What is protein?
Proteins are complex organic compounds. The basic structure of protein is a chain of amino acids. They provide energy for the body. 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 (chicken, beef, lamb and fish), and legumes like beans and lentils. These proteins are digested to release amino acids. In the body, amino acids are used to make new proteins, converted into hormones such as adrenalin or are used as an energy source.
What are amino acids?
Proteins are made up of chains of smaller building blocks called amino acids, which are chemically linked to each other. 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 between 50 and tens of thousands of amino acids. There are two broad classes of amino acids: those that can be made by the human body (non-essential amino acids) and those that can only be supplied by food (essential amino acids).
In the diet, protein sources are labeled according to how many of the essential amino acids they provide:
- A complete protein source is one that provides all of the essential amino acids. You may also hear these sources called high quality proteins. Animal-based foods; for example, meat, poultry, fish, milk, eggs, and cheese are considered complete protein sources.
- An incomplete protein source is one that is low in one or more of the essential amino acids. Complementary proteins are two or more incomplete protein sources that together provide adequate amounts of all the essential amino acids.
- Every function in the living cell depends on proteins.
- Motion and locomotion of cells and organisms depends on contractile proteins. [Examples: Muscles]
- The catalysis of all biochemical reactions is done by enzymes, which contain protein.
- The structure of cells, and the extracellular matrix in which they are embedded, is largely made of protein. [Examples: Collagens] (Plants and many microbes depend more on carbohydrates, e.g., cellulose, for support, but these are synthesized by enzymes.)
- The transport of materials in body fluids depends of proteins. [Blood]
- The receptors for hormones and other signaling molecules are proteins.
How much protein you need
The amount of protein you need in your diet depends on your weight, age and health. As a rough guide, the recommended dietary intake (RDI) for protein (measured in grams per kilogram of bodyweight) is:
- 0.75 g/kg for adult women
- 0.84 g/kg for adult men
- Around 1 g/kg for pregnant and breastfeeding women, and for men and women over 70 years.
So, for example, a 75 kg adult male would need 63 g of protein per day. It is recommended that 15 to 25 per cent of total energy intake per day is from protein sources.
The needs of children and adolescents also vary according to their age and weight. A full list of recommendations for dietary protein is available from the Australian Nutrient Reference Values (NRVs) website.
Most Australians eat far more protein than they actually need, so deficiencies are rare.
Sources of protein
Some sources of dietary protein include:
Beans (including soy)
- Tofu, ½ cup 20 grams protein
- Soy milk, 1 cup – 6 -10 grams
- Most beans (black, pinto, lentils, etc) about 7-10 grams protein per half cup of cooked beans
- Soy beans, ½ cup cooked – 14 grams protein
- Split peas, ½ cup cooked – 8 grams
Nuts & Seeds
- Peanut butter, 2 Tablespoons – 8 grams protein
- Almonds, ¼ cup – 8 grams
- Peanuts, ¼ cup – 9 grams
- Cashews, ¼ cup – 5 grams
- Pecans, ¼ cup – 2.5 grams
- Sunflower seeds, ¼ cup – 6 grams
- Pumpkin seeds, ¼ cup – 8 grams
- Flax seeds – ¼ cup – 8 grams
- Most fish fillets or steaks are about 22 grams of protein for 3 ½ oz (100 grams) of cooked fish, or 6 grams per ounce
- Tuna, 6 oz can – 40 grams of protein
- Steak, 6 oz – 42 grams
- Most cuts of beef – 7 grams of protein per ounce
- Chicken breast, 3.5 oz – 30 grams protein
- Chicken thigh – 10 grams (for average size)
- Drumstick – 11 grams
- Wing – 6 grams
- Chicken meat, cooked, 4 oz – 35 grams
- Pork chop, average – 22 grams protein
- Ham, 3 oz serving – 19 grams
- Bacon, 1 slice – 3 grams
- Egg, large – 6 grams protein
Recommended serves per day from the lean meat, poultry, fish, eggs, legumes and beans, and nuts and seeds food category:
- 3 serves for adult men
- 2½ serves for adult women
- 2½ to 3½ serves for breastfeeding and pregnant women
A standard serving size is one of the following:
- 65 g cooked lean red meats
- 80 g cooked poultry
- 100 g cooked fish fillet
- 2 large eggs
- 1 cup cooked dried beans, lentils, chickpeas, split peas or canned beans
- 170 g tofu
- 30 g nuts or seeds.
Adult men are recommended to consume 2½ serves of (mostly low-fat) dairy or alternatives per day and 2½ to 4 serves per day for adult women. A serve could include either:
- 250 ml (1 cup) milk
- 200 g (3/4 cup or 1 small carton) yogurt
- 40 g (2 slices) hard cheese
Do athletes require more protein?
In the last 20 years, detailed research has enabled scientists to measure protein metabolism during exercise and recovery, and to monitor protein balance in athletes. Endurance athletes in heavy training require extra protein to cover a small proportion of the energy costs of their training and to assist in the repair and recovery process after exercise. Strength athletes, who are interested in gaining muscle size and function, require more protein in the early stages of very intensive resistance exercise. However, strength athlete’s muscles seem to adapt to the stress of resistance exercise, so that the protein requirements to maintain protein balance in very well-trained athletes are only marginally greater than those of generally active people. Athletes, who are growing, such as adolescents, have additional protein requirements. The table below summarizes protein requirements for different types of athletes or exercise activities. Since athletes come in various shapes and sizes, it is easier to keep track of these requirements by relating them to the size (body mass or BM) of the athlete.
Table : Estimated protein requirements for athletes
|Group||Protein intake (g/kg/day)|
|Sedentary men and women||0.8-1.0|
|Elite male endurance athletes||1.6|
|Moderate-intensity endurance athletes (a)||1.2|
|Recreational endurance athletes (b)||0.8-1.0|
|Football, power sports||1.4-1.7|
|Resistance athletes (early training)||1.5-1.7|
|Resistance athletes (steady state)||1.0-1.2|
|Female athletes||~15% lower than male athletes|
(a) Exercising approximately four to five times per week for 45-60 min (b) Exercising four to five times per week for 30 min at <55% VO2peak
Source: Burke and Deakin, Clinical Sports Nutrition, 3rd Edition, McGraw-Hill Australia Pty Ltd, 2006
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 yogurt) 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.
- Fiber is largely a carbohydrate. Foods rich in carbohydrates (such as whole grains and legumes) are also rich in fiber. Avoiding these foods leads to an overall low-fiber 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.