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- What is protein?
- Classification of protein
- Sources of protein
- Complete and incomplete protein
- Protein quality
- Why is protein important?
- Protein for vegetarians
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 in a chain formation. Amino acids are what protein is broken down into, in the digestive system of the body. Amino acids are the building blocks of the body - most parts of the body require protein 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).
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
- 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). There are 6 conditionally essential amino acids
The main source of protein for non-vegetarians is meat and meat products:
Non-meat sources of protein are:
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.
Protein quality is deemed to be either:
- high quality protein, or
- low quality protein
The factors that determine whether a protein source is either high or low quality are:
- 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)
Meat eaters generally eat high quality protein foods, while vegetarians (and especially vegans) need to combine specific foods together in order to get a meal that has high quality protein - while not impossible, it does take extra work and some vegetarians may not be aware of protein quality being a factor in their dietary requirements intake.
The Committee on Dietary Intakes (USA) has developed a way to measure the protein quality in a food. This is determine by measuring the amounts of each amino acid in the food and comparing it against each of the amino acid requirements of pre-school age children.
A food high in protein is considered to be a complete protein if it contains all the essential amino acids in high amounts. Foods high in protein, but which do not have adequate amounts of the essential amino acids are considered incomplete protein.
Most meat / fish / eggs and other meat by-products are complete proteins, so most people will get adequate protein and essential amino acids they require, just from eating regular foods. Vegetarians, on the other hand, have a little more work to do in order to get all the protein and essential amino acids they require, as most legumes, beans, nuts, peas are incomplete proteins.
In order to make a complete protein, foods that are incomplete proteins need to be combined and eaten together. This is the way that vegetarians, especially strict vegetarians (vegans), normally get their daily protein requirements.
Protein is important because it is broken down into the various amino acids, which are required by the body to produce various essential proteins necessary for the body to function properly. Some of the vital proteins required in the body are: neurotransmitters, blood clotting factors and enzymes. Without protein in the diet, of which the body can absorb the amino acids, and synthesise various proteins, the body would simply not function properly and ill health would ensue.
Vegetarians need to ensure they get enough high quality protein in their diet from the food they consume.
Soy is the only plant food that is a high quality protein and it may sometimes be difficult (but not impossible) for some strict vegetarians (vegans) to get adequate protein from their dietary intake.
Lacto-ovo and lacto vegetarians (who consume either eggs and dairy or just dairy) have less difficulty in obtaining high quality protein because both milk and eggs in particular are very good high quality protein foods.
Vegetarians (or those who want to adopt a vegetarian diet) would do well to seek the assistance of a dietician to ensure they are consuming adequate amounts of all nutrients and specifically of high quality protein (to ensure they are consuming enough essential amino acids).
- McGuire M, Beerman KA, Nutritional Sciences: From Fundamentals to Food, 2007 Thomson Wadsworth USA
- Marieb EM, Hoehn K. Human Anatomy & Physiology. 7th edition, 2006. Benjamin Cummings Publishing
- Rolfes SR, Pinna K, Whitney E, Understanding Normal and Clinical Nutrition 7th Edition, 2006 Thomson Wadsworth USA
To learn more, go to the official USDA's Dietary Guidelines web site