Nutritional Aspects of Protein Catabolism

The remainder of this chapter takes on a more nutritional aura. It concerns the concepts of growth and maintenance. The material may be especially interesting to those involved in infant nutrition, animal science, and weight reduction by low-energy diets. The material reveals the techniques used for determining the protein requirement and the recommended daily allowance (RDA) for protein. Clinical issues involving protein nutrition, such as renal disease and gout, are also outlined. 

Nitrogen balance is the difference between the amount of nitrogen consumed per day and the amount of nitrogen excreted per day. This definition can be expressed by the formula 

Nitrogen balance = ( grams of nitrogen eaten - grams of nitrogen lost) 

A meaningful value for N balance cannot be determined by analyzing the food and excreta collected during 1 day. Because of the biological variability involved in experiments involving animals and humans, samples must be collected over several days. The study described next involved measiuement of the daily N balance over 5-day periods. 

Growth can be assessed by measuring the content of potassium ions in the body on the following basis. Potassium ions occur mainly in intracellular fluids, in which the K concentration is about 142 mM. Extracellular fluids, on the other hand, contain much lower levels of potassium, about 10 mM. Adipose tissue is largely water free and thus contains minimal amounts of fluids and salts. The potassium ion content may be determined in growing humans. How is it possible to measure the total content of K ions in a living person A fi action of the potassium in the 

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Toxicity and Packaging of Waste Nitrogen Illustrated by Experimental Nutrition Regulation of Protein Catabolism Nutritional Aspects of Protein Catabolism Nitrogen Balance Growth... 

Nutritional Aspects of Protein Catabolism 449 TABLE 8.7 Nitrogen Balance Experiment... 

Nutritional Aspects of Protein Catabolism 453 TABLE 8.8 In Vivo Half-Life of Various Proteins... 

In humans, the catabolism of macronutrients (carbohydrates, fats, and proteins) to supply energy is an important aspect of nutrition. In the United States, most diets provide more than an adequate number of nutritional calories. The typical American diet is high enough in fat that essential fatty acids (Section 21.6) are seldom, if ever, deficient. The only concern is that the diet contains an adequate supply of protein. If the intake of protein is sufficient, the supply of essential amino acids (Section 23.5) is normally also sufficient. Packaging on food items... 

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