Physiological economy in nutrition, with special reference to the minimal proteid requirement of the healthy man / an experimental study

by R. H. Chittenden

Summary

Certain general conclusions seem to be justified by the results reported. A healthy man, whose occupation is such as not to involve excessive muscular work, but whose activity is mainly mental rather than physical, though by no means excluding the latter, can live on a much smaller amount of proteid or albuminous food than is usually considered essential for life, without loss of mental or physical strength and vigor, and with maintenance of body and nitrogen equilibrium. This means that the ordinary professional man who leads an active and even strenuous life, with its burden of care and responsibility, need not clog his system and inhibit his power for work by the ingestion of any such quantities of proteid food as the ordinary dietetic standards call for. There is no real physiological need—that is apparent—for the adoption of such dietetic habits as ordinarily prevail, or as are called for by the dietary standards set by most authorities in this branch of physiology. There is no justifiable ground for the dictum, or the assumption, that the adult man of average body-weight needs daily 118 grams of proteid food for the maintenance of health, strength and vigor, or that there is need for the metabolism of at least 16 grams of nitrogen daily. If such were the case, how could these five subjects, whose experiences have been detailed in the foregoing pages, have maintained their body-weight, established nitrogen equilibrium, pursued their ordinary vocations without loss of strength and vigor, and kept in a perpetual condition of good health, with an average daily metabolism of from 5.4 grams of nitrogen to 8.99 grams of nitrogen for periods ranging from six to eighteen months? Surely, if 16 to 18 grams of nitrogen are a daily requisite for the healthy adult, there should have been some sign of nitrogen starvation during these long periods of low proteid diet, but the sharpest scrutiny failed to find it. On the contrary, there were not wanting signs of improved conditions of the body which could not well be associated with anything but the changes in diet.

Let us briefly consider the main facts. The writer, of 57 kilos body-weight, showed for nearly nine consecutive months an average daily metabolism of 5.7 grams of nitrogen. During the last two months the daily metabolism averaged 5.4 grams of nitrogen. As body-weight and nitrogen equilibrium were both maintained under these conditions, it is certainly fair to assume that the physiological needs of the body were fully met. These figures imply a metabolism, in the first instance, of 0.1 gram of nitrogen per kilo of body-weight, while the lower figure shows a metabolism of 0.094 gram of nitrogen per kilo of body-weight. We may call this latter amount the minimal nitrogen requirement for this particular individual, under which health, strength, and vigor can be fully maintained. This lower nitrogen figure shows that the needs of this particular individual for proteid material are met by the metabolism of 33.75 grams of proteid per day. Hence, one-third the usually accepted standard of proteid is quite sufficient for the wants of this particular person, and this too with a quantity of non-nitrogenous food far below the daily amount called for by ordinary physiological rules. A fuel value of 2000 calories per day was fully adequate to meet the ordinary wants of the body.

Dr. Mendel, with a body-weight of 70 kilos, showed for seven consecutive months an average daily metabolism of 6.53 grams of nitrogen, likewise with maintenance of health, strength, body equilibrium, and nitrogenous equilibrium. This figure implies a nitrogen metabolism of 0.093 gram per kilo of body-weight and shows that the wants of the body—in his case—can be fully met by a metabolism of 40.8 grams of proteid matter daily, and this likewise without increasing the amount of non-nitrogenous food ingested. Indeed, a total fuel value of 2500 calories per day was quite sufficient for all the needs of his body under the existing conditions.

Dr. Underhill, with a body-weight of 65 kilos, showed for six consecutive months a proteid metabolism equal to 7.81 grams of nitrogen per day, while for the last two months the daily average excretion of nitrogen was only 6.68 grams. These figures mean respectively a nitrogen metabolism of 0.120 and 0.102 gram of nitrogen per kilo of body-weight. Here, too, as in the preceding cases, this lowered rate of proteid metabolism was maintained without increasing the total fuel value of the food and with a continuance of health and strength.

Messrs. Dean and Beers, with body-weights of 64 and 61.5 kilos respectively, likewise kept up their health and strength for a long period of time with a nitrogen metabolism averaging 8.99 and 8.58 grams of nitrogen per day, i. e., with a metabolism of 0.140 and 0.139 gram of nitrogen per kilo of body-weight respectively, and this with a total fuel value in their daily food averaging not more than 2500 calories.

With this general concurrence of results, we are certainly warranted in the assertion that the professional man can safely practise a physiological economy in the use of proteid food equal to a saving of one-half to two-thirds the amount called for by existing dietary standards, and this without increasing the amount of non-nitrogenous food consumed. Indeed, the latter class of foods can likewise be diminished in amount without detriment to health or strength, where there is no call for great physical exertion. Lastly, the so-called minimal proteid requirement of the healthy man—which for this group of individuals we may place at the low level of 0.093 to 0.130 gram of nitrogen per kilo of body-weight—represents the real physiological needs of the system for nitrogen, and in so far as our present data show, anything beyond this quantity may be considered as an excess over and above what is required for the actual physiological necessities of the body. Naturally, however, there may be nothing detrimental in a slight excess of proteid beyond the daily needs. That is a subject, however, to be discussed later in connection with other results.

In view of the close agreement in the amount of nitrogen metabolized by these different individuals per kilo of body-weight, emphasis should be laid upon the fact that the results recorded were all obtained with perfect freedom of choice in the matter of diet, without prescription of any kind, so that the close concurrence in the final figures tends to strengthen the value of the data as pointing to a certain minimal requirement easily attainable, and fully adequate for meeting the needs of the body.