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 However, the outdated belief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):
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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