In my nursing training this topic made me cross-eyed.  

I trust my efforts have made this blog both palatable and useful for you.

Protein Utilization and Nitrogen Balance 

Metabolism: Using Amino Acids and Proteins in the Body

There is no pure storage form on body protein, therefore all body protein serves a physiological (functional) role. That is why we need protein in the diet each and every day.

Muscle is the major protein depot of the body. Expansion of body protein is due primarily to an increase in muscle protein. The converse holds true.

I love good choreography, whether it be classical ballet or a good martial art fight scene. When I follow chemical reactions I see a dancers with complex dance steps that need to be precise or the whole falters:

The Amino Acid Pool:

Amino acids are in the blood and body fluids, in a constant state of flux and chemically unstable (labile). You can see the importance of a healthy liver:

  1. Amino acids (protein) absorbed from the food in the gut.
  2. Synthesis (combination of various elements into a whole) into blood and tissue protein. We need enough amino groups from other amino acids to have the liver able to produce the non-essential amino acids.
  3. Liver: Most amino acids go to the liver (about 17 of the 20). The other 3 pass through the liver and are metabolized mainly in the muscle. The liver synthesizes its own proteins and a number of proteins for the blood stream. Principle site for synthesis of non-essential amino acids if these are not obtained in sufficient amounts in the diet. Delivers, by way of blood, a balanced mixture of amino acids for all other cells.
  4. Liver: Formation of other nitrogen-containing products needed by the rest of the body.
  5. Liver: Disposes excess amino acids. Liver enzymes with the nitrogen in the protein produce urea. After nitrogen is removed, carbon skeletons (called keto acids) are converted to fat, glucose and immediate oxidation to generate energy in the form of ATP (remember Part 1?).

The body proteins are dynamic, continuously being broken down and re-synthesized. This takes place in all cells. Approximately three grams of protein must be re-synthesized per kilogram body weight each day, thus needs to be consumed, to make up for protein that is catabolized (broken down). Thus we can see how important the amino acid pool and the health of the liver are. The liver is the major organ for amino acid metabolism. Old protein molecules break down (catabolize or degrade) and new protein (not different) molecules are produced.

Nitrogen Essentials:

  • Nitrogen is used by the liver to convert excess amino acids to urea.
  • As with everything else in life, balance is required. Nitrogen equilibrium is protein equilibrium normally occurring when protein intake is adequate.
  • A positive nitrogen balance, that is to say a gain of body protein, occurs under special circumstances: growth, exercise training, convalescence, pregnancy, period following under-nutrition. An excess of calories can increase nitrogen retention by the body. Extra protein is required for infectious diseases, particularly of the gastro-intestinal tract where there is difficulty with digestion and absorption. Also catch-up growth assume 18% of new tissue added is protein during pregnancy, especially 2nd and 3rd trimester and during lactation, nursing of baby.
  • A net loss of nitrogen from the body, called a negative nitrogen balance, occurs with wasting illnesses, immobilization, infections, emotional stresses, old age, reducing diets that have a large deficit between intake of food energy and expenditure of food calories or energy. Total body protein has decreased, prevails during starvation– muscle protein, certain blood and liver protein are preferentially catabolized so the carbon skeletons of the amino acids can be used as a source of glucose. Because of this, body energy supplies are more important than body protein supplies. therefore, in an emergency, it is the energy that counts, not maintaining muscle mass. Negative nitrogen balance can occur if diet contains insufficient quantity and quality of proteins and amino acids. Starvation is characterized by loss of muscle protein so that it can convert carbon skeletons of amino acids to glucose. 
  • About 16% of proteins is nitrogen.
  • About 82% of all of nitrogen losses are mainly in urea, but also in creatinine, uric acid, ammonia and creatine.
  • About 12% loss is mainly from undigested and unabsorbed protein and amino acids, intestinal cells and bacteria.
  • 3% skin; 3% sweat; hair, nails, saliva, blood in trace amounts. 

Quantity of Protein in Foods- average values

  • Chicken has an average value of 32% protein and 16 g/kcal food energy.
  • Fish is 18 g/100 g  and 12 g/100 kcal food energy.
  • Eggs are 13 g /100 g  and 8 g/100 kcal food energy.
  • White bread 9 g/100 g and 3 g/100 kcal food energy.
  • Peas are 6 g/100 g and 8 g/100 kcal food energy.

Animal protein has a complete pattern of essential amino acids. Plant proteins generally have completely or partially missing one or more essential amino acids. The greater the amount of nitrogen retained, the greater the biological value of the food in question.

Two protein containing foods that lack different essential amino acids can complement each other to produce a better quality overall protein; or one complete and one incomplete protein makes a protein that is better than the incomplete protein.

Soybeans are a good cash crop, high in quantity of protein, quality sufficient to maintain human life as a single protein food, but better if grain protein available  to compliment it.

Kwashiorkor is a disease in which there is adequate energy intake but the diet is deficient in protein. This usually affects children because their protein requirements are greater than those of adults.

Marasmus is a disease condition due to to insuffiecient calories.

It is common for athletes to take in diets containing amounts of protein greatly in excess of recommended amounts. If more protein rich food is selected, the athlete can easily get three times the recommended amount. In heavy resistance  training, assume athlete adds one pound (454 g) of muscle tissue per week as a result of training. 75% will be water. 25% or less will be protein. Only a small 25% increase in protein is required beyond the recommended level.

Nitrogen is lost in sweat during training, covered by only a few extra grams of protein each day. Heavy training or a sharp increase in intensity and duration of workouts should pay particular attention to the protein intake of the diet. Their needs may exceed 2.5 g protein/kg BW. Any drop in body weight and training performance indicates training is too severe and/or more dietary protein should be consumed. An increase in protein increases urea, increasing water loss into urine, which in turn may be overly taxing on the kidney. Increase water intake.

Janet Wiebe