Protein is the basic structural material of all cells. Biologically active proteins include enzymes, immunoglobulins, hormones, neurotransmitters, nutrient transport and storage compounds, and cell membrane receptors. Plasma proteins (e.g., albumin) contribute to oncotic pressure that directs the flow of fluid and metabolic waste from the intracellular compartment into the capillary venules. These proteins (e.g., hemoglobin) also contribute to plasma buffering capacity and oxygen-carbon dioxide transport (e.g., hemoglobin, myoglobin). Acute phase reactant proteins (e.g., ferritin, prealbumin) secreted by the liver bind minerals such as iron and zinc rendering them unavailable to support microbial proliferation.
Biological Value
Biological value of a dietary protein is determined by the amount and proportion of essential amino acids it provides. If any one of the essential amino acids is not available in sufficient amounts or is present in excessive amounts relative to other essential amino acids, protein synthesis will not be supported. Under these circumstances, labile body proteins such as plasma albumin will be catabolized to provide the limiting amino acid so that protein synthesis may continue.
Protein from animal sources (meat, fish, dairy products, egg white) is considered high biological value protein or a "complete" protein because all nine essential amino acids are present in these proteins. An exception to this rule is collagen-derived gelatin which is lacking in tryptophan.
Plant sources of protein (grains, legumes, nuts, and seeds) generally do not contain sufficient amounts of one or more of the essential amino acids. Thus protein synthesis can occur only to the extent that the limiting amino acids are available. These proteins are considered to have intermediate biological value or to be partially complete because, although consumed alone they do not meet the requirements for essential amino acids, they can be combined to provide amounts and proportions of essential amino acids equivalent to high biological proteins from animal sources.
Plants that are entirely lacking in essential amino acids are considered incomplete proteins or sources of low biological value protein. These sources include most fruits and vegetables. A low biological value 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.
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