Lactational efficiency complex of rats: provisional model for interpretation of energy balance data

In experiments to determine maintenance requirements and partial efficiencies of conversion of diet and body tissue to milk, correlations between independent variables interfered with multiple regression procedures usually used in analyzing nutritional energetic data. Therefore, an alternative of a largely deterministic model of energy transformations in lactating rats was developed. Food intake, initial and final body weights, and diet composition were inputs to the model. These inputs were partitioned among the several metabolic functions of lactating rats within the model, and estimates of milk energy, heat increment of production, energy used for maintenance, and heat increment of maintenance were computed. The model was validated with rats and diets not used in model development. Inferences were: (a) average efficiency of body energy conversion to milk is 83%; (b) average gross and net efficiencies of milk production on balanced rations are 57 and 80%; and, (c) maintenance requirements vary as a function of food intake. A logistic function relating maintenance to food intake was developed based on the postulate that changes in intake of food cause changes in weights of several vital organs changing the maintenance requirement. This postulate explains, in part, changes in maintenance requirements during long food restriction and during gestation and lactation.