not sure about heart rate mate,it never did me any harm tho. i remember training with it and i never use to feel sore the next day like i use to, and it did increase my performance weight lifting wise, i do no a lad who use to have it and it made him a bit edgy tho, u normally have a teaspoonful in a cup of water before and after ur workout. best thing to do is try it, see how u get on with it, and go from there, i swear by it tho
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