Daniel Tomé, Cécile Bos, Dietary Protein and Nitrogen Utilization, The Journal of Diet, Quantity 130, Difficulty 7, July 2000, Pages 1868S–1873S, https://doi.org/10.1093/jn/130.7.1868S
ABSTRACT
The dietary worth of dietary proteins in people depends upon their capacity to satisfy nitrogen and amino acid necessities for development and upkeep, however the degree and sort of those necessities stay unclear in people (Millward and Pacy 1995, Munro 1969, Rennie et al. 1994, Younger and Pellett 1988). Ranges of physique protein stay comparatively fixed all through grownup life, regardless of qualitative and quantitative variations in dietary protein consumption. Excessive protein diets present as a lot as 180–200 g/d in adults, whereas the minimal consumption is at present evaluated at 50 g/d. Beneath these circumstances, reaching a nitrogen stability includes variations that could be related to hostile metabolic penalties (Younger 1986, Younger et al. 1987). It’s effectively accepted that the dietary worth of proteins could differ considerably. Variable elements embody their important amino acid content material and digestibility. Primarily based on the totally different strategies used to evaluate dietary protein high quality in people, the protein digestibility–corrected amino acid rating (PDCAAS)3 takes account of those two parameters with respect to amino acid necessities (FAO/WHO, 1990). Nevertheless, the definition of protein and amino acid necessities is complicated and makes it troublesome to evaluate the dietary worth of proteins by utilizing scores as a way of predicting web protein utilization.
Nitrogen stability and nitrogen necessities
Nitrogen necessities embody what is required for each tissue protein synthesis and the manufacturing of a number of nitrogenous compounds concerned in a spread of capabilities (hormones, neurotransmitters, immune competence and peroxidative defenses). Beneath particular physiological and dietary circumstances, the dietary necessities for protein, amino acid and nitrogen are decided by the character of the metabolic demand that have to be glad. An analysis of protein high quality should due to this fact consider the totally different processes concerned in amino acid and nitrogen homeostasis. The achievement of nitrogen homeostasis includes a posh sequence of adjustments in charges of complete physique protein turnover, amino acid oxidation, urea manufacturing and nitrogen excretion throughout the fasting, fed, postprandial and postabsorptive durations of the day.
Human nitrogen necessities are normally decided from the nitrogen stability. The same old process is to regress nitrogen stability on consumption and to outline the requirement because the consumption degree that will produce a zero stability, i.e., equality of dietary N consumption and N losses (Fig. 1). Nitrogen losses happen in numerous methods. They primarily come up from urinary losses within the type of urea, ammonia and creatinine but additionally within the type of fecal and miscellaneous losses (Calloway and Margen 1971) (Desk 1). Minimal nitrogen losses [“obligatory nitrogen losses” (ONL)] had been measured in topics fed a protein-free weight loss program for 1 week. Beneath these circumstances, nitrogen losses had been estimated at 36 mg/kg/d in urine, 12 mg/kg/d in feces and eight mg/kg/d in miscellaneous nitrogen losses (sweat, sebum, desquamations, nails, hairs and saliva) (Manatt and Garcia 1992). Compulsory oxidative losses and indispensable ileal amino acid losses have been reported to achieve 162 and 18 mg/kg/d, respectively (Fuller et al. 1994, Younger et al. 1989) (Desk 2). We decided complete ileal nitrogen losses as reaching 9 mg/kg/d, i.e., 16% of ONL. Indispensable ileal amino acid losses symbolize ∼10% of compulsory oxidative losses. From these estimations, ONL symbolize 54 mg/kg/d and correspond to a protein requirement degree of 0.34 g/kg/d (FAO/WHO, 1985). As a result of dietary protein utilization doesn’t obtain 100% effectivity, it has been recommended that an consumption of 0.6 g/kg/d of well-balanced protein will obtain a zero nitrogen stability. The adequacy of this weight loss program has been reported in research performed over 2- or 3-mo durations (FAO/WHO 1985). A security coefficient is added to this determine in order that the ultimate advice for dietary protein is 0.75 g/kg/d. In kids, the necessities for development have to be built-in along with upkeep necessities.
It is very important emphasize that theses values symbolize the minimal really useful protein consumption. Research investigating the metabolic response to totally different protein intakes have thought-about the power of the physique to make metabolic changes to a variety of protein intakes (0.75–2 g/kg/d). As well as, there are numerous limitations to figuring out the nitrogen stability, and as not too long ago identified (Rand and Younger 1999): “Nitrogen balance estimates are highly dependent on the assumed amount of N miscellaneous losses… further studies on these losses and on the factors that influence them are essential.” First, there’s a slight distinction between massive values for N consumption and N losses. Second, it’s effectively acknowledged that the nitrogen stability method overestimates N consumption and underestimates N losses. That is primarily as a result of problem in assessing N fuel losses after denitrification by the colonic microflora, N losses via the pores and skin (urea) and expired air (ammonia) and the nitrate content material in meals and urine, which isn’t measured utilizing the Kjeldahl technique.
Will increase in protein consumption and nitrogen stability
Folks typically eat extra protein than the theoretical requirement based mostly on nitrogen stability estimations. The impact of an elevated protein consumption on the entire physique nitrogen stability and protein turnover have to be decided. Specifically, it is rather vital to elucidate the results of accelerating the nitrogen consumption as regards to totally different nitrogen pathways. The kind of protein and nitrogen swimming pools more likely to be modified by the extent of nitrogen consumption additionally have to be clarified. This results in the query of the optimum protein consumption and the chance that this optimum degree could also be increased than present suggestions (Millward, 1999).
Rising the protein consumption induces a sequence of adaptive processes (Fig. 2). Probably the most conspicuous adjustment is a rise in amino acid oxidation and in subsequent nitrogen excretion, primarily as urea and particularly pronounced within the fed state. There’s a pattern towards a rise within the nitrogen stability when nitrogen intakes are elevated (Value et al. 1994). Adjustments within the protein stability primarily outcome from the improved inhibition of protein breakdown by feeding and, to a lesser extent, from a rise in protein synthesis (Forslund et al. 1999) (Fig. 3). Complete physique protein synthesis might be delicate to amino acid availability, whereas degradation could also be delicate to an interactive impact by each the amino acid degree and insulin. The amplitude of diurnal physique protein biking will increase, with no clear change within the imply day by day protein turnover price (Fig. 4) (Pacy et al. 1994). As well as, excessive protein intakes have been proven to lead to a steady, constructive N stability of 1–3 g N/d, as proven in Desk 3(Cheng et al. 1978, Fisher et al. 1967, Forslund et al. 1999, Oddoye and Margen 1979, Value et al. 1994). Nevertheless, it isn’t clear whether or not this obvious retention is actual or originates from intrinsic errors in calculation of the N stability.
This can be very vital to find out whether or not the lean physique mass could be elevated by rising the protein consumption. Some current outcomes have strongly recommended that there in all probability is proscribed anabolic stimulation by excessive protein diets in wholesome younger adults. In distinction, this subject is especially related in populations comparable to aged individuals, athletes or sufferers with protein-energy malnutrition. We studied the opportunity of inducing an anabolic response by rising the protein consumption in reasonably malnourished, aged sufferers. Our outcomes confirmed {that a} shift from a spontaneous protein consumption of 0.9 g protein/kg/d to 1.4 g protein/kg/d over a 10-d interval utilizing dietary dietary supplements was related on this inhabitants with a rise in each the fat-free mass and web protein synthesis, as measured utilizing the 15N-glycine end-products technique in each the postabsorptive and fed states (Desk 4) (Bos et al. 2000). The potential website of deposition underneath each acute and persistent feeding circumstances stays to be decided. It’s also unclear whether or not rising the dietary protein degree causes a generalized improve in protein turnover. Adaptability might be enhanced if the turnover was extra fast.
Dietary protein nitrogen distribution throughout postprandial section
Dietary protein nitrogen and amino acids go transiently via the nitrogen metabolic swimming pools of the physique. It is very important contemplate the diurnal cycle of feeding and fasting durations that leads to postprandial dietary nitrogen beneficial properties and postabsorptive losses of physique proteins (Millward et al. 1974). Acute nitrogen deposition throughout the postprandial section is more likely to be significantly crucial when it comes to the deposition of dietary protein within the tissues. Evaluation of the postprandial utilization of dietary proteins is an applicable strategy, as a result of this parameter is thought to affect protein turnover (Marchini et al. 1993).
Research have been undertaken to evaluate the acute postprandial utilization of dietary protein throughout the repletion section of the diurnal cycle. The important thing steps in regards to the destiny of dietary nitrogen are thought-about to be i) the quantity of nitrogen really absorbed; ii) the quantity that has been deaminated and recovered, primarily within the type of urea; and iii) the extent of nitrogen retained within the physique. The issues of measuring the postprandial utilization of dietary protein nitrogen when it comes to ileal nitrogen digestibility and short-term retention of dietary protein nitrogen could be circumvented by means of 15N-labeled proteins. This system makes it potential to comply with the metabolic destiny of dietary nitrogen after its ingestion in people (Bos et al. 1999, Gaudichon et al. 1999, Gausserès et al. 1996, 1997, Mahé et al. 1992, 1994, Mariotti et al. 1999). Taking into consideration the totally different outcomes obtained relating to evaluations of complete nitrogen and protein metabolism in grownup people consuming 100–110 g/d of a well-balanced protein weight loss program, of the 300 g/d protein turnover, we measured that 75–80 g was misplaced via the oxidative pathways and 14 g was misplaced on the ileal degree (Fig. 5). The contribution of dietary nitrogen to the principal pathways was evaluated at 70–80 and 13–20 g/d, i.e., 30–40% and 17–25%, to anabolism and oxidative loss fluxes, respectively. This strongly suggests a preferential orientation of dietary nitrogen towards anabolic pathways. This preferential orientation towards physique protein synthesis is as a result of adequacy of the dietary protein amino acid profile relating to the physique protein and the compartmentation of protein metabolism. Indispensable amino acids equipped by dietary proteins equilibrate the free amino acid pool: the first-pass metabolism of dietary amino acids is especially associated to the splanchnic metabolism, whereas amino acids launched from the peripheral metabolism are used within the catabolic pathways (i.e., alanine, glutamine).
To enhance our understanding of the acute phenomena that happen after dietary nitrogen ingestion and as a consequence of restricted entry with the compartments of curiosity in human experiments, a compartmental modeling strategy may also be used. Compartmental modeling permits simulation of the distribution of exogenous nitrogen within the main physique nitrogen swimming pools (together with these not experimentally monitored) based mostly on experimental measurements. This instrument additionally permits a prediction of the long run evolution of the system (Fouillet et al. 2000). This has been made potential by the event and validation of an 11-compartment mannequin that makes a specific distinction between free and protein-bound amino acids in each the splanchnic and peripheral areas, to explain the cascade of transient metabolic processes that management the distribution of exogenous nitrogen all through the physique. The outcomes obtained by modeling the sample of dietary nitrogen distribution into the totally different physique compartments after the ingestion of a protein meal is an useful gizmo to allow additional definition of the notion of protein high quality in a interval of protein acquire as a result of it simulates the relative capacity of a protein supply to advertise dietary nitrogen retention in numerous organs. It may also be used to discriminate between totally different dietary circumstances (kind of the protein ingested, power content material of the meal) and to explain the processes concerned within the differential metabolic utilization of assorted protein meals.
Nitrogen metabolism and dietary protein traits – “does protein have nitrogen”
The basic strategy to measurement of web nitrogen retention is normally based mostly on nitrogen stability information measured in topics after adaptation to totally different protein ranges over durations of a number of days (Millward and Pacy 1995, Munro 1964). Our present information is considerably restricted. We all know that diets containing poor high quality protein are related to a rise in nitrogen losses as a result of inefficient utilization of indispensable amino acids. It’s essential to make clear which proportions of dietary and intestinal nitrogen are absorbed as amino acids or excreted within the feces, urine or different pathways and, most vital, to find out which portion is utilized for protein synthesis and retained in physique proteins.
One of many main limitations to using the basic nitrogen stability technique is the existence of diurnal biking for a transition between the fasted and fed states, which leads alternately to nitrogen postprandial accretion and postabsorptive loss phases. Due to this, retention calculated every day is decrease than that derived from the postprandial section (Millward et al. 1974), and underneath these circumstances, dietary protein utilization calculated because the day by day acquire needs to be decrease than the postprandial acquire. Moreover, the connection between protein traits and protein consumption requires further examine. It has been demonstrated that variations within the gastric emptying price of dietary proteins are related to extremely variable charges of amino acid absorption within the small gut (Gaudichon et al. 1994, Mahé et al. 1996). These variations are additionally related to vital variations in amino acid oxidation and nitrogen postprandial accretion (Boirie et al. 1996).
Strategies based mostly on digestibility and short-term protein retention are of curiosity when trying on the short-term utilization of dietary proteins, however few protein retention values can be found in people. Internet postprandial protein utilization (NPPU) is calculated utilizing true ileal digestibility and true 15N-labeled protein deamination parameters and including the dietary nitrogen collected within the urine and that retained within the physique within the type of urea, as follows:
Utilizing this strategy, we calculated NPPU values of 80 and 72% for milk protein and soy protein, respectively, measured throughout the 8 h after the ingestion of a regular meal by wholesome human topics (Fig. 6). These information strongly counsel the existence of sure variations between the dietary worth of proteins. These variations needs to be taken into consideration when calculating amino acid scores. In keeping with the tactic used at current, PDCAAS values which can be >1 are rounded off to 1, based mostly on the argument that (digestible) important amino acid concentrations in a protein that exceed these within the reference amino acid sample don’t present any further dietary worth. The truth might be extra complicated, and one of the best strategy would in all probability be to match totally different calculations of PDCAAS values with out there in vivo outcomes obtained in people. Research are in progress to find out each the ileal digestibility and metabolic destiny of the person 15N-amino acid ingested from milk, soy and wheat protein and to measure the NPPU of those proteins in topics tailored to regular (1 g/kg/d) or excessive (2 g/kg/d) protein ranges of their weight loss program. As proven in Determine 7, preliminary outcomes point out that each the protein supply and the recurring degree of protein degree consumption affect the effectivity of postprandial dietary nitrogen accretion. As anticipated, the principal variations come up from modulations to the splanchnic destiny of nitrogen. A protein supply–dependent distinction in interorgan amino acid metabolism has additionally been described in pigs after the infusion of both soy or casein (Deutz et al. 1998).
Protein and nitrogen homeostasis is achieved through a posh sequence of adjustments to the charges of complete physique protein turnover, amino acid oxidation, urea manufacturing and nitrogen excretion that happen throughout the postprandial and postabsorptive durations of the day. The power of people to adapt to diets containing a variety of protein ranges have been the topic of appreciable examine, and tips have been drawn up relating to the minimal ranges required to take care of well being within the normal inhabitants (FAO/WHO, 1985). Though it’s clear that grownup people in numerous cultures survive on a broad vary of protein intakes (from 0.6 g/kg/d to ≥5 occasions that quantity), the results of excessive protein diets are nonetheless poorly understood. Thus, the importance of amino acid oxidation nonetheless requires clarification when it comes to their roles as vitamins used to supply power. The power of excessive protein diets to extend nitrogen retention and the protein turnover price stays unclear. So far as an evaluation of protein high quality is worried, it appears vital to think about the extent of protein within the weight loss program. From a qualitative perspective, the usual strategies used to judge protein primarily allow a discrimination between poor (unbalanced) and top quality protein diets. A comparability of comparatively well-balanced protein diets stays troublesome. Nevertheless, in vivo strategies can be found that allow the correct measurement of smaller variations between protein sources and the affect of different elements (different vitamins within the meal, recurring protein consumption) on nitrogen metabolic utilization. The postprandial retention of dietary N may due to this fact symbolize a reference technique for additional validation of the PDCAAS technique.
LITERATURE CITED
Abbreviations
“does protein have nitrogen”