Protein forms structural tissues and compounds throughout the body, and they are continually being broken down and formed into new proteins, assisting our bodies to repair and grow. Proteins also form our genes, enzymes and also critical messenger molecules in our bodies such as hormones and neurotransmitters. They are also needed for making white blood cells, antibodies and immune chemical messengers such as interferon (an antiviral substance produced by T lymphocytes and macrophages). For absorption and use in the body, proteins are broken down into amino acids, from which proteins are formed. Amino acids meanwhile are classed as being essential, non-essential or conditionally essential
Essential and non-essential amino acids
An amino acid is essential (needed in dietary intake) or non-essential (can be manufactured within the body) is determined by whether the body can or cannot synthesize the amino acid within the body. The issue of whether the body is given the right conditions to either manufacture or absorb amino acids (ie from food) brings the label of “essentiality” into question. Instead, conditional essentiality becomes a more functional perspective with which to view amino acids.
How a non-essential amino acid becomes “conditionally essential”
Conditional essentiality pays more attention to what is happening both outside and within the body: the conditions the body finds itself in. It is these conditions that will determine whether or not the body is either manufacturing or absorbing enough amino acids for the physiological activities they are needed for. For example, Arginine is used in wound healing, detoxification reactions, immune functions. Use of Arginine by the body could increase due to any of the above activities becoming more pronounced such as during growth phases, such as childhood and adolescence or muscular strength training. Arginine is made within the body, however, under these conditions, use by the body would outstrip supply and become conditionally essential – needed in food intake
Protein quality and bioavailability
Protein quality is determined by the number of essential amino acids present in a food. All amino acids required for forming body proteins must be present and absorbed from food in the same meal. A complete protein source is thus food that delivers the amino acids in the right qualities. Meat, fish and dairy and foods that are complete protein sources. Vegetable sources are often incomplete and thus foods must be combined (eg rice and beans) in order to become a complete protein source.
Bioavailability (or Biological Value: BV) refers to the ability of the body to absorb and make use of the protein and amino acids in a food source. Whey protein and eggs have the highest BV, and milk, fish, beef and soybeans have extremely high BV, making them the best food sources of protein.
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