Nitrogen balance in college women: the effect of coffee on the nitrogen balance

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Virginia Polytechnic Institute


Protein has long been recognized as an essential dietary substance, necessary for the building and repair of body tissue. However, in recent years we have learned even more of the versatility of protein in body processes. In addition to the need of protein for growth and repair of tissue breakdown, protein compounds are now known to serve also as parts of hormones to regulate body processes, enzymes to digest foods, plasma albumin to maintain water balance and blood volume, globulins to resist infection, and hemoglobin to transport oxygen to the tissues.

In times of food shortages, protein deficiency is more likely to occur than that of any other of the important dietary essentials. This is due to the fact that there is a shortage of the foods that furnish protein of high biological value, i.e., the proteins of meat, milk, eggs and cheese. At the present time, where food shortages exist, these foods are scarce and when available expensive. Because of this situation, research concerned with protein metabolism is especially significant today.

Proteins differ from one another on the basis of the number and kind of amino acids of which they are made. Since there are nine (possibly 10) essential amino acids, that cannot be synthesized in the body in adequate amounts from other amino acids, they must be present in the food. These essential amino acids are found in protein of animal origin. Plant protein is good for supplementing other protein, but alone, it is inadequate for maintenance and growth in the body. With complete protein foods both scarce and expensive, a knowledge of the protein requirement, the factors that affect its utilization, and how the body needs are met are of prime importance.

The protein requirement has been determined from the data obtained from protein (or nitrogen) balance experiments in man. However, due to the nature of the data and the many variable factors involved in the amino acid content of various dietaries, there have been disagreements as to the real requirement, some advocating a high intake, others suggesting a lower intake. Today a compromise of the two extremes is generally advocated. The National Research Council has suggested an adult protein allowance of 60 to 70 gm. per day, from a diet of mixed protein (both animal and plant foods).

In view of the fact that the number of studies to determine the protein requirement of human beings has of necessity been small, this problem was originally undertaken to observe the effect of the variation of caloric intake on protein use by means of the nitrogen balance experiment.

In developing a low protein diet for the above study, coffee was suggested by one of the subjects to make the diet more palatable. This raised the question of whether or not coffee might in some way affect protein utilization or nitrogen excretion. A search was made for data on this point. In reviewing the available literature it was found that workers between 1853 and 1912 had done some work on the effect of caffeine on the nitrogen in the urine of men and dogs, but their results were contradictory, and in most cases they had used varying and unspecified amounts of caffeine. Since that time this problem has apparently been neglected.

In the hope of obtaining further information concerning the effect of coffee on nitrogen metabolism, this study was planned with the following purposes: To

  1. Develop dietary and laboratory technique for the nitrogen balance experiment.

  2. Attempt to reach the point of nitrogen equilibrium in subjects on a known protein and caloric intake.

  3. Determine the effect, if any, of the ingestion of a known amount of coffee on the total nitrogen, urea and ammonia excretions.