4.2 Proteins

Like carbohydrates and fats, proteins contain carbon, hydrogen and oxygen, but they all also contain nitrogen. Nitrogen is a key additional element in proteins to the extent that intake of nitrogen is often equated with protein intake.

Proteins exist in thousands of different varieties, so there is little point in trying to list them all here! We find protein in plant and animal sources including eggs, animal meat and plant seeds. Wheat gluten, wheat gliadin, maize gluten, zein (from maize) are all individual types of plant proteins. Egg albumin is an example of an individual animal protein and myosin is an important protein of muscle meat, e.g. found in steak or chicken fillets.

The important nutritional point about proteins is that they are all comprised of building blocks called amino acids. There are twenty-two types of amino acid:

• Alanine
• Arginine
• Asparagine
• Aspartic Acid
• Cysteine
• Cystine
• Glutamic Acid
• Glutamine
• Glycine
• Histidine
• Hydroxyproline
• Isoleucine
• Leucine
• Lysine
• Methionine
• Phenylalanine
• Proline
• Serine
• Threonine
• Tyrosine
• Tryptophan
• Valine

For the purposes of this foundation course, you do not need to remember all these names. Simply consider amino acids as important building blocks with each one joining together in a specific order thereby forming chains that fold to form protein structure. These chains, which you can think of as similar to beads on a necklace, can be very long indeed, and in some cases the chains themselves may have to aggregate together to form the complete protein. A typical protein may be made up from between 500 and 5000 of these amino acids joining together in sequence; although there are many proteins both smaller and larger than this range.

Animal proteins, and some vegetarian sources including quinoa, contain a mix of all 22 types of amino acids, repeated many times in the chain to make up what we term complete protein. Other plant sources of protein may contain just a few of the 22 amino acids, and are termed incomplete protein.

Protein is broken down via the action of stomach acid and certain digestive enzymes (proteases) in the small intestines to its constituent amino acids. These are then absorbed through the digestive tract to be used to manufacture our own human proteins that make up the body tissues and enzymes. Human proteins are usually quite distinct from the proteins of other species and from the proteins in food due to their own characteristic arrangements, or sequences, of amino acids within the previously described amino acid chains.

Of the 22 amino acids, 8 are essential in our diet, i.e. there is no way we can make them for ourselves in the body. They are known as the essential amino acids and include:

• Lysine
• Leucine
• Isoleucine
• Phenylalanine
• Methionine
• Valine
• Threonine
• Tryptophan

A few other amino acids are termed conditionally essential meaning that there are circumstances in which the amino acid is essential in the diet and other circumstances in which it is not. For example, cysteine can be made from methionine if there is an excess of the latter in the body, but otherwise cysteine becomes essential. We can however inter-convert cysteine and cystine. Tyrosine can be made in the body if there is plenty of spare phenylalanine, but not otherwise. The adult body also has only limited ability to make arginine and histidine but is essential in the young.

All the other amino acids can be made within the body provided there is some spare nitrogen to make them from. These are termed non-essential amino acids. In practice, the main nitrogen source is other amino acids,  those amino acids which are non-essential can be produced provided there is a surplus of other amino acids, either essential or inessential, from which to make them. This is achieved by biochemical processes to break down the surplus amino acids, splitting off the nitrogen and then re-incorporating this nitrogen into the amino acids that the body needs.

Since we must have a protein supply with which to first build and then to repair and replace the tissue proteins of our own body, it follows that a certain level of protein in the diet is essential. Some authorities put this from as low as 35g per day up to 65g per day (g = gramme). As we shall see, the requirements of individuals vary depending upon the quality of the protein they eat and how well they digest it, but probably in average circumstances, about 45g per day is enough for many adults. Body weight and gender is also a factor since a larger body requires more amino acids for maintenance and repair.

The best quality proteins are those in which the percentage of each of the essential amino acids approaches most closely to the percentages in our own bodies. This quality factor is termed the protein’s biological value. This is an established measure of the efficiency of any particular protein in supporting the growth of young animals that have requirements similar to human ones.

When proteins are available in amino acid ratios closely approaching the balance in our own bodies, then they can be used with maximum efficiency for the synthesis of our own tissue proteins. From this point of view, egg protein shows the most favourable balance, or biological value, of all. Most animal-derived proteins also have a very favourable biological value.

Plant proteins have very variable biological values with many plant proteins relatively lacking in one or more of the essential amino acids. They are therefore often referred to as second class (or incomplete) protein. However, this effect can be minimised by mixing different plant protein sources together that are known to contain a mix of all of the 22 amino acids. Examples of good complementary proteins, in terms of amino acids, are soya and sesame proteins, which complement each other well. These two can be combined to give a mixture having 90% of the value of egg protein and as good as that of beef. Maize protein and bean protein is another combination, which is better than either separately.

Some dieticians and practitioners think that the more protein there is in the diet the better. However, this is not realistically the case as it takes a lot of the body’s energy to digest protein therefore reducing the available energy for other purposes, like detoxification and ATP producing cellular processes. Excess protein in the body is converted by the liver into urea, ready for excretion by the kidney. The synthesis of urea also takes up energy, so the excessive intake of protein makes many energy demands upon the body, not only for digestion but also for elimination of the waste products of its breakdown. The left over carbon and hydrogen components from urea production can be utilised by the cells for energy or else converted to energy stores in the form of either glycogen or fat.

Hence we believe that the correct amount of protein is enough to supply all the amino acid requirements for that individual, and no more. Excess protein may overwhelm a weakened digestive system, allowing undigested protein fragments to pass through the intestines. Protein digestion yields a greater diversity of end products than carbohydrates or fat, including short chain fatty acids (SCFAs), amines, phenols, indoles, thiols, carbon dioxide (CO₂), hydrogen (H₂) and hydrogen sulphide (H₂S), many of which have toxic properties if not eliminated efficiently. We will cover the effects of incomplete protein digestion on gut health in Module 7.

Please read the following reference by clicking on the link below:

Yao CK, Muir JG, Gibson PR. insights into colonic protein fermentation, its modulation and potential health implications. Aliment Pharmacol Ther. 2016 Jan;43(2):181-96. Full paper