L. Preston Mercer

The determination of nutritional requirements: mathematical modeling of nutrient-response curves.

The Saturation Kinetics Model (SKM) is useful in describing physiological responses as functions of a limiting dietary nutrient. We have recently expanded the SKM to predict the inhibited portions of the nutrient-response curve. By using the SKM, nutrient requirements can be predicted analytically by, requirement = (K0.5 x KS)0.5. It is also possible to set an upper and lower dietary nutrient concentration which encompasses the 100% response range for each response, thereby giving an inhibition or toxicity index. This index allows one to set nutritional requirement levels precisely, optimizing responses without moving into inhibiting or toxic ranges of nutrients. The model equation can also be converted to a three-dimensional representation by graphing each parameter as a function of time. This allows one to visualize a three-dimensional response surface, showing response as a function of time and dietary nutrient concentration.

The problem of human protein requirements: Some kinetic and metabolic considerations

Estimated human protein requirements have been substantially lowered by FAO/WHO expert committees over the past two decades. The estimates and methods of calculation are considered in the light of the kinetics of response to protein intake, body protein turnover, amino acid flows in the body, and the concept of nitrogen (N) steady state. Whereas traditional methods of estimation have assumed an essentially linear (first order) response of N retention to absorbed N, animal studies show that response to graded protein intakes obeys saturation kinetics. Corrections for protein quality have also assumed a linear relation between response and supply of limiting amino acid, while animal experiments indicate that this response likewise follows saturation kinetics. Evidence is lacking that the present minimum protein standards for humans can support acceptable internal nitrogen steady states at any age above infancy or foster normal growth in the child. New research approaches to determination of protein requirements are suggested , including study of the kinetics of human response to graded protein intakes and graded variations of quality; development of indicators of nitrogen steady state and correlation with clinical status; and determination of optimum protein-energy ratios by age and sex.

METABOLIC ADAPTATION TO PROTEIN DEFICIENCY IN RATS: HISTIDINE

Abstract The relationship between serum and brain concentrations of histidine and methionine were detewrmined in rats fed graded levels of protein, histidine and methionine. In each case, changes in tissue levels of the two amino acids were inversely proportional. In protein-deficient rats, histidine increases rapidly in brain and serum while methionine decreases. Glycine and methionine supplementation of protein-deficient diets lessen increases in histidine concentrations while glycine supplementation increases growth of depleted rats placed on normal diets. These findings implicate the pathway for one-carbon metabolism in the large increases seen in histidine levels in protein deficiency.

Protein Nutritional Quality: A Modeling Approach

The protein efficiency ratio (PER) is the official method for protein quality evaluation in the United States and Canada. Two other widely used indices of evaluation are the slope ratio (SR) and net protein ratio (NPR) methods. Each of these methods has problems associated with its calculation and interpretation. In this paper, a new index, actual protein utilization (APU), is discussed and its relationship to the other indices is examined. Each of the indices of protein quality evaluation is given a theoretical basis by relating it to the four parameter mathematical model for physiological responses. The derivation of the four parameter model is given, along with its mathematical and physiological significance. To compare the indices, growth bioassays were carried out using male rats (40g, 60g, 115g) consuming three different proteins, casein, lactalbumin, and soy and an amino acid mixture. Dose-response curves were generated for each diet and the indices PER, NPR, SLOPE, and APU were calculated. APU was shown to have certein characteristics which make it superior to the other methods of protein quality evaluation, i.e., it closely approximates the growth response curve and it incorporates a term for the protein intake required for maintenance.

Brain resistance to protein loss on restricted protein intake in weanling rats

Abstract A multi-disciplinary study involving anatomy, pharmacology, biochemistry, and computer sciences was carried out to determine the effect of protein deprivation on the free amino acid levels in the brain of rats who were protein deprived in the weanling stage. The literature has many references to studies of protein and other dietary deprivation in the dam prior to and during pregnancy and also in the preweanling. Few references are available about protein deprivation effects on the weanling rat. Once the species has reached a stage of neural development where the essential elements for higher function are in place, what happens then to the brain and its function during dietary deprivation, particularly protein? If there is any great change, how are the neurotransmitters, which have been identified, affected? In this experiment, the first of a series, an attempt is made to first identify what changes may occur. Specific patterns of change, especially of the neurotransmitters, were followed in the brain as a whole and in three subdivisions, fore-, mid-, and hind-brain. The results of the biochemical assay indicate that once the brain has reached a stage of relative maturity there is an inherent ability of that organ to resist great and obvious changes through some, as yet unknown, mechanism. It would also suggest that studies of protein deprivation beyond the term of this study are in order, for the overt behavior of the deprived animals was not so greatly different from that of the non-deprived. Also, in terms of behavior, it is interesting to note that the total food intake was directly proportional to the amount of protein in the diet, i.e., the less protein in the diet, the less total food intake or conversely, the more protein in the diet, the more the total intake.

The determination of nutritional requirements: A mathematical modeling approach

The Saturation Kinetics Model (SKM) has been shown to describe the functional relationship between physiologic responses and nutrients fed in graded amounts. The SKM is based on the concept that an organism is characterized by a sequence of homeostatically constrained steady states. Responses are the result of a series of enzymatically mediated steps, one of which is rate limiting and displays saturation kinetics. The model is descriptive of a wide range of physiological responses and the model equation is continuous in its derivatives. This study examines the usefulness of the application of the SKM to the determination of nutritional requirement levels for growth in the young rat, and provides a basis for the rational formulation of complex mixtures of nutrients (diets) which are designed to optimize some measured performance characteristic in an animal. Using the model, theoretical response curves were constructed based on dietary nutrient concentration, daily weight gain (dW/dt) and daily food intake (dF/dt). Theoretical slopes (first derivative) and efficiency curves were also constructed. Parameters derived from the application of the SKM to rat growth experiments were used to formulate a complete dietary amino acid mix for weanling rats. Dietary ratios and concentrations for indispensable amino acids and arginine were calculated using the parameter K.5. A curve-shift technique was used to determine dietary concentrations for conditionally dispensable and dispensable amino acids. The model was also used to determine a dietary ratio of (IAA + Arg)/(CAA + DAA). Using the dietary amino acid concentrations suggested by the model and an (IAA + Arg)/(CAA + DAA) ratio of 1, a growth response curve was constructed and compared to a similar curve using the amino acid mix of Rogers and Harper. The modeling approach produced a 10-15% improvement in growth over the Rogers and Harper mix. The SKM is discussed in terms of calculating an ideal nutrient ratio and choosing of a desired response level. It was demonstrated that the model can rapidly produce accurate estimates for dietary amino acid levels, while minimizing required numbers of vertebrate animals.

The Biosynthesis of Riboflavin: Affinity Chromatography Purification of GTP-Ring-Opening Enzyme

The ability of guanine compounds to serve as precursors of riboflavin has been well established (1,2). This ability was demonstrated in two different organisms in which purine-purine interconversions were blocked to insure retention of radioactive label in the specified molecule. The direct participation of the ribosyl moiety of a guanine nucleoside or nucleotide has not been established. The observation that loss of carbon 8 of guanine precedes riboflavin biosynthesis has led to its comparison with pteridine biosynthesis, which has a similar beginning (3,4). While the ribose moiety of GTP has been shown to be incorporated into pteridines (5) it does not appear to be directly incorporated as the ribityl side chain of riboflavin (6). Therefore, it is not known whether expulsion of carbon 8 of the imidazole portion of GTP represents a common first step in the biosynthesis of both vitamins or the initiation of two independent but similar pathways. (Figure 1.)