Methods for Determining Metabolic Utilization of Proteins, Carbohydrates, and Fats
Nitrogen Excretion Can Be Used to Assess Protein Metabolism.
The average protein contains about 16 per cent nitro-gen. During metabolism of the protein, about 90 per cent of this nitrogen is excreted in the urine in the form of urea, uric acid, creatinine, and other less important nitrogen products. The remaining 10 per cent is excreted in the feces. Therefore, the rate of protein breakdown in the body can be estimated by measuring the amount of nitrogen in the urine, then adding 10 per cent for the nitrogen excreted in the feces, and multiplying by 6.25 (i.e., 100/16) to determine the total amount of protein metabolism in grams per day. Thus, excretion of 8 grams of nitrogen in the urine each day means that there has been about 55 grams of protein breakdown. If the daily intake of protein is less than the daily breakdown of protein, the person is said to have a negative nitrogenbalance, which means that his or her body stores ofprotein are decreasing daily.
“Respiratory Quotient” Is the Ratio of CO2 Production to O2 Utilization and Can Be Used to Estimate Fat and Carbohydrate Utilization. When carbohydrates are metabolized withoxygen, exactly one carbon dioxide molecule is formed for each molecule of oxygen consumed. This ratio of carbon dioxide output to oxygen usage is called the respiratory quotient, so the respiratory quotient forcarbohydrates is 1.0.
When fat is oxidized in the body’s cells, an average of 70 carbon dioxide molecules are formed for each 100 molecules of oxygen consumed. The respiratory quo-tient for the metabolism of fat averages 0.70. When pro-teins are oxidized by the cells, the average respiratory quotient is 0.80. The reason that the respiratory quo-tients for fats and proteins are lower than that for carbohydrates is that a large share of the oxygen metab-olized with these foods is required to combine with the excess hydrogen atoms present in their molecules, so that less carbon dioxide is formed in relation to the oxygen used.
Now let us see how one can make use of the respira-tory quotient to determine the relative utilization of dif-ferent foods by the body. First, it will be recalled that the output of carbon dioxide by the lungs divided by the uptake of oxygen during the same period is called the respiratory exchange ratio. Over a period of 1 hour or more, the respiratory exchange ratio exactly equals the average respiratory quotient of the metabolic reactions throughout the body. If a person has a respiratory quotient of 1.0, he or she is metabolizing almost entirely carbohydrates, because the respiratory quotients for both fat and protein metabolism are con-siderably less than 1.0. Likewise, when the respiratory quotient is about 0.70, the body is metabolizing almost entirely fats, to the exclusion of carbohydrates and proteins. And, finally, if we ignore the normally small amount of protein metabolism, respiratory quotients between 0.70 and 1.0 describe the approximate ratios of carbohydrate to fat metabolism. To be more exact, one can first determine the protein utilization by measuring nitrogen excretion and then, using the appropriate mathematical formula, calculate almost exactly the uti-lization of the three foodstuffs.
Some of the important findings from studies of respi-ratory quotients are the following:
a. Immediately after a meal, almost all the food that is metabolized is carbohydrates, so that the respiratory quotient at that time approaches 1.0.
b. About 8 to 10 hours after a meal, the body has already used up most of its readily available carbohydrates, and the respiratory quotient approaches that for fat metabolism, about 0.70.
c. In untreated diabetes mellitus, little carbohydrate can be used by the body’s cells under any conditions, because insulin is required for this. Therefore, when diabetes is severe, most of the time the respiratory quotient remains near that for fat metabolism, 0.70.