What is Creatine?
Creatine is a naturally occurring compound synthesized endogenously from the amino acids glycine, arginine, and methionine in the kidneys, liver, and pancreas, and is also obtained exogenously through dietary intake of meat and fish. 1, 2
Biochemical Properties and Distribution
Creatine is predominantly stored in skeletal muscle (approximately 95% of total body stores), where it exists in two forms: free creatine (approximately 40%) and phosphorylated creatine phosphate (approximately 60%). 2, 3
Total body creatine content in a 70 kg male is approximately 120 grams, with a daily turnover rate of approximately 2 grams per day, which is met equally through dietary intake and endogenous synthesis. 2, 4
Creatine concentration is higher in fast-twitch muscle fibers compared to slow-twitch fibers, though slow-twitch fibers demonstrate greater resynthesis capability due to their increased aerobic capacity. 2
Physiological Functions
Creatine serves four primary roles during physical performance: temporal energy buffering (providing rapid ATP resynthesis), spatial energy buffering (transporting high-energy phosphates), proton buffering (reducing acidosis), and glycolysis regulation. 2
When phosphorylated to creatine phosphate, it functions as a high-energy phosphagen in the ATP-CP energy system, providing rapid energy production crucial for speed and power activities. 2, 4
Creatine prevents ATP depletion, stimulates protein synthesis or reduces protein degradation, and stabilizes biological membranes. 3
Dietary Sources and Metabolism
Dietary creatine intake comes primarily from skeletal and cardiac muscle tissue (cooked meat), with vegetarians typically having lower initial total creatine stores. 1, 2
Creatine is eliminated from the body through glomerular filtration and tubular secretion, with creatinine (the breakdown product of creatine) serving as a marker of kidney function. 1
Creatine production is directly proportional to skeletal muscle mass in stable adults who are neither anabolic nor catabolic and maintain constant protein intake. 1
Clinical Significance
Factors that acutely increase creatinine (creatine's metabolite) include dietary creatine intake from meat meals and increased creatinine generation from rhabdomyolysis. 1
Chronic factors affecting creatinine levels include increased generation in individuals with muscular body habitus and Afro-Caribbean ethnicity, while chronic reduction occurs with low dietary protein intake, reduced muscle mass (elderly, females, muscle-wasting conditions, amputation, malnutrition), and critical illness. 1
The creatinine index (creatinine synthesis rate) is used clinically to assess dietary skeletal muscle protein intake and skeletal muscle mass, and can estimate edema-free lean body mass. 1