Beware of your average over the counter sports supplement. Most are very sketchy. For instance, your average protein powder has less than half of the stated protein content. The rest of the powder comes from a variety of fillers used (intentionally) to offset the deficit. These fillers can vary, but sugary substances such as corn starch are not uncommon (despite a low carb count on the label).
When buying supplements be sure to buy pharmaceutical grade supplements that are independently tested to insure that you are getting what you are supposed to be getting.
For advice on specific products feel free to contact me using the e-mail link.