Protein, made up of amino acids, is essential for growth and repair of all human tissues. Not all amino acids are equal, however! Well that's not completely true, but in the first of a series of posts about proteins and amino acids, I explain why some amino acids are more essential in the diet than others. Tomorrow Rachael will look at Branched Chain Amino Acids, a subgroup of essential amino acids.
What are proteins, peptides and amino acids?
Proteins are organic compounds that form a vital component of every living cell. Proteins protect and provide structure to the body in the form of skin, cartilage, tendons, muscles, callus and hair; regulate catalyze and protect body chemistry in the form of enzymes, antibodies, hormones and globulins, and enable the transport of oxygen and other chemicals in the form of haemoglobin, myoglobin and lipoproteins. Consuming protein regularly is necessary for growth and repair of body cells and tissues, general health, and regulation of homeostasis. Excess protein is used as a source of energy, or is converted to urea and excreted from the body.
Amino acids are the compounds from which peptides, and proteins are constructed. Each amino acid consists of amine and carboxylic acid groups, along with a specific side-chain which determines which defines the amino acid. Amino acids contain nitrogen, oxygen, carbon and hydrogen, though some amino acids contain other elements in their side-chains. A proteins properties is determined by the amino acids that it contains, and the sequence of amino acids within its structure. Amino acids are crucial in both their role as protein building blocks, and as intermediates in metabolism.
What are essential amino acids?
Of the 22 biologically available amino acids, nine must be consumed in the diet. This does not mean that they are more important human tissues than the other 11, but unlike the non-essential amino acids, they cannot be made from other amino acids, or produced from other substances within the body. Argining, for example, is crucial in the formation of bone, among other things, but is not essential as it can be produced from citrulline Leucine, important in muscle synthesis, and isoleucine, important in both muscle and haemoglobin synthesis and blood sugar regulation, on the other hand are essential amino acids and must be consumed in the diet.
Where can I find essential amino acids?
As a rule, animal proteins are better sources of essential amino acids than vegetable proteins, though soy is a notably useful plant source of essential amino acids. Eggs and milk are the particularly good, and both contain all nine essential amino acids, along with a number of non-essential and semi-essential ones. Vegetarians and those who can't consume diary products or eggs should ensure that they obtain protein from a variety of sources, including soy.
|Eggs are an excellent source of essential amino acids|
Combining certain vegetable proteins can ensure that essential amino acids needs are met. Indigenous staples in a number of developing countries cleverly achieve this in the absence of animal foods. Examples include Mexican corn and beans, Cajun red beans and rice, and Japanese soybeans and rice.
The following are the essential amino acids and some of their top food sources*:
Histidine - eggs, soy protein, sesame, peanuts, parmesan
Isoleucine - eggs, soy protein, parmesan, pork, whitefish
Leucine - eggs, soy protein, sesame seeds, parmesan, whitefish
Lysine - eggs, soy protein, parmesan, whitefish
Methionine - eggs, soy protein, sesame seeds, whitefish
Cysteine - eggs, soy protein, sesame seeds, peanuts, mustard seeds
Phenylalanine - eggs, soy protein, sesame seeds, peanuts, whitefish
Tyrosine - eggs, soy protein, sesame seeds, parmesan
Threonine - eggs, soy protein, sesame seeds, whitefish
Tryptophan - eggs, soy protein, sesame seeds, chia seeds
Valine - eggs, soy protein, sesame seeds, parmesan, beef
*Note the these are the top food based on available protein per 100 grammes, but eating 100 g of sesame seeds or parmesan cheese is a lot more difficult than drinking 100 ml of milk.
How often should protein be consumed?
The body is constantly growing and repairing it tissues, and has a constant demand for amino acids. Small amounts of protein should be consumed with each meal (this doesn't need to be a steak or chicken breast). The best way to ensure that your essential amino acid needs are always being meet, is to ensure that you eat an egg or drink a glass of milk daily, or that you consume protein from a variety of sources. Different foods will have different minerals and vitamins, so eating foods from a variety of sources has additional benefits.
- Eggs or milk should form part of your diet
- Protein should be consumed from a variety of sources
Useful references and resources
- Young VR, Pellett PL (1994). Plant proteins in relation to human protein and amino acid nutrition. The American Journal of clinical Nutrition 59: 1203S-1212S.