Protein and its role as an energy source.
Proteins are made up of amino acids linked end to end. There are twenty different amino acids that can combine in a number of ways to form proteins. We can obtain most of these amino acids from our diet but 12 of these amino acids can be synthesised by the body if necessary. Eight amino acids are knwon as essential amino acids. This means that the body can not synthesise these amino acids and so they must be ingested. Proteins make up between 12% and 15% of the body mass. Among their many functions they are involved in building tissue structures such as tendons, ligaments and also involved in the control of chemical reactions that take place in the body. Continual turnover of protein takes place and an adequate dietary protein intake is important in replenishing the old protein being degraded.
The diagram on the right shows the results of a study conducted to measure urea content in sweat. Urea is formed from the breakdown of protein and as such its concentration in sweat can be used to predict the rate of protein breakdown. Sweating does not only aid in cooling the body but is also an avenue for the excretion of urea which is formed from protein metabolism during exercise.
Notice the sharp rise in protein breakdown during exercise in a carbohydrate depleted athlete. This may indicate that carbohydrate may modify the demands on protein as a fuel source.
Protein appears to play a role as a source of energy during endurance events where glycogen is depleted. Eating a high carbohydrate diet minimises the breakdown of muscle protein in athletes who train for long periods of time in endurance events.
At rest protein breakdown (catabolism) accounts for between 2% and 5% of energy production. Unlike fat and glucose the body does not store protein for the purpose of conversion into energy. All protein in the body is accounted for as structural material and in its many other functions such as metabolic control.
Athletes who engage in intense training for more than 2 hours per day should consume between 1.2 and 1.8 grams of protein per kilogram of body mass each day.
Dietary intake of protein substantially above the recommended levels can prove harmful to proper liver and kidney function. This is because the breakdown of proteins into useable energy sources requires the removal of nitrogen from its amino acids. This nitrogen is then removed from the body in the form of urea. Production of urea and its excretion places undue stress on the liver and kidneys respectively. The carbon skeleton that remains after the nitrogen is removed from the amino acids is used for the formation of glucose or fat.
Most body builders ingest more protein than their body needs. They would argue that it is necessary for extreme muscle growth and good results can not be achieved without the ingestion of extra protein.
Sam is on a strict diet to reduce calorie intake. His diet consists of little carbohydrate. He exercises intensely for more than 2 hours per day. What is the expected outcome for his muscle mass?