Creatine

Creatine is a endogenous amino acid derivative produced by vertebrate animals and occurring primarily in muscle cells, according to the National Institutes of Health.

Basically it's an acid that your body can produce, by way of your kidneys, liver, and pancreas after you eat protein. Animal proteins, particularly red meat and fish, contain creatine naturally, but you'd need to eat almost inhuman amounts of each to hit the level available in most creatine supplements.

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