QUOTE : A Protein Is Not a Protein Companies are touting protein as a cure-all and for weight loss/muscle gain. They’re selling protein shakes, protein cookies, protein snack bars, even protein coffee. It’s true that protein is neither carbohydrate nor sugar nor fat, and you need it to maintain normal growth. However, your kidneys have a limited capacity to excrete the metabolic by-products of protein metabolism, and overexcretion can cause kidney damage. Therefore, protein quality is as important as protein quantity. For example, eggs and beans both contain protein, but are very different in quality. Dietary protein is made up of twenty separate amino acids strung together in different combinations and amounts. One of those amino acids, tryptophan, is rarer and therefore more important than others, because it’s the precursor of serotonin, an important brain neurotransmitter (see Chapter 19). Eggs, poultry, and fish are the best sources of this amino acid, while beans have very little. On the other hand, additional protein is needed if you’re building muscle, especially branched-chain amino acids (BCAAs; leucine, isoleucine, valine), which are 20 percent of muscle (see Chapter 18). BCAAs are in high concentration in corn products, and are what’s in those tubs of protein powder at the health food store. If you’re a bodybuilder, you need them; if you’re not a gym rat and consume excess BCAAs, your liver will take the amino groups off and turn them into organic acids, which will either be diverted into liver fat (through DNL) or into excess glucose, either of which can generate hyperinsulinemia and drive chronic disease. The goal is to get more tryptophan and less BCAAs in the protein you consume.