Nutritional value of protein

A protein’s nutritional value is measured by the quantity of essential amino acids it provides. Different foods contain different numbers and amounts of amino acids. Generally speaking:

• Animal products (such as chicken, beef or fish) contain all of the essential amino acids.
• Soy products and the seed of a leafy green called Amaranth (consumed in Asia and the Mediterranean) also contain all of the essential amino acids. However, plant proteins usually lack at least one amino acid.

People following a strict vegetarian or vegan diet need to choose a variety of protein sources from a combination of plant foods throughout the day to get an adequate mix of amino acids. For example, a meal containing cereals and legumes, such as baked beans on toast, provides all the essential amino acids found in a typical meat dish.

Digestion of proteins

A protein-rich food, such as meat, is broken down into individual proteins by the gastric juices in your stomach. Pancreatic enzymes released into the first portion of your small intestine (duodenum) split the proteins into their separate amino acids. The amino acids are absorbed by the small finger-like projections (villi) lining the intestine walls, and are taken to the liver via the bloodstream.

How amino acids are used

The human body uses amino acids in three main ways:

• Protein synthesis – new proteins are created constantly. For example, as old, dead cells are sloughed off the skin surface, new ones are pushed up to replace them.
• Precursors of other compounds – a range of substances are created using amino acids, (for example, the brain chemical (neurotransmitter) serotonin and the ‘fight or flight’ chemical adrenalin).
• Energy – although carbohydrates are the body’s preferred fuel source, about 10 per cent of energy is obtained from protein.

Ammonia – a toxic byproduct of protein

One of the byproducts of protein metabolism is ammonia. In high levels, ammonia is extremely dangerous to the body and so is converted into urea. This water-soluble chemical is collected by the kidneys and eliminated from the body in our urine. The more protein we eat each day, in excess of our needs, the more work our kidneys must do to expel ammonia.

Amount of protein needed each day

Some people, such as growing children, pregnant women and breastfeeding mothers, need slightly more protein than the recommended daily intake (RDI). However, most Australians consume more than enough dietary protein, so deficiencies are rare.

From 50 years onwards, ageing is associated with loss of skeletal muscle, a condition known as sarcopenia, which, in the elderly is worsened by chronic illness, poor diet and inactivity. It is likely that protein intake at the upper end of the RDI range can maintain muscle mass and strength, which is vital for walking ability.

It is also important for the elderly to eat protein ‘effectively’, which means to consume high-quality protein foods, such as lean meats.

Strenuous exercise doesn’t mean you need extra protein

Contrary to popular belief, people who exercise vigorously or are trying to put on muscle mass do not need to consume extra protein. Studies show that weight-trainers who do not eat extra protein (either in food or protein powders) still gain muscle at the same rate as weight-trainers who supplement their diets with protein. A very high-protein diet can strain the kidneys and liver, and prompt excessive loss of the mineral calcium.

Timing of protein consumption

Soon after exercising (either resistance or aerobic), it is recommended consuming a high-quality protein source (such as a glass of milk or tub of yoghurt) combined with a carbohydrate meal to help positive protein balance. Studies have shown this to be beneficial for maintaining protein balance even when following low to moderate aerobic exercise (such as walking), particularly for older adults.

Symptoms of protein deficiency

The human body cannot store protein, so it must be supplied on a daily basis from the foods we eat. Strict vegetarians who do not consume any animal products at all are at increased risk of protein deficiency if they do not eat a wide range of plant proteins.

Symptoms of protein deficiency include:

• Wasting and shrinkage of muscle tissue
• Oedema (build-up of fluids, particularly in the feet and ankles)
• Anaemia (the blood’s inability to deliver sufficient oxygen to the cells, usually caused by dietary deficiencies such as lack of iron)
• Slow growth (in children).

Very high protein diets are dangerous

Some weight trainers and bodybuilders believe that high protein diets lead to increased muscle mass. High protein diets promote intakes of protein of between 200 and 400 g per day, which equates to approximately 5 g/kg each day (more than five times the RDI). This belief is false. It is the stimulation of muscle tissue through exercise, not extra dietary protein, that leads to muscle growth.

The RDI for protein provides adequate protein to build and repair muscles even for body builders and athletes. Fad diets that favour very high protein and fat intake, combined with very low carbohydrate intake, may be harmful.

Some of the problems with very high protein diets (more than 35 per cent of total daily intake) include that:

• They usually promote a very low intake of carbohydrates. Glucose, made when your body breaks down dietary carbohydrate, is your body’s preferred fuel source. If your body does not receive enough dietary carbohydrate, it will break down muscle tissue to make glucose. This causes muscle wastage, reduced metabolism and a build-up of ketones.
• Fibre is largely a carbohydrate. Foods rich in carbohydrates (such as wholegrains and legumes) are also rich in fibre. Avoiding these foods leads to an overall low-fibre intake, which can result in constipation, bowel disorders and increased risk of colon cancer.
• There is evidence to suggest that the heart may not function as well if its main source of fuel is ketones.
• High intake of animal products (which is usually recommended in such diets) can also be high in saturated fats and cholesterol, which is associated with a range of conditions including heart disease.
• The liver and kidneys are put under strain because they have to detoxify and eliminate unusually high quantities of protein byproducts. Kidney problems may be exacerbated in people with diabetes.
• There is an increased risk of developing gout and gall bladder colic.
• Greater losses of body calcium may increase the risk of osteoporosis.
• High protein diets can cause mild dehydration due to increased water loss through urine. Increased risk of dehydration puts the body under pressure.
• Recent research shows that weight loss over one year is not greater on a high protein diet when compared to safer, low fat eating patterns.

Things to remember

• Proteins are made up of chains of smaller chemicals called amino acids.
• The human body cannot store amino acids, so it must be supplied daily from the foods we eat.
• Very high protein diets combined with very low carbohydrate intakes are not recommended.