here i will cite the article.

this is from the December 2006 issue, page 58.

"You may want to avoid supplements that contain L-Arginine. Syracuse University researcher's recently found that the amino acid deminishes the normal post exercise release of growth hormones, which signal your body to build muscle and burn fat. When men in the study popped an L-Arginine capsule before pumping iron, their growth hormone production fell 50% compared to when they hit the weights without the pill."