To the layman one of the most obvious indices of the value of a given diet would be the changes in body-weight, for it is commonly believed that any increase in body-weight indicates ample, if not excessive, nourishment, and that a decrease is evidence of insufficient nourishment. For experiments of long duration, such as are commonly made on domestic animals when feeding or fattening for market, this is a remarkably good index.
With man, however, the experiments must of necessity be of short duration, since a routine diet can not be adhered to for so long a time as with animals, and fluctuations in body-weight are, therefore, by no means a proper index of gain or loss of body-material.
Index of Body Condition
Factors involved in change of body-weight.–An increase in body-weight is a resultant of a number of factors. There may have been actual additions to protein, fat, carbohydrate, and water, these four being the principal ingredients of the body, or there may have been an increase in two or three of these compounds with an actual decrease in the others, and there may have been losses from three of them compensated by a gain in the fourth.
It is evident, therefore, that a gain in body-weight is of real service only when it indicates increase of material other than water, and yet a large majority of the fluctuations in the body-weight can be attributed to material changes in the water-content of the body. When it is considered that some 60 per cent of the total weight of the body is water, and this water is easily lessened or increased, it can be seen that the gain or loss of a few pounds by the body may be very largely due to fluctuations in water-content and in no wise gives a true idea of the addition to or loss from the store of organic body-material.
This fact has frequently been the source of considerable error in experiments of short duration, in that a diet manifestly inadequate for maintenance has actually been partaken of for 3 or 4 days while the body-weight of the subject of the experiment has remained practically unchanged. As a result of this apparent constancy in body-weight, a diet obviously deficient has been thought ample for the needs of the body.
Influence of Diet on Body Weight
Nearly all experimental diets differ widely from the normal. In some diets there is a large preponderance of carbohydrates, or of fat; occasionally there is a large proportion of protein. Rarely is an experimental diet so evenly adjusted as to correspond exactly to the diet on which people commonly live. A striking series of experiments has demonstrated very clearly that a change from a diet poor in carbohydrates to one rich in carbohydrates is accompanied by a considerable retention of water by the tissues of the body.
Conversely, it is shown that when a change is made from the rich carbohydrate diet and a fat diet is substituted, there is a considerable loss of water to the body. It is obvious, therefore, that if a change is made from a normal diet to one containing an excessive proportion of carbohydrates, even though the total nutrients in the food may be insufficient for the maintenance of the body, the excess carbohydrates may cause the retention in the body of a sufficient amount of water to more than make up for the loss in body-material resulting from the decrease in the total food-supply.
Moreover, the body must draw upon its body-material, chiefly fat, and with a diet such as is under discussion, the loss of 100 grams of fat, furnishing some 900 calories of energy, may be compensated by the addition of 100 grams of water to the body. These facts will not be seen from a mere observation of change in body-weight, and one must be very careful in drawing deductions from such change charges, particularly in experiments of short duration.
Effect of Transition
The influence of marked changes in diet upon the body-weight has been shown in connection with a series of experiments conducted in the laboratory of Wesleyan University. 1 The diet of the subject in this series was for three days largely carbohydrate. It was then suddenly changed to a diet having equal energy which, however, was derived in large part from fat.
The changes in body-weight during the series were most remarkable and interesting. The series consisted of work experiments, and the amount of energy in the diet was, therefore, large. During the carbohydrate period there were ingested about 970 grams of solid matter each day and sufficient water in food and drink to make the total weight of food and drink about 4500 grams per day. During 3 days on this diet, the body-weight as determined by a platform balance increased on the average 61 grams per day. The more accurate determinations of the gains and losses of body-material calculated from the amounts of protein, fat, carbohydrates, water, and ash katabolized showed an average gain of 88 grams per day.
On the fourth day of the series, the diet was so changed that the greater part of the energy came from the fat rather than the carbohydrates. The fat diet contained about 750 grams of solid matter and sufficient water to make the total weight of the materials ingested equal 3860 grams on the first day and 4900 grams on each of the other 2 days, the average for the 3 days being 4550 grams per day.
Although the total weight of food and drink ingested during the fat period was somewhat greater than during the carbohydrate period, there was actually a very marked loss to the body, averaging 914 grams per day, as determined by the balance with which the subject was weighed. The computations of the gains and losses of protein, fat, carbohydrate, water, and ash showed an average daily loss of 974 grams.
That this loss in weight was in large part water is shown by an examination of the data, by which it is seen that during the fat period there was an average loss from the body of 12 grams of protein, 47 grams of fat, 2.5 grams of carbohydrate, 906 grams of water, and 7 grams of ash per day. Even on the third day of the period the results show a loss of some 800 grams of water.
Although there are differences between the balance as found by means of the actual weights of the subject and that calculated from the materials gained or lost, it is reasonable to suppose from a careful inspection of the analytical data, that the losses in weight as computed from the gains and losses of material are somewhat more accurate than those obtained from the weights of the man. The apparatus then in use for weighing the man, while satisfactory for long experiments, has since been much improved, as its accuracy was by no means all that could be desired in studying such a problem as this. Fortunately, we have the direct chemical data indicating the katabolism of the protein, fat, carbohydrates, water, and ash.
The total energy furnished by both diets was substantially the same and the amount of external work performed was identical in both experiments. The heat eliminated by the body was on the average 70 calories larger in the experiment with the fat diet. It is significant that during the 6 days with the two diets, the body sustained a continuous loss of energy approximating 500 calories per day, and that during the 3 days with the carbohydrate diet the subject gained on the average about 60 grams per day.