Deficiencies: Deficiencies of
protein or of one or more of the indispensable amino acids will lead to
reduced growth in children or loss of muscle mass in adults.
Susceptibility to disease will be increased, and in extreme cases, death
may result. Protein-energy malnutrition is often described in its two
extreme forms: marasmus which is a generalized wasting due to a
deficiency of both protein and energy, and kwashiorkor which is
characterized by edema and a deficiency of both protein quantity and
quality (i.e., poor pattern of indispensable amino acids).
Food sources: Virtually all
unprocessed foods contain protein, and the protein of these foods is
characterized by the quantity and ratio of indispensable amino acids (protein
quality) contained therein. Animal proteins (meat, milk, eggs) are
considered to be excellent in both protein quantity and quality. Plant
proteins vary in protein quantity and quality. Members of the bean
family: soybeans, kidney beans, navy beans, and lima beans are rich in
protein, and the protein quality is quite good as well. Split peas and
lentils fall into the same category. Potatoes are rich in protein
quality but not in quantity. Breads, cereals, fruits and most leaf and
stem vegetables are not considered good sources of high-quality protein.
Combining certain foods together (e.g. beans with corn) results in a
complementary effect in which the protein quality of the combination is
superior to either food consumed alone.
Diet recommendations: Recommended
dietary allowances attempt to make allowances for protein quality
differences in foods typically consumed by the US population. Surveys
have indicated that animal-source proteins contribute 65% of the daily
protein ingested by humans in the US. Protein requirements increase with
advancing age up to 50 years (63 g/day for males, 50 g/day for females)
and then plateau. Children of both sexes between 7 and 10 years of age
are estimated to require 28 g/day. Additional protein is needed for
pregnancy (+10 g/day) and lactation (+15 g/day). Dietary surveys
indicate that adult Americans consume more-than-adequate amounts of
protein (men, 90 g; women 70 g).
For further information:
Fuller, M. F. & Garlich, P. J. (1994) Human amino
acid requirements: can the controversy be resolved. In: Annual Review of
Nutrition (Olson, R. E., Bier, D. M. & McCormick, D. B., eds.), vol. 14,
pp. 217-241. Annual Reviews, Inc. Palo Alto, CA.
Emmert, J. L. & Baker, D. H. (1995) Protein quality
assessment of soy products. Nutr. Res. 15: 1647-1656.
