Lawrence Silver

The caloric value of labile body tissue in obese subjects.

The caloric balance normally varies from moment to moment. Food is taken intermittently, absorbed, stored, and used as needed for the continuous requirements of metabolism. Variations of diet and of physical activity on successive days cause temporary inequalities of energy intake and output. Long-term balance requires some system to smooth these fluctuations. In the present work we have endeavored to define one aspect of the system by correlating small changes in body weight with known variations of caloric intake. The ratio of these two quantities -the caloric equivalent of labile body weightsets an upper limit to the energy value of primary storage tissue. The caloric equivalent would be easy to measure if all changes in weight were due to caloric imbalance. In reality, however, various other processes-menstrual changes or venous congestion, for instance-cause body fluid to vary without relation to the caloric status of the individual. Any single measurement of energy balance and weight change fails to define the caloric equivalent because it provides no means to distinguish a specific change of weight from an accidental fluctuation of water balance. Measurement of the total fluid balance cannot resolve the difficulty, however, because a considerable amount of water is held intracellularly with glycogen and protein. If these substances are used for temporary storage of energy their accumulation and discharge will cause an obligate variation of fluid balance, quantitatively related to each caloric change, and thus part of the caloric equivalent. Possibly the volume of extracellular fluid also varies with the caloric response. In any case, the distinction between caloric and noncaloric changes of hydration can be made only by repetition of the measurement, since a specific response to caloric variation is defined solely by the fact of its consistent occurrence. Standard methods for measurement of heat production-a calorimeter room (1) or an exact determination of water loss (2)-would be inconvenient for use in prolonged experiments such as are required by repeated changes of dietary intake. Fortunately, the average energy balance and the caloric equivalent can be measured more easily, and with greater accuracy, by an analysis of the weight response alone. Two known levels of dietary intake are equivalent in this case to the usual pair of data: one known level of intake and the measured rate of heat production. An additional advantage of the method is that it measures the caloric equivalent and the average daily requirement for maintenance under conditions that simulate normal variations of diet.