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 physiological responses as functions of a limiting dietary nutrient. Physiological responses also vary with time, and, by graphing each parameter of the SKM as a function of time, the model equation can be used to produce a three-dimensional response surface, allowing the investigator to predict requirement as a function of both dietary nutrient concentration and time. To test this hypothesis, rats were fed diets containing graded levels of indispensable amino acids (0-10 g/100 g). The inhibition form of the SKM was able to predict the complete response range of weight gain, food intake, weight-specific weight gain and weight-specific food intake for each amino acid on a day-by-day basis. With a complete response range established, nutrient requirements (defined as maximum responses) could be determined explicitly by the equation: Requirement = (K0.5.KS)0.5, where K0.5 and KS are parameters of the SKM. No arbitrary decisions (such as break points, slopes, percentages of maximum, etc.) were necessary to accomplish this. This approach allows one to set nutritional requirements and optimize responses in a dynamic system without causing inhibiting and/or toxic responses. In each case (valine, methionine, threonine and histidine), the dietary amino acid 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 not.