Physical Education · Ch 5 — Sports and Nutrition
Proteins
Proteins
5.2.2 Proteins
Proteins are organic compounds that contain nitrogen, besides carbon, oxygen and hydrogen. They have a complex structure and are built from nitrogen-containing amino acids, which link together in chains to make the many different proteins of the body. From hair to nails, muscles to skin, organs to blood, and hormones to enzymes, protein is a major structural and functional component of the body.
There are around 20 amino acids that join in varying sequences to form different proteins. Nine of these cannot be synthesised by the body and must be supplied by the diet — these are the Essential Amino Acids (EAA). The rest are non-essential amino acids, because the body can make them itself.
| Essential | Nonessential |
|---|---|
| Histidine | Alanine |
| Isoleucine | Arginine |
| Leucine | Asparagine |
| Lysine | Aspartate |
| Methionine | Cysteine |
| Phenylalanine | Glutamate |
| Threonine | Glutamine |
| Tryptophan | Glycine |
| Valine | Proline |
| Serine | |
| Tyrosine |
Quality of protein foods
Depending on which essential amino acids they contain, protein foods are classified as complete, partially complete or incomplete:
- Complete protein foods contain all the essential amino acids in adequate amounts. These include animal sources such as eggs, milk and milk products, and meat and meat products, and one plant source — soybean. The presence of complete protein foods in the diet determines protein quality and improves the absorption and utilisation of protein.
- Partially complete protein foods lack any one essential amino acid — for example, cereals lack lysine and pulses lack methionine. Taking cereals and pulses together in a meal, or combining them with a complete protein food, improves the overall protein quality.
- Incomplete proteins lack more than one essential amino acid; an example is maize protein.