Researchers have developed the protein digestibility-corrected amino acid score, or PDCAAS. The PDCAAS is the main measure used by those establishing the protein values listed on food labels. This system of measurement is geared toward protein needed to support the maintenance of body tissue of adults.

All things being equal, a protein that supplies all the essential amino acids in exactly the right proportions will be most completely used. Chances are that if a diet provides enough of the essential amino acids, then it will meet all the protein needs of the body.

Digestability is also a critical element in evaluating protein sources for human consumption. Simple measures of the total protein contained in a food are not useful by themselves, since, according to those alone, even animal hair or hooves would receive a top score. As the words of its name suggest, the PDCAAS takes into account the digestibility of a protein as well as its amino acid balance. To obtain the PDCAAS, the food is first given a score based on its amino acid balance. Then the score is adjusted to account for the food's digestibility. The following table shows some selected foods and shows their PDCAAS score (the highest possible score is 100%).

* An example of mutual supplementation. Combining whole wheat and pea flours yields a protein with a higher PDCAAS than that of either product alone.

Why do people take protein or amino acid supplements? Athletes take them to build muscle. Dieters take them to spare their bodies’ protein while losing weight. People also take individual amino acids, mixtures of two or more amino acids, or products that combine amino acids with other nutrients. Some takers believe the products will cure herpes, induce restful sleep, or relieve pain or depression. Do protein and amino acid supplements really do any of these things? Almost never. Are they safe? No.

Protein supplements are less well digested than protein-rich food and they cost more than food, too. When used as a replacement for such food, they are often downright dangerous. The “liquid protein” diet, advocated some years ago for weight loss, caused deaths in many users. Even the physician-supervised, protein-sparing, modified fast, also based on liquid protein, can cause abnormal heart rhythms.

Amino acid supplements are also unnecessary. The body is designed to handle whole proteins best. It breaks them into manageable pieces (dipeptides and tripeptides), then splits these a few at a time, simultaneously absorbing them into the blood. This bit-by-bit absorption is ideal, because groups of chemically similar amino acids compete for the carriers that absorb them into the blood. An excess of one amino acid can tie up a carrier, and temporarily prevent the absorption of another similar amino acid. When carriers are tied up dealing with an overdose of some one or a few amino acids, some other needed amino acids may pass through the body unabsorbed. The result is a deficiency. The human body is meant is live without encountering highly concentrated amino acids in the unbalanced arrays found in supplements, and therefore lacks equipment with which to handle them.

Recently the FDA asked a panel of scientists for a well-known scientific research group to review the safety of amino acid supplements. When the scientists began to search the literature for well-encountered studies on the supplements, they found next to none. The panel did find evidence of adverse health effects from amino acids, however, and they therefore concluded that, without appropriate scientific research, no level of intake of these supplements could be considered safe. They also warned that any use of amino acids in dietary supplements is inappropriate, for two reasons. First, some (serine and proline) present a high risk of toxicity. Second, no amino acid supplement performs any nutrient function in the human body.

The panel also singled out some groups of people they consider to be at especially high risk of harm from amino acid supplements:

• All women of childbearing age
• Pregnant or lactating women
• Infants, children, and adolescents
• Elderly people
• People with inborn errors of metabolism that affect their bodies’ handling of amino acids
• Smokers
• People on low-protein diets
• People with chronic or acute mental or physical illnesses who take amino acids without medical supervision

Anyone considering taking amino acid supplements should check with a physician first.

Enthusiastic reports about two amino acids have led to widespread public use. One is lysine, popularly recommended to prevent or relieve the infections that cause herpes sores on the mouth or genital organs. The other is tryptophan, popularly recommended to relieve pain, depression, and insomnia. Lysine does not prevent or cure herpes infections, and if long-term use helps prevent outbreaks, it does so only in some individuals and with unknown associated risks.

Some people who elected to take tryptophan developed a blood disorder (EMS, short for eosinophilia-myalgia syndrome). EMS is characterized by severe muscle and joint pain, limb swelling, an elevated white blood cell count, extremely high fever, and, in at least 15 cases, death. Some evidence suggests that changes in procedures at a major Japanese tryptophan processing plant may have introduced contaminants that contributed to the as-yet-incurable disease. FDA has requested a recall of tryptophan supplements made by the company and of all formulas to which they were added. If you own a bottle of tryptophan or a formula containing it, throw it out. It is safer to derive your amino acids from protein-rich foods taken with a little carbohydrate to facilitate their use. A glass of milk or a turkey sandwich is a good choice.