DETERMINATION OF NUTRITIONAL-REQUIREMENTS IN RATS - VARIATION WITH TIME OF WEIGHT-GAIN RESPONSES TO INDISPENSABLE AMINO-ACIDS

The Saturation Kinetics Model (SKM) can be used to describe physiologi cal responses as functions of a limiting dietary nutrient. Physiologic al responses also vary with time, and, by graphing each parameter of t he SKM as a function of time, the model equation can be used to produc e a three-dimensional response surface, allowing the investigator to p redict requirement as a function of both dietary nutrient concentratio n and time. To test this hypothesis, rats were fed diets containing gr aded levels of indispensable amino acids (0-10 g/100 g). The inhibitio n form of the SKM was able to predict the complete response range of w eight gain, food intake, weight-specific weight gain and weight-specif ic food intake for each amino acid on a day-by-day basis. With a compl ete response range established, nutrient requirements (defined as maxi mum responses) could be determined explicitly by the equation: Require ment = (K0.5.K(S))0.5, where K0.5 and K(S) are parameters of the SKM. No arbitrary decisions (such as break points, slopes, percentages of m aximum, etc.) were necessary to accomplish this. This approach allows one to set nutritional requirements and optimize responses in a dynami c system without causing inhibiting and/or toxic responses. In each ca se (valine, methionine, threonine and histidine), the dietary amino ac id concentration required for maximal weight gain exceeded current NRC recommendations, which may be low because they fall in the more time- dependent portion of the response curve whereas maximum responses do n ot.