Relationship between creatine and creatinine

• creatine is a nitrogenous acid that is synthesised in the liver or ingested as dietary protein
  • creatine is converted to creatinine
    • creatine is transported to skeletal muscle and stored in the form of creatine phosphate, where it restores ATP after muscle contraction, before being converted to creatinine by non-enzymatic dehydration
    • creatinine is mainly filtered in the glomerulus, with a small proportion excreted by active tubular secretion. Almost no tubular reabsorption occurs, so the final creatinine levels in the blood and urine represent, in most situations, the amount filtered at the glomerulus, although the relation is not linear
      
      
• creatine and effects on the kidney
  • after increased intake, muscle concentrations of creatine and creatine phosphate remain raised for several weeks
    • there is a small increase in body weight, due to increased muscle mass and fluid retention within the muscle. There is no evidence that creatine supplementation results in more serious harm, including renal impairment, but no long term studies exist (1)
    • as creatine is converted to creatinine in proportion to its concentration (and hence its intake), a high creatine intake may lead to high serum creatinine and a reduced estimated GFR (as calculated from serum creatinine) despite normal kidney function. A similar effect will occur with a high intake of protein supplements or a meat meal
    • the impact of changes in muscle mass on serum creatinine concentration is well recognised
      • as muscle mass represents roughly 98% of total body creatine, people with substantial muscle will have naturally higher serum creatinine levels than those without
        • thus a creatinine concentration of 100 µmol/l may represent entirely normal renal function in a healthy 90 kg young man, or impaired renal function in a frail 50 kg woman.

Reference:

• 1) Willis J et al. Protein and creatine supplements and misdiagnosis of kidney disease. BMJ. 2010 Jan 8;340:b5027.
• 2) van der Meijden WA, Smak Gregoor PJ. Impaired renal function: be aware of exogenous factors. Ned Tijdschr Geneeskd. 2013;157(24):A5944.