What is Protein?
Protein is one of the three macronutrients required to be eaten in relatively large amounts each day (the other two are carbohydrates and fats). Protein is basically a whole lot of amino acids joined together. The digestive system breaks protein down in to amino acids. Amino acids are the building blocks of the body - most parts of the body require protein from the amino acids in order to function properly, and not just the muscles.
Protein serves as a source of energy for the body and to make up various structural components of the body (such as muscle, bones, fingernails, hair and skin).
Protein also has important functional roles in the body - the body uses the protein from foods to create the multitude of protein required in the body. Some of the actions of protein in the body are: to activate enzymes, move skeletal muscles, transportation of various important substance through cell membranes, activate communication of various hormones, provide blood clotting, as well as the regulation of fluid balance and pH.
Protein makes up at least 50% of the human body (that is not made up of water).
Essential and Non-Essential Amino Acids
Protein contains various amino acids, which are required to maintain life. There are 20 amino acids that are found in protein. The amino acids come in three forms:
Essential amino acids - these amino acids cannot be produced by the body (either at all or not enough to be effective) and must be consumed through the diet. There are 9 essential amino acids: - Histidine
- Isoleucine
- Leucine
- Lysine
- Methionine
- Phenylalanine
- Threonine
- Trytophan
- Valine
Non-essential amino acids - these amino acids are able to be synthesised from the essential amino acids (and also from glucose) so are not necessary to be consumed through the diet. There are 11 non-essential amino acids Conditionally essential amino acids - these amino acids are normally non-essential ones, but they become conditional essential in special circumstances (babies / infants cannot make their own taurine, phyenylketonuria makes the amino acid phenylalanine essential for the individual with this disease as they cannot synthesise it, plus there are other times when some non-essential amino acids may become essential).
Sources of protein
Animal sources of protein usually have higher levels of some essential amino acids than plant sources of amino acids - except for soy (a type of legume), which contains all the essential amino acids. Other legumes (such as dried beans, lentils, peas) and peanuts also have balanced levels of the essential amino acids and are considered a good source of protein, especially for vegetarians.
Fruits, vegetables and grains do not contain a great deal of protein in comparison to meat sources of protein.
Protein quality
Protein quality is deemed to be either:- high quality protein, or
- low quality protein
Protein quality is determined to be high or low quality due to:- Digestibility (or bioavailability) - how easily a protein source is digested and absorbed in the gastrointestinal system is one factor
- Essential amino acids - a food source that contains all the essential amino acids in plentiful amounts and at a similar level, is another factor (complete proteins have all essential amino acids in the correct combination and levels, while incomplete proteins do not)
Complete and incomplete protein
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