. The fastest of all is a single chemical step in which phosphocreatine hands its phosphate to ADP. Creatine and phosphocreatine work as a buffer for ATP and as a shuttle that moves phosphate from the mitochondria to the sites where it is used. 1

General education. The article explains physiology and research. It is not individual advice on supplements.

Model 1 · The phosphate buffer

One reaction keeps ATP topped up.

Follow the phosphate from the mitochondrion to the contracting filaments and back. Select a step.

Schematic by ZACH based on Brosnan and Brosnan (2007). Molecules are drawn as symbols and are not to scale. The moving dots show the direction of the reactions, not their speed.

  1. A loop between two places.

    The filaments spend ATP (1). Creatine kinase rebuilds it from phosphocreatine on the spot (2). Creatine travels back to the mitochondrion, where it is loaded with a new phosphate (3). When work is too hard for the mitochondria to keep up, phosphocreatine runs down (4).

  2. The filaments spend ATP.

    Each cycle of a myosin head splits one ATP into ADP and phosphate. The energy released moves the filament. ADP has to be turned back into ATP before the next cycle.

  3. Phosphocreatine gives its phosphate.

    The enzyme creatine kinase moves the phosphate from phosphocreatine to ADP. The result is ATP and free creatine. The reaction is a single step, which is why it is so fast. 1

  4. Creatine goes back to be reloaded.

    Next to the mitochondria, a second form of creatine kinase runs the reaction the other way. ATP from the mitochondria loads creatine with phosphate again. Phosphocreatine then carries the phosphate back to the filaments. This is the shuttle described by Brosnan and Brosnan. 1

  5. Hard work empties the reserve.

    In an all-out effort the filaments use ATP faster than the mitochondria can reload creatine. Phosphocreatine falls, and power output falls with it. The next section shows how fast that happens.

Measured in muscle

Six seconds can use half the reserve.

Who took part
8 men in a study by Gaitanos and colleagues. 2
What was done
Ten 6-second all-out sprints on a bike with 30 seconds of rest between them. Small samples of thigh muscle were taken before and after the first and the tenth sprint.
What they found
In the first sprint, phosphocreatine fell by 57%. Phosphocreatine and glycolysis supplied about equal shares of the energy. In the tenth sprint, glycolysis added almost nothing, yet power was still 73% of the first sprint. The authors suggested that phosphocreatine and aerobic metabolism carried most of the work.
What it cannot say
How the pattern looks in women or in longer efforts. Eight participants is a small group.
57% fall in phosphocreatine during the first 6-second sprint
73% of the first sprint's power left in the tenth sprint
30 seconds of rest between sprints

Where creatine comes from

Part is made in the body, part comes from meat and fish.

Creatine breaks down by itself into creatinine, which leaves the body in urine, so the store has to be replaced every day. The body makes some of it from the amino acids glycine, arginine and methionine. The rest comes from food, mostly meat and fish. Vegetarians get almost none from their diet. 1

The amounts in food are small compared with the doses used in studies. Harris and colleagues worked out that 5 g of creatine, the dose they gave several times a day, matches the creatine in 1.1 kg of fresh, uncooked steak. 3

Model 2 · Creatine on the plate

How much steak is a study dose?

Move the amount of raw steak and compare it with the doses used in the studies on this page.

Worked example by ZACH using the ratio given by Harris et al. (1992): 5 g of creatine per 1.1 kg of fresh, uncooked steak, about 4.5 g per kg. Cooking and other foods are not included. The marks are the doses used in the studies cited here, not recommendations.

200 g of raw steak contains about 0.9 g of creatine. A 5 g dose matches about 1.1 kg.

Model 3 · Filling the store

Fast or slow, the store rises by about a fifth.

Choose a pattern of intake and follow the creatine content of muscle over two months.

Based on Hultman et al. (1996), 31 men. Dots mark the measured time points and the reported changes. The lines between them are drawn by ZACH to connect the points and do not show measured day-by-day values.

  1. 20 g a day for 6 days.

    Muscle total creatine rose by about 20% after 6 days. Most of the uptake happened early. In an earlier study, uptake was greatest in the first 2 days, and a large part of each dose left in the urine. 3 4

  2. Then 2 g a day.

    When intake continued at 2 g a day for another 30 days, the higher level stayed. 4

  3. Or nothing.

    Without the 2 g a day, the level drifted down and was back at the starting value 30 days after the last dose. 4

  4. 3 g a day from the start.

    A daily 3 g gave the same rise of about 20%, but it took 28 days to get there. The authors concluded that 3 g a day is likely to work as well as loading in the long run. 4

Variation between people

Some muscles barely take up more.

The average rise hides wide differences. In an early study of supplementation, the rise was largest in people who started with a low creatine content, and in some it reached 50%. 3

Who took part
8 people in a study by Greenhaff and colleagues. 5
What was compared
The same thigh muscle before and after 5 days of 20 g creatine a day. Phosphocreatine was measured during recovery from intense contractions produced with electrical stimulation.
What they found
In 5 of 8 people, total creatine rose by 15–32% and phosphocreatine was rebuilt faster after the effort. In the other 3, it rose by only 5–7% and recovery did not change.
What it cannot say
How common low responders are in general. With 8 people, the split of 5 and 3 is not a reliable proportion.

Data · Responders and low responders

Rise in muscle total creatine after 5 days of loading.

Data: Greenhaff et al., American Journal of Physiology 1994. The bars show the reported range in each group. Individual values were not reported in the abstract.

Five people: 15 to 32 percent. Three people: 5 to 7 percent.

Performance and muscle

The effect shows up with training.

A meta-analysis of 53 randomised trials found that creatine improved upper-body strength in exercises lasting less than 3 minutes, such as the bench press, compared with placebo. 6 For lean body mass, which includes muscle and water, a later meta-analysis pooled 35 trials.

Who took part
1,192 people without a specific disease in 35 randomised trials, pooled by Delpino and colleagues. 7
What was compared
Creatine against placebo, with and without a training programme.
What they found
On average, creatine added 0.68 kg of lean body mass. With resistance training it added 1.10 kg, whatever the age. Without training there was no clear effect.
What it cannot say
How much of the gain is muscle protein and how much is water, because lean body mass includes both. The trials were also different in dose, length and method.

Data · Lean body mass with creatine

Difference against placebo, in kilograms.

Data: Delpino et al., Nutrition 2022. Mean difference with 95% confidence interval. An interval that crosses zero is compatible with no effect.

All trials 0.68 kg (0.26 to 1.11). Resistance training 1.10 (0.56 to 1.65). No exercise 0.03 (−0.65 to 0.70). Men 1.46 (0.47 to 2.46). Women 0.29 (−0.43 to 1.01).

The split between men and women should be read with care. A wide interval that crosses zero means uncertainty, not proof that there is no effect. 7 In older adults, a meta-analysis of 8 trials with 482 participants found small added gains in leg strength and lean mass when creatine was combined with resistance training. 8

Safety

Well studied, with a few things to know.

The International Society of Sports Nutrition reviewed the research in 2017. It concluded that short- and long-term use, up to 30 g a day for 5 years, has been safe and well tolerated in healthy people and in several patient groups. 9 A review from 2021 by researchers from the same society went through common concerns, among them water retention, kidney function, hair loss, dehydration and cramping. It described creatine as relatively well tolerated at the usual doses of 3–5 g a day or 0.1 g per kg of body mass. 10

Two practical points follow from the physiology. Creatine breaks down to creatinine, the substance doctors measure in blood to estimate kidney function, so a test result can be affected and the doctor should know about the supplement. And people with kidney disease, or who take medicines that affect the kidneys, should talk to their doctor before using it.

What the evidence supports

Creatine raises the store in muscle by about a fifth on average and, combined with resistance training, adds strength and lean mass in many people.

What it does not support

Effects without training, the same response in everyone, or that a diet without supplements leaves muscle unable to work normally.

What this article cannot tell you

The muscle measurements come from small groups, mostly young men, and the meta-analyses pool trials that differ in dose and length. None of it can say how one person's muscle will respond, or whether a supplement suits someone with a medical condition. That is a question for a doctor.

Sources

Follow the evidence.

  1. Brosnan JT, Brosnan ME. Creatine: endogenous metabolite, dietary, and therapeutic supplement. Annual Review of Nutrition. 2007;27:241–261. PubMed record Supports: the buffer and shuttle functions, continuous loss to creatinine, synthesis from glycine, arginine and methionine, and the low intake of vegetarians.
  2. Gaitanos GC, Williams C, Boobis LH, Brooks S. Human muscle metabolism during intermittent maximal exercise. Journal of Applied Physiology. 1993;75(2):712–719. PubMed record Supports: the 57% fall in phosphocreatine in a 6-second sprint, the equal share with glycolysis and the 73% power in the tenth sprint.
  3. Harris RC, Söderlund K, Hultman E. Elevation of creatine in resting and exercised muscle of normal subjects by creatine supplementation. Clinical Science. 1992;83(3):367–374. PubMed record Supports: 5 g of creatine as the content of 1.1 kg of raw steak, the largest rise in people with low starting values, rises up to 50%, and the greatest uptake in the first 2 days.
  4. Hultman E, Söderlund K, Timmons JA, Cederblad G, Greenhaff PL. Muscle creatine loading in men. Journal of Applied Physiology. 1996;81(1):232–237. PubMed record Supports: the 20% rise with 20 g a day for 6 days, maintenance with 2 g a day, the fall after stopping, and the same rise with 3 g a day over 28 days.
  5. Greenhaff PL, Bodin K, Söderlund K, Hultman E. Effect of oral creatine supplementation on skeletal muscle phosphocreatine resynthesis. American Journal of Physiology. 1994;266(5 Pt 1):E725–E730. PubMed record Supports: the split into 5 responders and 3 low responders and the faster phosphocreatine recovery in responders.
  6. Lanhers C, Pereira B, Naughton G, et al. Creatine supplementation and upper limb strength performance: a systematic review and meta-analysis. Sports Medicine. 2017;47(1):163–173. PubMed record Supports: 53 trials and improved upper-limb strength in exercises under 3 minutes.
  7. Delpino FM, Figueiredo LM, Forbes SC, Candow DG, Santos HO. Influence of age, sex, and type of exercise on the efficacy of creatine supplementation on lean body mass: a systematic review and meta-analysis of randomized clinical trials. Nutrition. 2022;103–104:111791. PubMed record Supports: 35 trials, 1,192 participants and all the lean body mass estimates in the data figure.
  8. Liu S, Huang N, Wu W, et al. The impact of creatine supplementation associated with resistance training on muscular strength and lean tissue mass in the aged: a systematic review and meta-analysis. European Review of Aging and Physical Activity. 2025;22(1):26. PubMed record Supports: 8 trials, 482 older adults and small added gains in leg strength and lean mass.
  9. Kreider RB, Kalman DS, Antonio J, et al. International Society of Sports Nutrition position stand: safety and efficacy of creatine supplementation in exercise, sport, and medicine. Journal of the International Society of Sports Nutrition. 2017;14:18. PubMed record Supports: safety of up to 30 g a day for 5 years in healthy people and several patient groups.
  10. Antonio J, Candow DG, Forbes SC, et al. Common questions and misconceptions about creatine supplementation: what does the scientific evidence really show? Journal of the International Society of Sports Nutrition. 2021;18(1):13. PubMed record Supports: the usual doses of 3–5 g a day and the review of common concerns.

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Where to go from here.

Health Map See energy and nutrition among the connected systems. Thirty topics on one interactive map, with nutrition, strength and recovery alongside each other. Next article Three muscles, one tendon. How the Achilles tendon is built, how it stiffens with load and what trials show. Newsletter One mechanism at a time. New explanations by email when they are published.

All Insights · Muscle · How to read evidence

General education, not a diagnosis or an individual treatment plan. Models are schematic unless a data source is stated beside the figure. Zacharias Razvi is a final-semester physiotherapy student and is not yet an authorised physiotherapist.