You've seen the ads in health magazines—the ones proclaiming that you can't get enough essential amino acids in your diet, that you need polar bear-size portions of meats and fish to get the protein you need to replenish your body after exercise. You may even have heard terms like arginine and leucine being bandied about the gym by the buff set. Should we all be supplementing our diet with amino acid pills, powders, and potions? Not according to Jackie Berning, R.D., a professor and registered dietitian at the University of Colorado, who believes that people are pissing away…