The main function of protein in the diet is to perform repair of body tissues and structures. They are also vital in the formation of enzymes and hormones. They can also be used as an energy source, should there by insufficient energy or carbohydrates in the diet.
Protein Structure
The building blocks of proteins are called amino acids. There are 20 amino acids that can be combined in different combinations to make up many different types of proteins and also perform a wide variety of functions. Amino acids are either classified as essential or non-essential. Essential would be amino acids that are needed to be taken in by the diet as they cannot be manufactured in the body. Non-essential amino acids are what the body is able to manufacture from Nitrogen in the diet and certain carbohydrate and fat metabolites. Two amino acids, namely Arginine and Histidine are used often in the body, that at some times they can be considered as conditionally essential.
Essential Amino Acids
1. Isoleucine
2. Valine
3. Tryptophan
4. Lysine
5. Methionine
6. Phenylalanine
7. Leucine
8. Threonine
Non-Essential Amino Acids
1. Alanine
2. Tyrosine
3. Aspargine
4. Serine
5. Cysteine
6. Glutamic Acid
7. Aspartic Acid
8. Glycine
9. Proline
How are Proteins digested, absorbed and used in the body?
Proteins are made up of amino acids as discussed, and they are joined by what is known as a peptide bond. Many amino acids joined together are therefore called a polypeptide. Two peptides are called a dipeptide and three are call a tripeptide. Before a protein can be used by the body, it needs to be broken down into its individual amino acids.
Digestion begins in the stomach where the addition of stomach acid (HCl) starts to break down the protein’s structure (called denaturing). Ultimately polypeptides are broken down into amino acids and absorbed in the small intestine into the blood and then transported through to the liver. Beyond this they are then able to perform their necessary functions.
What is a protein?
- Animal Source (high bioavailability)
- Beef, lamb, mutton, chicken, fish, yoghurt, maas, amahewu, milk, cheese, paneer, cottage cheese, egg.
- Vegetable Protein (low bioavailability)
- Lentils, soya, chickpeas, mycroprotein products.
- Other Sources (smaller amounts)
- Carbohydrate foods, vegetables and fruits, nuts, seeds.
Vegetarian Protein combining
For a vegetarian source of protein to have a higher bioavailability they need to be combined with other sources such as carbohydrates such as rice and beans or peanut butter on bread. A wide variety of amino acids from various sources will be pooled together in an amino acid pool, and drawn upon when needed in the body for various functions or uses.
Factors affecting protein requirements
Total Energy Needs
When total energy needs are decreased, the amount of protein will increase due to its same need for a certain amount of amino acids to perform the functions as explained earlier. Protein can be used to provide satiety in lower energy diets. It is for this reason that carbohydrates are called “protein sparing” as they allow the protein in the diet to perform their function without needing to be used as an energy source. A high protein diet without a carbohydrate source will not however produce long term sustainable results.
Exercise
The oxidation of amino acids is increased during exercise, and it is particularly higher when combining resistance training and cardiovascular training. Protein turnover and muscle recovery post aerobic and anaerobic exercises are different in both, which is why there may be a greater need for protein in individuals participating in both types of exercise.
Carbohydrates and Protein: Mass Building
Sportsmen as well as the general public, including bodybuilders should aim to include approximately 50-60% of their diet as carbohydrate, only 20-30% fat and 12-20% protein. Because carbohydrates are the main fuel of muscle tissue a deficit in carbohydrates in the diet will cause chronic fatigue. The school of thought that says that an individual must eat large quantities of protein in order to build muscle is incorrect. Body builders need only to consume the recommended 12-20% of energy as protein. Food quantity is often increased due to increased training time, increased muscle mass and consequently metabolism. Because % of total energy remains the same BUT because energy intake has increased, the actual grams of protein will also increase BUT at the same time, so will carbohydrate and fat intake.
During competition however, body builders will adjust their diet to lose excess stored water and to promote muscle definition. During this time, they will also increase their protein intake due to their limiting of carbohydrates. This is not for an extended period but rather only shortly prior to competition. If they did this on a regular basis, it would be very expensive, could tax the kidneys with excess excretion of urea (a byproduct of protein digestion), it can lead to gout, arthritis and osteoporosis. Body Builders should also be careful not to limit too much fat in their diet, and rather focus on health unsaturated fats (see section on fats). If they consume a diet that provides less than 30% fat, their intake of essential fatty acids is compromised, which is a concern as essential fatty acids are important in keeping blood cholesterol normal, preventing dry skin and for the absorption of fat soluble vitamins. Research actually shows that as sportsmen get fitter; their bodies become more efficient in using fat as a source of fuel, so that a little extra fat in the diet will not make them fatter, but actually help them to feel more energetic.
Concerns with a High Protein Diet
- Associated with a higher intake of saturated fats
- Lower fibre intake
- Increased urea production therefore increase stress on kidneys
- Increased calcium losses
- Increased fluid requirements
- Decreased glycogen stores
- Possible dehydration
- Impaired physical performance
Recommended Protein Intake
The RDA (recommended daily allowance) states that an adult requires 0.8g Protein/kg/day and this should be about 10 – 35% of total energy intake. Below is a table depicting these requirements for different activity levels.
| Activity Level | Grams of protein per kg body weight/day |
| Sedentary | 0.8 |
| Strength Athletes | 1.2 – 1.7 |
| Endurance Athletes | 1.2 – 1.4 |
Protein Properties & Review
- One gram protein provides 17kJ (4kcal) {same as carbohydrates}.
- Protein must have peptide bonds broken to release amino acids before they can be used by the body.
- Energy requirements are 10 – 35% of total energy.
- General guidelines stipulate 0.8g Protein/kg/day for an adult.
- Requirements may increase depending on activity level.
- Amino acids can be spared by carbohydrates for use as muscle repair and building, hormone and enzyme production.
- High intake over the recommendations can be dangerous and result in many negative and adverse results in the body.
- In the absence of sufficient carbohydrates in the diet, protein can be used as an energy source.
- There are different sources of protein – animal and vegetarian. By protein combining vegetarian sources you can achieve a full amino acid profile successfully.