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

Partition of Nitrogen Excretion in Urine and the Feces of Holstein Replacement Heifers

Increasing public concern has been focused on animal production systems as a major nonpoint source of pollution. These studies were conducted to further our understanding of whole-animal N metabolism, N excretion, and its partition between feces and urine in growing dairy heifers. Isocaloric diets [2.31 Mcal of metabolizable energy (ME)/kg of dry matter (DM)], ranging from 12.4 to 34.2 g of N/kg of DM, were fed to Holstein heifers in 2 experiments at approximately 1.8 times maintenance. Diets were formulated to provide 54 to 143% of the ruminal ammonia requirements as predicted by the Cornell Net Carbohydrate and Protein System. Increasing the N content of the diet increased urinary N excretion and N balance, but did not affect fecal N excretion. Holstein heifers fed low N diets were able to maintain growth rates consistent with current recommendations while at the same time reducing N excretion, in particular nitrogenous compounds that are readily converted to ammonia. However, more research is needed before this type of diet is recommended for growing heifers because of possible changes in body composition that may affect future milk production and performance.

Statistical analysis of nitrogen balance data with reference to the lysine requirement in adults

Statistical analysis of nitrogen balance data is an important approach to the estimation of human nutrient requirement. The usual procedure is to regress nitrogen (N) balance on intake and to define the requirement as intake that would produce zero balance. In the actual application of this methodology, there are a number of options, and in the present study we explore the sensitivity of the regression approach to those options. To put this problem into a realistic context, we examine the current controversy over the lysine requirements of healthy adults. From early N balance studies, investigators concluded that the mean requirement was generally less than 10 mg. kg(-1). d(-1), whereas based on recent (13)C-tracer and metabolic studies, we propose a tentative mean requirement of approximately 30 mg. kg(-1). d(-1). Jones et al. (1956) conducted careful N balance studies from which they derived an estimate of lysine requirement of less than about 8 mg. kg(-1). d(-1). We reanalyzed these data with different choices of modes of analysis, mathematical models, and different assumptions concerning the magnitude of miscellaneous N losses. We find that for these data the choice of a specific mathematical model has only a small effect on resultant estimates of requirement, while estimated requirements are very sensitive to amount of unmeasured losses that are assumed and how the model is applied (whether the aggregate data are fitted in one pass to a single model, or the data for each individual subject are fitted to that individual's unique model). Moreover, our reanalysis suggests that the population requirement for lysine is in the range of 17 to 36 mg. kg(-1). d(-1) and strongly supports a lysine requirement value of about 30 mg. kg(-1). d(-1). In general, our results indicate that whenever possible, N balance data should be analyzed using a square root model fitted to individual data and that the median of the individual requirements so derived be used as the estimate of population requirement. Moreover, clearly any statistical analyses of N balance data should include a sensitivity analysis to determine the influence of underlying assumptions. Finally, the finding that these estimates are highly dependent on the assumed amount of N miscellaneous losses recommends that further studies on these losses and of the factors that influence them are essential.

Lysine supplementation of cereal foods: a retrospective

During the period 1950-1975, protein and amino acid metabolism and its application to world nutrition problems constituted a major focus of nutrition research. A large part of this research was concerned with the potential benefits to be expected from lysine supplementation of cereal foods, stimulated by the availability, beginning in 1955, of commercial quantities of L-lysine monohydrochloride from industry. Events during this era and lessons learned are reviewed by the author, a participant in the commercialization of lysine.

Lysine deficiency reduces transcription activity and concentration of chromatin proteins reversibly in rat liver

The reversible effect of dietary lysine deficiency was studied in young adult rats. During 6 days on a lysine diet the rats maintained the same body weight. During 2 days of recovery body weight gain was that of the controls. Liver nuclei were isolated, incubated with micrococcus nuclease and chromatin fractionated in to a 2 000 X g pellet, 102 000 X g pellet and supernatant fraction. Chromatin-bound RNA polymerase 1 plus III activity decreased by 15% per mg of fractional and nuclear DNA and by 30% per total liver. The corresponding decrease of RNA polymerase II activity was 30% and 40%. Recovery from lysine deficiency was complete after 2 days of refeeding the amino acid. Chromatin proteins of the 102 000 X g pellet were characterized by polyacrylamide gel electrophoresis in sodium dodecylsulfate and by 2-dimensional gel electrophoresis. Quantitative but no qualitative differences between the proteins of the dietary groups were observed. Relative to DNA the non-histone proteins decreased in the lysine deficient group by 43% and histones by 10%. It is concluded that RNA synthesis is restored to its original level within 2 days of refeeding lysine after 6 days of lysine deficiency.

Experimental studies on the quality of food proteins

1. This paper reviews chemical and biological assays for measuring the quality of food proteins. 2. The availability of amino acids in a protein is determined by digestability and the capacity of the recipient animal to absorb the amino acids. 3. Intracellularly amino acid concentration influences enzyme activities associated with amino acid and nitrogen metabolism. 4. Amino acids utilized for metabolism affect the cells at the gene level: transcription of DNA to RNA and translation of RNA to protein. 5. More accurate measurements of amino acid utilization for metabolism will follow from further understanding of basic cellular metabolic events.

Choline/methionine nutrition of starting broiler chicks. Three models for estimating the choline requirement with economic considerations

Experiments were conducted to test the hypothesis that metabolically labile methyl groups and not methionine per se are limiting the growth of starting broiler chicks fed corn-soy-grease diets formulated to meet NRC (1977) recommendations (except for methionine). The basal diet contained approximately 23% protein, 3200 kcal ME/kg. 32% methionine, .42% cystine, and 1300 ppm choline. In the first two experiments the diets were fed to quadruplicate lots of 10 chicks in battery brooders. Average weight gain (g) and feed per unit gain for supplements of methyl group sources were: 1) methionine: 424, 1.48; 2) choline Cl: 406, 1.49; and 3) betaine HCl: 416, 1.48. These values were not significantly different from one another (P greater than .05) but were significantly different (P less than .05) from the values for the basal (362, 1.57) and from the DL-homocystine supplemented groups (376, 1.56). This indicated that methyl group utilization was not impaired by a lack of the carbon skeleton of methionine. Five 3-week assays with five groups of 10 chicks per treatment (1050 chicks) were conducted to determine the choline requirement using the basal diet without supplemental methionine. Results from the five assays were pooled in computing regressions. Significant regressions (P less than .05) were found for broken-line linear (R2 = .630), quadratic (R2 = .556), and saturation kinetic (R2 = .642) models. The requirement was determined to approximately 2130 ppm by the broken-line linear model. The most economic feeding levels were estimated to be 4100 and 1910 ppm by the quadratic and saturation kinetic models, respectively. The economic implications of the three models are discussed.

Some thoughts on amino acid supplementation of proteins in relation to improvement of protein nutriture

Supplementation of diets with free amino acids has not routinely been beneficial and poses a potential risk. Thus, mixing proteins to increase the quality of the dietary protein is attractive. The slope-ratio technique for evaluation of protein quality has serious drawbacks, because the growth response is not linear and animals appear to adapt to the diet and thus make more efficient use of diets low in protein or low in protein quality. Apparently, the relative value of proteins is not constant, but varies with the amino acid spectrum as well as intake. Thus, evaluation of proteins or protein mixtures should be determined under the conditions (e.g. maintenance or maximum gain) of their potential use. This approach could lead to the development of a protein replacement equivalents system similar to the starch equivalents system in the evaluation of the net energy of animal feeds.