1. Four normal adults were given [15N]glycine in a single dose either orally or intravenously. Rates of whole-body protein turnover were estimated from the excretion of 15N in ammonia and in urea during the following 9 h. The rate derived from urea took account of the [15N]urea retained in body water. 2. In postabsorptive subjects the rates of protein synthesis given by ammonia were equal to those from urea, when the isotope was given orally, but lower when an intravenous dose was given. 3. In subjects receiving equal portions of food every 2 h rates of synthesis calculated from ammonia were much lower than those from urea whether an oral or intravenous isotope was given. Comparison of rates obtained during the post-absorptive and absorptive periods indicated regulation by food intake primarily of synthesis when measurements were made on urea, but regulation primarily of breakdown when measurements were made on ammonia. 4. These inconsistencies suggest that changes in protein metabolism might be assessed better by correlating results given by different end-products, and it is suggested that the mean value given by urea and ammonia will be useful for this purpose.

BackgroundCombinations of foods that provide suitable levels of nutrients and energy are required for optimum health. Currently, however, it is difficult to define numerically what are ‘suitable levels’.ObjectiveTo develop new metrics based on energy considerations—the Nutrient Balance Concept (NBC)—for assessing overall nutrition quality when combining foods and meals.MethodThe NBC was developed using the USDA Food Composition Database (Release 27) and illustrated with their MyPlate 7-day sample menus for a 2000 calorie food pattern. The NBC concept is centered on three specific metrics for a given food, meal or diet—a Qualifying Index (QI), a Disqualifying Index (DI) and a Nutrient Balance (NB). The QI and DI were determined, respectively, from the content of 27 essential nutrients and 6 nutrients associated with negative health outcomes. The third metric, the Nutrient Balance (NB), was derived from the Qualifying Index (QI) and provided key information on the relative content of qualifying nutrients in the food. Because the Qualifying and Disqualifying Indices (QI and DI) were standardized to energy content, both become constants for a given food/meal/diet and a particular consumer age group, making it possible to develop algorithms for predicting nutrition quality when combining different foods.ResultsCombining different foods into composite meals and daily diets led to improved nutrition quality as seen by QI values closer to unity (indicating nutrient density was better equilibrated with energy density), DI values below 1.0 (denoting an acceptable level of consumption of disqualifying nutrients) and increased NB values (signifying complementarity of foods and better provision of qualifying nutrients).ConclusionThe Nutrient Balance Concept (NBC) represents a new approach to nutrient profiling and the first step in the progression from the nutrient evaluation of individual foods to that of multiple foods in the context of meals and total diets.

The impact of ingesting various foods on psychological and behavioural functions is a topic of both interest and concern to the general public. In this article, the scientific literature concerning demonstrated cause-and-effect relationships is reviewed, beginning with methodological considerations specific to the quantification of particular behaviours and psychological events. The essential function of food is to satisfy hunger and the need for essential nutrients. The contributions of macronutrients to appetite and satiety are described, as well as their impact on metabolism and energy balance. Functional properties of macronutrient substitutes (high intensity sweeteners, fat replacers) and flavour enhancers are examined in relation to their contribution to hunger, satiety, and energy balance. The effects of foods and individual nutrients on the performance of diverse psychomotor tasks are studied with consideration given to the various validated quantitative tools used to assess behaviour. The effects of food components on activation, sedation, and affective states such as dysphoria are also reviewed, with special attention given to brain function and neuroactive substances such as serotonin and the endorphins. The case of hyperactivity in children is given special emphasis with reference to the potential influence of sugar and food additives. Safety issues related to food constituents and additives are discussed. Finally, a set of criteria is proposed for the evaluation and elaboration of studies in the behavioural and psychological fields, along with suggestions for future research.

The rate of protein synthesis in the whole body was measured in one fed subject with seven 15N-labelled amino acids (intravenous and oral doses) and two 15N protein mixtures (oral doses only). The rates were determined individually from the urinary excretion of ammonia and total urea over a 12 h experimental period. Except with oral glycine and alanine, the synthesis rates given by ammonia and urea were appreciably different within each study when calculated on the assumption of a single pool of metabolic nitrogen in the body. In general, intravenous administration of the tracers gave higher rates with urea and the oral route gave higher rates with ammonia. The differences between intravenous and oral doses of 15N could be reduced significantly by calculating synthesis rates from either the arithmetic or harmonic average of flux rates given by ammonia and urea. The averages correspond to estimates of the total flux in a two-pool model of metabolic nitrogen when it is assumed either that both pools receive an equal amount of tracer (arithmetic) or that both have the same rate of nitrogen turnover (harmonic). By so reducing the effect of physical separation of nitrogen in the body, the metabolic aspects of compartmentation of the tracer could be examined. The results show that the absolute value obtained for protein synthesis depends on the source of labelled nitrogen. The data are discussed in this empirical context.

The effect of 100 mumol of leucine on protein synthesis in several tissues was assessed in the intact rat. Leucine had no immediate effect on protein synthesis in gastrocnemius muscle, heart, jejunal serosa, jejunal mucosa or liver in rats which were fed, starved for 2 days or deprived of dietary protein for 9 days. Leucine treatment for 1 h also failed to stimulate protein synthesis in tissues of 2-day-starved animals.