Category: transport
Explains the 1-3 kg scale jump and the persistent 'pumped' feel that most lifters notice in week 1 of creatine loading or in week 2-3 of a maintenance-dose protocol — the most observable user-felt effect of the most-used legal ergogenic. Creatine enters skeletal muscle via SLC6A8 (CRT1), a Na+/Cl⁻-coupled cotransporter, and is phosphorylated in situ by creatine kinase to phosphocreatine. Hultman 1996 showed that loading (20 g/day × 5-7 days) raises intramuscular total creatine by ~20% in responders; lower maintenance doses (3-5 g/day × 3-4 weeks) reach the same plateau more slowly. Creatine + phosphocreatine are osmotically active solutes; their accumulation inside the myocyte pulls water across the sarcolemma — Powers 2003 demonstrated this directly with total body water + ICW/ECW measurement, showing that creatine raises total body water but does not alter the ICW:ECW ratio (i.e., water tracks the intracellular osmotic load rather than expanding ECW). This intracellular volume expansion is the 'fullness': individual myocytes swell ~3-5% and the muscle bellies look and feel rounder/firmer; the scale registers it as ~1-3 kg added body mass within the loading week — fluid, not fat or contractile protein. The effect saturates at the loading plateau; further intake is excreted in urine as creatinine (raising serum Cr without indicating renal impairment — a common lab-test confounder). Kreider 2017 ISSN position stand is the authoritative safety/efficacy summary; Volek 1999 covers the body-composition + training adaptation arc. Non-responders (~20-30%) have lower baseline transporter capacity / already-high muscle creatine and show smaller fullness effects.