WJEC Level 3 Certificate in Food Science and Nutrition

59 The ‘free’ amino acids are then absorbed into the bloodstream and are taken to the liver, where they are reorganised into new protein molecules that are needed by the body – see diagram. Most amino acids can be made in the body by reorganising those that are absorbed during digestion, and are called non- essential amino acids . Some amino acids cannot be made in the body and must be supplied ready-made by food. They are called essential amino acids . There are also two amino acids that are called conditionally essential . In adults, these two amino acids can be made in sufficient amounts to meet their needs. However, because children and adolescents are growing rapidly, they cannot make enough of them in their body, so they must get these amino acids from food at that particular stage of their lives. The chart below lists the essential, non-essential and conditionally essential amino acids: Essential amino acids Non-essential amino acids Isoleucine Leucine Lysine Methionine Phenylalanine Threonine Tryptophan Valine Alanine Asparagine Aspartic acid Cysteine Glutamic acid Glutamine Glycine Proline Serine Tyrosine Conditionally essential amino acids (sometimes called semi-essential) Arginine Histidine Biological value of proteins Proteins are found in a wide variety of animal and plant foods. Depending on the number of essential amino acids they contain, protein foods are classified as having either high biological value (HBV) if they contain all the essential amino acids, or low biological value (LBV) if they are missing one or more essential amino acids. These names can give the impression that HBV proteins are more important for the body than LBV proteins, but this is not the case. In a mixed diet, where a variety of either just plant foods or plant and animal foods are eaten, all the essential amino acids (and the non- and conditionally essential amino acids) will be provided. The missing essential amino acids in one protein food will be provided by other protein foods. This is called the complementary action of proteins (sometimes called protein complementation). Sources of proteins AC 2.2 The chart on the following page shows examples of some foods that are sources of proteins. Free amino acids travel from small intestine to liver in the bloodstream Food enters stomach Protein broken down into free amino acids in stomach and small intestine (ileum) Liver Food Conditionally essential amino acid – amino acids that must be obtained ready-made from food for a particular stage of life, e.g. the growing stages of childhood and adolescence Essential amino acid – amino acids that cannot be made in the body and must be supplied ready-made by food Non-essential amino acid – amino acids that can be made in the body Key terms The chemical structure of the macronutrients