It is obvious that heat can be supplied to the body by external application, including the entry of hot air into the lungs, by ingesta, and by chemic change - practically entirely oxidation - within the body. As, even when the outer air exceeds the normal temperature of the body, the temperature is reduced to normal by evaporation of sweat, the external supply of heat can act only locally or in a negative way by diminishing the amount of radiation that would otherwise take place. As the ingesta can not be administered at a temperature of much over 50 degrees centigrade (122 F.), i. e., 13 degrees above the normal body temperature, and as they amount to 4 liters at most, and consist of water to the extent of 3 liters, it is plain that not much over 50 calories can be supplied in this way. Conversely, if every particle of food and drink were administered at zero (freezing point), the loss in warming the ingesta would amount to only about 4x37, or, approximately 150 calories. As a matter of fact, cold and warm ingesta ordinarily about balance each other so that the modification of temperature and supply or loss of heat units from this source is insignificant. Thus, practically all the heat and energy of the body are due to the oxidation of food.

Vierordt has calculated that the discharge of heat units is divided as follows:

Practically all authorities agree that about 88% of the loss of heat units occurs through the skin in one way or another, including that required in muscular exercise. The same fact may also be stated in other words that, with slight muscular exercise the total discharge of heat units through the skin is about 2200 calories or 100'0 - 1200 calories per square meter, the total skin area of an adult being about 2 square meters.

A very practical point in the maintenance of adequate nutrition and in the reduction of obesity is the fact that the evaporation from the skin of a liter of water, signifies the loss of 580 calories (Weintraud). This fact is an additional reason for administering water in the obese, but, in the present connection, it may be pointed out that any excess of water drunk on account of muscular exercise is a rough index to the requirement of calories beyond the standard light exercise ration which does not usually cause sensible perspiration. By measuring the ingested water, the urine and faeces, and allowing 500 - 600 c.c. of elimination of water by the lungs, we can get a fairly accurate idea of the difference which must pass out by the skin, remembering, of course, to allow for the water of solid food and for the inclusion of the evaporation of insensible perspiration in the standard table above. Various fallacies are involved in such an estimate but it is approximately' correct.

If the patient is not taking an excess of watery vegetables and fruits, we may estimate approximately that the watery contents of the solid foods and the water produced by oxidation, balance the loss by the lungs and bowel - providing that there is no diarrhoea. Then, measuring the beverages, we subtract 2000 c.c. for the amount ordinarily eliminated by the skin and kidneys, the caloric loss of which is included in the standard table of Vierordt. " The excess represents excess of caloric loss, and is, indeed, very directly seen to be connected with muscular labor and perspiration; every liter of this excess represents a demand for 580 calories.

The nitrogenous output may be estimated approximately by the ureometer. Practically no nitrogen is eliminated by the lungs, (the elemental nitrogen of respired air is, of course, not considered) or by the skin. About 1/7 of the amount of urinary nitrogen is ordinarily eliminated in the faeces but part of this is alimentary waste. The Hinds modification of the Doremus ureometer estimates the urea about as accurately as any method depending upon the liberation of nitrogen gas, and it should be remembered that such instruments - barring imperfection of reactions - include all non-protein urinary nitrogen with that formed from urea. Thus, when the instrument reads such a per milleage of urea, it signifies total nitrogen read as urea. 28/60 of the weight of urea is nitrogen. Hence, allowing for the additional loss of body nitrogen in the faeces, every gram of so-called urea eliminated in the urine, represents fully 1/2 gram of nitrogen, and each gram of nitrogen corresponds to 6.25 grams of protein consumed in the body. Thus, if the total quantity of Urine and the ureometer reading indicate a loss of 20 grams of so-called urea, there has been an oxidation of about 62.5 grams of protein.

As is discussed elsewhere, the nitrogen elimination is very important in observing the course of diabetes, on account of the tendency to oxidize the tissues. But, in the great majority of cases, those free from wasting disease, and not losing weight rapidly, the ureometer is a valuable clinical index to the amount of protein actually assimilated by the body. We may even go further and say that unless there is an obvious overeating of protein foods or marked intestinal indigestion, as evidenced by the passage of milk curds, masses of muscle, etc., or by putrefactive signs the ureometer is a fairly good measure of the protein ingested. Often, indeed, it is easier to check the protein ration by the ureometer than by direct weighing and analysis of the ingesta. It should be remembered that, on a standard mixed diet, there is a physiologic waste of about 10% of the ingested protein.

With the qualifications suggested, and allowing for daily variations, which are not usually great, unless irregular habits give warning, it may be said that a ureometer estimation of less than 20 grams of so-called urea per diem, shows that the protein assimilation is nearly at the physiologic minimum and that an elimination of 30 grams represents the full allowance of about 93 grams. Under most conditions, the diet should be regulate,! so as to keep the "urea " elimination close to 20 grams.