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During digestion, the proteins in our food are broken down into constituent amino acids absorbed by the blood capillaries and transported to the liver. The amino acids are then synthesized into proteins or stored as fat or glycogen for energy. Each gram of protein produces approximately four Calories. Many proteins function as enzymes, and others:

  • Form the structural framework of various parts of the body - Keratin in skin and hair
  • Function as hormones - Insulin
  • Serve as antibodies
  • Transport vital substances throughout the body - haemoglobin
  • Serve as contractile elements in muscle tissues - actin & myosin

Amino Acids

Amino acids are the building blocks of protein. Protein formation can result in dehydration because water molecules are lost as amino acids combine to form more complex molecules.

The body requires 20 amino acids; eight are referred to as essential amino acids that the human body cannot synthesize. Animals and plants manufacture proteins that contain these essential amino acids. The body can synthesize non-essential amino acids, but this does not mean they are unimportant. It is just that the body can produce sufficient to meet the demands for growth and tissue repair. Therefore, our diet must contain appropriate levels of protein.

The essential Amino Acids (Bean 1993)[1] are Histidine, lsoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan and Valine.

The non-essential Amino Acids (Bean 1993)[1] are Alanine, Arginine, Asparagine, Aspartic acid, Cysteine, Glutamic acid, Glutamine, Glycine, Proline, Serine and Tyrosine.


Recommended Protein Intake

Despite many coaches and athletes' beliefs, eating excessive protein provides little benefit. Muscle mass does not increase by eating high-protein foods. Protein intake significantly above the recommended values can prove harmful because excessive protein breakdown strains the liver and kidney functions by producing and eliminating urea and other solutes.

The recommended daily allowance for men and women (McArdle et al. 2000)[2]:

  • Adolescent - 0.9 gram of protein per kg body weight
  • Adult - 0.8 gram of protein per kg body weight

Training and Protein Needs

Research suggests that protein breakdown increases during and immediately after exercise and that protein manufacturing slows down simultaneously. The more intense the activity, the higher your protein breakdown will be, and the higher your needs will become. If you train to increase muscle mass, your protein needs will be more significant still. The extra protein will be needed to compensate for protein breakdown, for a new protein to be made, and for muscle growth. It is essential to realize that a high-protein diet alone will not increase strength or muscle size. When combined with heavy resistance exercise, additional protein can cause this to happen.

Over the past ten years, research has indicated that athletes engaged in intense training must ingest 1.5 to 2 times the recommended intake to maintain a positive protein balance. Bear in mind that excess protein is converted to fat and stored.

Optimum Protein Nutrition

There is more to protein nutrition than just eating the optimum amount; the timing of consumption and the type of protein selected can impact nitrogen balance. Many nutritional 'co-factors' are either essential or helpful in promoting optimum protein metabolism within the body. It is especially true where carbohydrate is concerned because building or maintaining lean tissue mass is an 'energy-intensive' process.

Increasing protein intake at the expense of carbohydrate can be a bad strategy for athletes engaged in heavy training because without sufficient carbohydrate the body switches to other fuels for energy, and amino acids from protein (particularly the branched-chain amino acids, leucine, isoleucine and valine) provide a ready source of energy!

Muscle tissue is a relatively rich source of branched-chain amino acids (BCAAs). It tends to undergo breakdown during high-energy demand periods when carbohydrates or the amino acid pool becomes depleted. Furthermore, carbohydrates stimulate insulin release, a highly anabolic hormone that helps drive glucose and amino acids into muscle cells. Athletes seeking to optimize their protein metabolism should ensure a carbohydrate intake commensurate with training volume.

The following are some steps you can take to optimise your protein nutrition:

  • Ensure an adequate intake of dietary protein - i.e. a minimum of 1.5g of high-quality protein per kg of body weight per day. Power/strength athletes, or those engaged in intense training, should consider increasing this to 2g/kg/day
  • Ingest protein-carbohydrate drinks after exercise rather than protein alone. Ideally, consume a drink made up of about 1g per kg of carbohydrate and 0.5g per kg of protein within 30 minutes of training, and eat a high-carbohydrate meal within two hours
  • Consume a light pre-exercise snack: 50g of carbohydrate and 5-10g of protein taken before a training session can increase carbohydrate availability towards the end of an intense exercise bout and increase the availability of amino acids to muscles. However, make sure your snacks are low in fat to allow for rapid gastric emptying
  • Use protein/carbohydrate drinks during very long events: a solution containing 73g carbohydrate and 18g protein per litre, consumed at a rate of 1ml per kg of body weight per minute, may delay the onset of fatigue and reduce muscle damage
  • Consume quick-digesting proteins such as soy and whey immediately after training: this may be especially important for older athletes
  • At other meals, consume a mix of proteins to promote a more sustained release of amino acids into the body
  • Adding branched-chain amino acids (BCAAs) to your normal protein intake may be useful for athletes undergoing prolonged or heavy training, and this may be particularly true for events/sports requiring large amounts of mental agility and motor coordination
  • HMB supplementation, at 3g per day, maybe a useful additional strategy for novice athletes or those returning to training after a layoff (HMB is a leucine metabolite)
  • Ensure that your overall diet is of high quality and as whole and unprocessed as possible: this will ensure adequate intakes of other nutrients essential for protein metabolisms, such as zinc and B vitamins

Source of Protein

Proteins that contain all the essential amino acids in approximately the right proportions for your body's requirements are sometimes called 'high-biological-value' proteins. These are found in foods derived from animals: meat, fish, eggs, milk and dairy products.


  1. BEAN, A (1993) The complete guide to Sports Nutrition. 1st ed. London: A & C Black
  2. McARDLE, W.D. et al. (2000) Training muscles to become stronger. In: McARDLE, W.D. et al., 2nd ed. Essentials of Exercise Physiology, USA: Lippincott Williams and Wilkins, p. 56

Page Reference

If you quote information from this page in your work, then the reference for this page is:

  • MACKENZIE, B. (2001) Proteins [WWW] Available from: [Accessed