The essential amino acids are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Another amino acid, histidine, is considered semiessential because the body does not always require dietary sources for it. The nonessential amino acids are arginine, alanine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, proline, serine, and tyrosine. Other amino acids, such as carnitine, are made from the combination of other amino acids, in this case lysine and methionine.

Where are they found? Foods of animal origin, such as meat, fish, poultry, eggs, and dairy products, are the richest dietary sources of the essential amino acids. However, the outdated belief that vegetarians need to be concerned about combining certain foods to obtain enough essential amino acids has now been disproved by research¹ and is almost universally rejected by scientists. Part of the reason that vegetarians do not need to “balance” amino acids is that the body’s requirement for essential amino acids now appears to be much less important than researchers once believed, especially in adults. In fact, research indicates that protein deficiencies rarely occur in people who simply eat enough calories. As a result, the old scientific term for protein deficiency, “kwashiorkor,” has been dropped from use and replaced by the term “protein-calorie malnutrition.”

Amino acids have been used in connection with the following conditions (refer to the individual health concern for complete information):

Who is likely to be deficient? The vast majority of Americans eat more than enough protein and also more than enough of each essential amino acid for normal purposes. Anyone not consuming an adequate number of calories, dieters, and some strict vegetarian body builders may not consume adequate amino acids. In these cases, the body will break down the protein in muscle tissue and use those amino acids to meet the needs of more important organs or simply not build more muscle mass despite increasing exercise.

Supplements of individual amino acids are recommended by nutritionally oriented doctors for specific purposes, such as lysine for herpes or phenylalanine for pain.

Are there any side effects or interactions? Most diets provide more protein than the body needs, causing excess nitrogen to be excreted as urea in urine. The excess nitrogen has been linked in some studies with reduced kidney function in old age. Moreover, several studies have found that when people have impaired kidney function, restricting dietary intake of protein improves their health status.³

Excessive protein intake also can increase excretion of calcium, and some evidence has linked high-protein diets with osteoporosis,⁴ particularly regarding animal protein.⁵ On the other hand, some protein is needed for bone formation. Double blind evidence indicates that elderly individuals who eat barely below the recommended amount suffer less bone loss when supplemented with an additional 20 grams of protein per day.⁶ A nutritionally oriented doctor can help people assess their protein intake.

Refer to the individual amino acids for information about drug interactions.

References:

1. Young VR, Pellett PL. Plant proteins in relation to human protein and amino acid nutrition. Am J Clin Nutr 1994;59(suppl):1203S–12S.
2. Lemon P. Is increased dietary protein necessary or beneficial for individuals with a physically active life? Nutr Rev 1996;54:S169–75.
3. Sitprija V, Suvanpha R. Low protein diet and chronic renal failure in Buddhist monks. BMJ 1983;287:469–71.
4. Heaney R. Protein intake and the calcium economy. J Am Diet Assoc 1993;93(11):1259–60.
5. Abelow BJ, Folford TR, Insogna KL. Cross-cultural association between dietary animal protein and hip fracture: a hypothesis. Calcif Tiss Int 1992;50:14–18.
6. Schürch M-A, Rizzoli R, Slosman D, et al. Protein supplements increase serum insulin-like growth factor-I levels and attenuate proximal femur bone loss in patients with recent hip fracture. Ann Intern Med 1998;128:801–9.

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