In judging of the adequacy or inadequacy of a diet, transitory variations in body-weight should not be emphasized unduly. The body contains a large proportion of water, which may be very rapidly lost or gained. In pathological cases, with edema, very large amounts of water may be stored in the body. With normal individuals, the most striking examples of variations in weight which are without significance as indicating actual loss of body-tissue occur with athletes during severe muscular exercise. Professor William G. Anderson,1 of the Yale University Gymnasium, states that a football player in 1 hour and 10 minutes of exercise lost 6.4 kg. It is not uncommon for Marathon runners in 3 hours of running to lose 3.9 kg., while a member of a college boat crew in a 22-minute race is known to have lost 2.5 kg. An analysis of this loss, based upon the known metabolic activities during severe muscular work, shows clearly that in so short a period as 22 minutes it would be utterly impossible to have disintegrated 2.5 kg. of either protein, fat, or carbohydrate. In experiments with a bicycle rider riding to the limit of human endurance, with a runner, or with a man walking, it has been found that 200 grams of carbon dioxide per hour represent approximately the maximum excretion. This would correspond to not far from 100 grams of dry organic material, or about one-quarter pound. In instances like these the body-weight is very rapidly regained inside of 48 hours, showing that the large changes in body-weight were chiefly due to fluctuations in water content of the body.

1 Benedict and Joelia. Carnegie Inst. Wash. Pub. No. 176, 1912. p. 96.

With the average individual leading either a sedentary or a moderately active life the same holds true, although in less degree. Frequent weighing on delicate scales throughout a 24-hour day shows a tendency for the body-weight to fall, with recovery when food or drink is taken, and sharp loss when urine or feces are passed.1 Hence fluctuations in body-weight that appear from day to day have practically no significance, and only the average weight for a week or more is of practical value. A progressive loss in weight extending over 7 or more days indicates that drafts are being made upon organized body-tissue and not simply upon the water content of the body. Failure to recognize this fact has frequently led to erroneous assumptions regarding the adequacy or inadequacy of a given diet. An individual may not vary in body-weight or may even increase in weight and yet actually lose body-substance because of an inadequate diet. This can be readily seen when we consider that with diets predominatingly carbohydrate there is a strong tendency for the body to retain water, while with diets predominatingly fat there is a distinct tendency for the body to lose water.2 Under these circumstances the use of the body-weight as an index of the adequacy of the fuel value of the diet is open to grave criticism - a criticism that can be overcome only by continuing the observation throughout several days, if not weeks.

The fact that the large majority of adult individuals retain their average body-weight constantly for long periods of time is, as stated, 'prima facie evidence of the adequacy of the diet from the fuel standpoint, and a strong indication that the appetite instinctively adjusts the intake to the needs. Accordingly, since there is this automatic adjustment of intake to needs, we are primarily interested in the need for fuel rather than in the actual food intake and are particularly interested in the fuel demands of the ordinary individual, what determines them, and to what extent, if any, they may be decreased. In addition to the abstractly scientific side of this problem we had at the time of the investigation a question of tremendous and immediate national importance, since any scientific study of the fuel needs of the body which would contribute towards the solution of the possibility of decreasing the need for fuel required immediate investigation. The fuel need of the body may be considered from one point of view as that required for the maintenance of vital activity at its lowest ebb under normal conditions, i. e., "basal metabolism," plus the fuel required for extraneous muscular activity. The latter varies with the external activity of the individual, for obviously the sedentary college professor requires much less energy in his diet than the Canadian lumberman.

1 See Benedict and Joslin, Carnegie Inst. Wash. Pub. No. 176, 1912, fig. 1, p. 90.

2 Benedict and Milner, U. S. Dept. Agr., Oflfice Exp. Sta. Bull. 175, 1907, p. 225. Also cited by Benedict and Joalin, Carnegie Inst. Wash. Pub. No. 176, 1912, p. 92.

Since basal metabolism has not, at present, known, definite relationships to the state of well-being and the efficiency of the organism, other criteria for judging the adequacy of the state of nutrition must be employed. The condition of the fundamental physiological processes and of the more complex neuro-muscular processes which have a most intimate bearing upon muscular and mental work are determinable by well-attested laboratory techniques and in any comprehensive investigation should be considered.