Plant Protein
Plant sources of protein (grains, legumes, nuts, and seeds) are
generally deficient in one or more essential amino acids.
The exception is the soybean; all essential amino acids are
present.
Protein synthesis occurs only to
the extent that the limiting
amino acids are available.
If they are relatively low, but not lacking in a limiting amino
acid, they can meet the requirements for essential amino acids if ingested in large quantities.
Different plant proteins can also be combined to provide
amounts and proportions of essential amino acids equivalent to high biological proteins from animal sources. This is discussed in more detail below.
Such proteins are considered to
have intermediate biological
value or to be
partially complete
.
Plants that are entirely lacking in one or more essential amino
acids are considered of low biological value or to be
incomplete
.
These sources include most fruits and many vegetables.
A low biological value also means that it is difficult or
impossible to compensate for insufficient amounts of essential amino acids by combining different sources as with partially complete proteins.
Examples of high-, intermediate-, and low-quality proteins are
listed in
Table 8
.
High/Complete
Intermediate/Partially Complete
Low/Incomplete
Milk, cow
Soy
Peas
Eggs, chicken
Sunflower seeds
Cornmeal
Milk, human
Rice
Flour, white wheat
Beef
Potatoes
Cassava
Fish
Oats
Gelatin
Peanut
butter
Beans
Table 8. Protein quality. Examples of high-, intermediate-, and low-quality proteins.
Combining Complementary Proteins
Vegetarians and others often de
pend upon incomplete proteins
such as nuts, grains, and legumes.
By eating a balanced diet of grains and legumes with each meal
or over the course of each day incomplete proteins can be combined to provide all amino acid requirements.
For example:
-^
The amino acids from beans taken at one meal can complement the amino acids from rice eaten at another meal that day.
-^
Macaroni complements cheese. The essential amino acids from the cheese allow the body to better use the amino acids in the macaroni.
-^
Peanut butter complements wheat bread.
Amino Acid Score
The amino acid balance concept:
-^
Protein synthesis is limited unless all amino acids are present in appropriate amounts.
-^
Protein synthesis is limited by the essential amino acid present in the least amount in relation to the amount required.
Nutrition for Sports, Essentials of 59