How full are your muscles?

Constants used

• Muscle, steady route: 0.10 g/kg/day, floor 3 g.
• Muscle, loading route: 0.30 g/kg/day for five to seven days, split into four doses.
• Muscle, holding after a load: 0.03 g/kg/day, floor 3 g.
• Brain range, low end: 0.10 g/kg/day, never below 8 g, never above 12 g.
• Brain range, high end: 0.15 g/kg/day, never below 10 g, never above 15 g.
• Acute single dose: 0.35 g/kg, rounded to the whole gram. All daily figures round to the nearest 0.5 g.
• Baseline muscle creatine, men: 120 mmol/kg dry muscle, roughly 75 to 80% of the ceiling.
• Baseline, women: about 10% higher at rest.
• Maximum saturation: about 160 mmol/kg dry muscle.
• Daily breakdown: about 1.7% of the muscle pool per day, with a reported range of 1 to 2%.

The brain range has a floor because no study has measured a change in brain creatine below roughly 8 to 10 g/day, and a cap because above 15 g there is no additional tissue the transporter at the blood-brain barrier can reach. Brain mass barely changes with body size, so body weight enters that number as a small correction, not a scaling law. And when your muscle dose reaches the brain range, the range is withdrawn rather than faked: past that point there is no separate higher figure to give.

The saturation curve

The curve uses a first order saturation model:

S(t) = S_max − (S_max − S₀) · e^−kt

S₀ is your starting saturation, S_max is the steady state your dose can hold, and k is the uptake rate fitted to the saturation timelines in the research:

• Loading at 0.3 g/kg/day: full in about 6 days, k ≈ 0.54 per day.
• 10 g/day for a 70 kg adult: full in about 11 days, k ≈ 0.29 per day.
• 5 g/day for a 70 kg adult: full in about 25 days, k ≈ 0.13 per day.
• 3 g/day for a 70 kg adult: full in about 32 days, k ≈ 0.10 per day.

Doses in between are interpolated linearly in dose per kilogram, which mirrors how the creatine transporter behaves at typical intakes. When you pick the loading week, the curve runs on loading kinetics until the tank is full or day seven, whichever comes first, then switches to your daily dose.

The milestone squares are read off the curve. Halfway and 90% are the days on which the gap between your starting point and a full tank is half closed and 90% closed. “Full effect” is the day you cross 99% saturation, the same definition behind the day counts in the list above.

Missed dose decay

When dosing stops, the stored creatine above your baseline decays exponentially at the daily breakdown rate:

S(t) = S_base + (S_start − S_base) · e^−0.017t

That reproduces the four to six week washout reported in the literature.

Sources

• Hultman, E. et al. (1996). Muscle creatine loading in men. J Appl Physiol 81.
• Harris, R.C. et al. (1992). Elevation of creatine in resting and exercised muscle. Clin Sci 83.
• Kreider, R.B. et al. (2017). ISSN position stand: safety and efficacy of creatine supplementation. JISSN 14.
• Candow, D.G. and Forbes, S.C. (2024). Creatine for brain and bone health. Adv Exerc Health Sci.
• Smith-Ryan, A.E. et al. (2021). Creatine supplementation in women’s health. Nutrients 13.
• Forbes, S.C. et al. (2022). Effects of creatine supplementation on brain function and health. Nutrients 14.
• Roschel, H. et al. (2021). Creatine supplementation and brain health. Nutrients 13.

Built from the kinetic data summarised in the Fitness ETC internal research brief, April 2026. The full citation chain is available on request.

Your muscles can only hold so much creatine, about 160 mmol per kilogram of dry muscle. Most people walk around at 75 to 80% of that ceiling. Supplementing fills the rest of the tank.

Loading just means using a high dose for the first week to fill the tank fast, which is the “load the first week” option above. There is no magic in it. Skip it, take your daily dose from day one, and you reach exactly the same level a couple of weeks later.

Once the tank is full, a small daily dose covers the breakdown and holds it there; the figures above do that math for your weight. The benefits are the same no matter how you got there.