Protein

Brain cells, muscle, skin, hair and nails are just some of the body parts that are protein-based. Estimates suggest that about half of the human body's dry weight is made up of protein. Many of the foods we eat contain protein, particularly flesh foods such as chicken, beef, lamb and fish and legumes like beans and lentils.

For protein to be used by the body it must first be broken down into its smaller components, called amino acids. Amino acids are used to repair and maintain the body. As a rough guide, the recommended daily allowance (RDA) for dietary protein of high nutritional quality such as egg or beef is 0.75 gram per kilogram of bodyweight for young healthy adults and 1gram per kilogram for older adults. For example, a 60kg person would need 45g of protein per day. However, most Australians eat far more protein than they actually need, so deficiencies are rare.

Sources of protein
Some sources of dietary protein include:
Meat, poultry and fish
Eggs
Dairy products
Seeds and nuts
Beans and lentils
soy products.
Grains, especially wheat, less so for rice, barley and corn

Amino acids explained
Proteins are made up of chains of smaller chemicals called amino acids. There are about 20 different amino acids that, in different combinations, make up the countless millions of proteins available in nature. A protein can consist of anywhere between 50 to tens of thousands of amino acids, linked together by a kind of 'glue' called peptide bonds. There are two broad classes of amino acid: those that can be made by the human body ('non-essential' amino acids) and those which can only be supplied by food ('essential' amino acids). These include:
Non-essential amino acids - alanine, arginine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine, tyrosine.
Essential amino acids - arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, trytophan, valine.
Tyrosine is not an essential amino acid because it can be formed from the essential amino acid, phenylanine. It can become "conditionally essential" if phenylalanine cannot be converted to tyrosine, as in the disease PKU or phenylketonuria.

Nutritional value explained
A protein's nutritional value is judged on how many of the essential amino acids it provides, and in what quantity. 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. Plant proteins usually lack at least one amino acid, except soy and the seed of a leafy green called Amaranth (consumed in Asia and the Mediterranean). Strict vegetarians can solve this dietary problem by eating a combination of plant foods, for example, a single meal containing cereals (lacking the amino acid lysne) and legumes (lacking the amino acid methionine) 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 gastric juices in the stomach. Pancreatic enzymes released into the first portion of the 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 passed to the liver via the bloodstream.

How amino acids are used
The human body uses amino acids in three main ways, including:
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 adrenaline.
Energy - although carbohydrates are the body's preferred fuel source, about 10 per cent of energy is derived from protein.

The toxic byproduct ammonia is excreted
One of the byproducts of protein metabolism is ammonia. In high levels, ammonia is extremely dangerous to the body, so it is converted into urea. This water soluble chemical is collected by the kidneys and eliminated from the body via urine.

How much is enough?
Some groups - such as growing children, pregnant women and breastfeeding mothers - need slightly more protein than the recommended daily allowance of one gram per kilogram of bodyweight. However, most Australians consume more than enough dietary protein, so deficiencies are rare.

Contrary to popular belief, people who exercise vigorously or are trying to put on muscle mass don't need to consume extra protein. Studies show that weight-trainers who don't 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.
For elderly people it is likely that the upper end of the recommended dietary intake range is desirable, about 1g/kg body weight, to maintain muscle mass and strength - a crucial component of walking ability.

Symptoms of protein deficiency
The human body can't store protein, so it must be supplied on a daily basis from the foods we eat. Strict vegetarians who don't consume any animal products at all are at increased risk of protein deficiency if they don't eat a wide range of complementary 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).

High protein diets are dangerous
Some weight-trainers and bodybuilders believe that high protein diets lead to increased muscle mass. This isn't true - it is stimulation of muscle tissue through exercise, not extra dietary protein, that leads to muscle growth. Fad diets, such as the Atkin's diet, that favour very high-protein (>35% energy) and fat and very low-carbohydrate (25% energy) intake may be are harmful. The drawbacks of very high protein diets include:
Very High protein diets usually advocate very low intake of carbohydrates. Glucose, made when the body breaks down (metabolises) dietary carbohydrate, is the body's preferred fuel source. If the body doesn't 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 (by-products of protein metabolism).
Low fibre intakes can result in consitpation, 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.
A high intake of animal products are usually recommended which can be high in saturated fats and cholesterol, 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.
Increased risk of developing gout and gall bladder colic
Increased losses of body calcium may increased risk of osteoporosis
Increased risk of dehydration
Moderate-high protein diets

Moderate-High protein diets
Moderate-High protein diets have recently been shown by the CSIRO in Adelaide to be a safe and effective option to conventional high carbohydrate weight loss diets in 100 overweight women.
The moderate-high protein diet contained moderate amounts of carbohydrate (46%energy) and protein (34% protein) and low amount of fat (20% energy).
Sample diet: 35g wholegrain breakfast cereal, 250ml low fat milk, 2 slices wholegrain bread, 2 fruits, 100g chicken/fish/meat for lunch, 200g beef/lamb for dinner, 2.5 cups vegetables, 200g diet yoghurt, 3 teaspoons canola oil; 2 glasses wine a week (optional).

The high carbohydrate diet was high in carbohydrate (63% energy), low in fat (20%) and protein (17%). Sample diet: 35g wholegrain breakfast cereal, 250ml low fat milk, 3 slices wholegrain bread, 3 fruits, 80g chicken/fish/meat for dinner, 3 low fat biscuits, 120g cooked pasta/rice, 2.5 cups vegetables, 200g diet yoghurt, 3 teaspoons canola oil, 2 glasses wine a week (optional).

Over 12 weeks the women on the high protein diet lost 8kg weight (especially from the abdomen) versus 6kg on the high carbohydrate diet. Both diets lowered blood lipids and insulin/glucose levels but women with high blood triglycerides lowered these to a greater extent (28% lower) on the high protein diet compared with the high carbohydrate diet (10% lower). The drop out was three times greater on the high carbohydrate diet. The researchers conclude high protein diets are a safe and effective weight loss program for those people showing symptoms of metabolic syndrome (increased blood fats and glucose) and that there does not appear to be any adverse effect on kidney function and bone.

Where to get help
Your doctor
Dietitian
Dietitians Association of Australia www.daa.asn.au Tel. (02) 6282 9555

Things to remember
The human body must first break down dietary protein into its smaller components, called amino acids, before protein can be used.
Proteins are made up of chains of smaller chemicals called amino acids.
The human body can't store protein, so it must be supplied on a daily basis from the foods we eat.
Very high protein diets combined with very low carbohydrate intakes are not recommended.
Moderate-high protein/carbohydrate diets appear to be more effective than high carbohydrate diets for weight loss (especially from the abdomen) especially for people showing symptoms of metabolic syndrome and with elevated triglycerides

Article co-authored by Better Health Channel
(Australian -Victorian Government website)

September 2003