It is widely recommended to consume protein with carbohydrate after a workout because it stimulates protein synthesis and enhances
. This is due in part to the anabolic hormone response created by protein and carbohydrate, most notably the increase in insulin. However, contrary to what you might think, several studies have shown that the anabolic hormone testosterone in the blood is actually decreased when food is consumed after resistance exercise. The reason for this decrease in circulating levels is not known, but it could be due to a decrease in the rate testosterone is made and/or an increase in testosterone uptake into tissues. If greater uptake in muscle is the explanation, this is actually a good response because it means more testosterone is acting on muscle to stimulate growth. In order for testosterone to be taken up, it has to bind to an androgen receptor. The androgen receptor is critical in order for testosterone to elicit its anabolic effects. No matter how much testosterone you have, if it does not bind with androgen receptors in muscle then the testosterone will not promote growth. In a recent study in our laboratory, subjects who consumed a protein and carbohydrate shake after a bout of resistance exercise experienced a greater reduction in testosterone as expected, but this was also associated with a greater increase in androgen receptor content. So the conclusion was that consuming protein and carbs after a workout had a positive effect on testosterone by increasing the amount of androgen receptors in muscle. The study also looked at the effects of carnitine supplementation
on testosterone and androgen receptors because prior work has shown carnitine may play a role in regulation of testosterone level and its biological effects. Carnitine did not result in a greater exercise-induced androgen receptor response, but it did result in greater androgen receptors at rest (before exercise). When you consider what is going into muscle, the findings indicate that carnitine helps increase androgen receptors before exercise, whereas protein and carbohydrate enhanced post-exercise androgen receptors.
Kraemer WJ, et al. Effects of L-carnitine-L-tartrate supplementation on testosterone and muscle androgen receptor content after resistance exercise. Federation of American Societies for Experimental Biology, Washington D.C., April, 2004. Abstract #851.8.