Nitrogen excretion in rats on a protein-free diet and during starvation

Nitrogen balances (six days) were determined in male Wistar rats during feeding a diet with sufficient protein or a nearly protein-free diet (n = 2 x 24), and then during three days of starvation (n = 2 x 12). The objective was to evaluate the effect of protein withdrawal on minimum nitrogen excretion in urine (UN), corresponding to endogenous UN, during feeding and subsequent starvation periods. The rats fed the protein free-diet had almost the same excretion of urinary N during feeding and starvation (165 and 157 mg/kg W(0.75)), while it was 444 mg/kg W(0.75) in rats previously fed with protein, demonstrating a major influence of protein content in a diet on N excretion during starvation. Consequently, the impact of former protein supply on N losses during starvation ought to be considered when evaluating minimum N requirement necessary to sustain life.     

Nitrogen excretion in rats on a protein-free diet and during starvation

Nitrogen balances (six days) were determined in male Wistar rats during feeding a diet with sufficient protein or a nearly protein-free diet (n = 2×24), and then during three days of starvation (n = 2×12). The objective was to evaluate the effect of protein withdrawal on minimum nitrogen excretion in urine (UN), corresponding to endogenous UN, during feeding and subsequent starvation periods. The rats fed the protein free-diet had almost the same excretion of urinary N during feeding and starvation (165 and 157 mg/kg W^0.75), while it was 444 mg/kg W^0.75 in rats previously fed with protein, demonstrating a major influence of protein content in a diet on Nexcretion during starvation. Consequently, the impact of former protein supply on Nlosses during starvation ought to be considered when evaluating minimum N requirement necessary to sustain life.     

The effect of high and low dietary crude protein and inulin supplementation on nutrient digestibility, nitrogen excretion, intestinal microflora and manure ammonia emissions from finisher pigs

A 2 × 2 factorial experiment was performed to investigate the interaction between a high- and low-crude-protein (CP) diet (200 v. 140 g/kg) and inulin supplementation (0 v. 12.5 g/kg) on nutrient digestibility, nitrogen (N) excretion, intestinal microflora, volatile fatty acid (VFA) concentration and manure ammonia emissions from 24 boars (n = 6, 74.0 kg live weight). The diets were formulated to contain similar concentrations of digestible energy and lysine. Pigs offered the high-CP diets had a higher excretion of urinary N (P < 0.001), faecal N (P < 0.01) and total N (P < 0.001) than the pigs offered the low-CP diets. Inulin supplementation increased faecal N excretion (P < 0.05) and decreased the urine N : faeces N ratio (P < 0.05) compared with the inulin-free diets. There was no effect (P > 0.05) of dietary treatment on N retention. There was an interaction (P < 0.05) between dietary CP concentration and inulin supplementation on caecal Enterobacteria spp. Pigs offered the diet containing 200 g/kg of CP plus inulin decreased the population of Enterobacteria spp. compared to those with the inulin-supplemented 140 g/kg CP diet. However, CP level had no significant effect on the population of Enterobacteria spp. in the unsupplemented diets. Inulin supplementation increased caecal Bifidobacteria (P < 0.01) compared with the inulin-free diets. There was no effect of inulin supplementation on VFA concentration or intestinal pH (P > 0.05). Pigs offered the 200 g/kg CP diets had higher (P < 0.05) manure ammonia emissions from 0 to 240 h of storage than pigs offered the 140 g/kg CP. In conclusion, inulin supplementation resulted in an increase in Bifidobacteria concentration and a reduction in Enterobacteria spp. at the high CP level indicating that inulin has the ability to beneficially manipulate gut microflora in a proteolytic environment.     

Effect of low protein and low energy diet on physiological status and digestibility of F344 rats.

A long-term raising study was carried out on male F344/DuCrj rats with three low protein (Crude Protein (CP); 14.5, 11.5, 8.5%) and low energy (Digestible Energy (DE); 2.0 kcal/g) diets from 4 to 104 weeks of age. In rats fed the 8.5% CP diet, body weight and digestible crude protein (DCP) consumption at 10 weeks of age were lower (P < 0.05) but the body weight at 50 weeks of age was higher (P < 0.05) than in the other groups. In rats fed the 8.5% CP diet the crude fat digestibility was higher (P < 0.05), and the CP/nitrogen-corrected metabolizable energy (MEn) ratio was low. On the other hand, the mean survival time at 80 weeks of age was shorter in rats fed the 8.5% CP diet (P < 0.05).     

The effect of starvation on oxygen consumption and nitrogen excretion inLepidogalaxias salamandroides (Mees)

Summary Measurement of the rates of oxygen consumption and nitrogen excretion in fed and starvedLepidogalaxias salamandroides showed that starvation has no effect on these basal metabolic parameters. A low metabolic rate (50 ml O₂ kg^–1 h^–1) is suggested to account for the lack of such an effect.A high proportion of the total waste nitrogen is excreted as urea and the magnitude of this fraction is also unaffected by starvation. The importance of urea synthesis is consistent with this species' ability to aestivate.The results are discussed with reference to current concepts in fish aestivation.     

Effects of adding dietary lysine to a low gluten diet on the brain protein synthesis rate in aged rats

The purpose of this study was to find whether the addition of dietary lysine affected the rate of brain protein synthesis in aged rats fed on a gluten diet. Experiments were done on two groups of aged rats (30 wk) given the diets containing 5% gluten or 5% gluten + 0.3% lysine for 10 d. The fractional rates of protein synthesis in brain, liver, and kidney increased with an addition of dietary lysine. In brain, liver, and kidney, the RNA activity [g protein synthesized/(g RNA x d)] was significantly correlated with the fractional rate of protein synthesis. The RNA concentration (mg RNA/g protein) was not related to the fractional rate of protein synthesis in any organ. The results suggest that the addition of the limiting amino acid for the low quality protein elevates the rate of protein synthesis in the brain of aged rats, and that RNA activity is at least partly related to the fractional rate of brain protein synthesis.     

The renal response of sheep to a low dietary nitrogen intake

Renal functions were tested in sheep fed on a low nitrogen diet (LN sheep), with a daily N intake of 4.7 g (gross energy 17.76 . 10(6) J). Sheep given a high nitrogen diet (HN sheep) with 21.2 g N (24.12 . 10(6) J) acted as the control. The functions of the left kidney were measured in anaesthetized animals by the standard clearance technique. A comparison of HN and LN sheep showed that a low nitrogen intake led to a drop in the plasma urea level (from 5.91 +/- 0.35 to 2.87 +/- 0.36 mmol.1-1, (P less than 0.001), the glomerular filtration rate (GFR, from 36.6 +/- 3.6 to 20.7 +/- 2.4 ml.min-1, P less than 0.005), amount of urea excreted (from 106.7 +/- 18.1 to 15.7 +/- 3.3 mumol.min-1, P less than 0.001), fractional urea excretion (from 51.0 +/- 3.0 to 24.6 +/- 3.1 %, P less than 0.001) and the absolute tubular reabsorption of urea (Reaburea/GFR (from 3.06 +/- 0.27 to 2.12 +/- 0.28 mumol.ml-1, P less than 0.05), without a significant change in the effective renal plasma flow (182.6 +/- 20.0 and 138.5 +/- 21.0 ml.min-1, non-significant - N.S.) and in sodium and potassium excretory function. Free water clearance rose in LN sheep (from -0.53 +/- 0.11 to -0.19 +/- 0.06 ml.min-1, P less than 0.05) owing to inhibited urea excretion. A regression analysis of the relationship of the tubular reabsorption of urea to the amount of filtered urea (both normalized to the GFR) showed that the urea transport capacity of the tubules of LN sheep was significantly higher.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of total starvation and very low energy diet in lean men on kinetics of whole body protein and five hepatic secretory proteins

It is unclear whether the rate of weight loss, independent of magnitude, affects whole body protein metabolism and the synthesis and plasma concentrations of specific hepatic secretory proteins. We examined 1) whether lean men losing weight rapidly (starvation) show greater changes in whole body protein kinetics, synthesis, and circulating concentrations of selected hepatic secretory proteins than those losing the same amount of weight more slowly [very low energy diet (VLED)]; and 2) whether plasma concentrations and synthetic rates of these proteins are related. Whole body protein kinetics were measured using [1-(13)C]leucine in 11 lean men (6 starvation, 5 VLED). Fractional and absolute synthetic rates of HDL-apolipoprotein A1 (apoA1), retinol binding protein, transthyretin, alpha(1)-antitrypsin (alpha(1)-AT), and transferrin were measured using a prime-constant intravenous infusion of [(13)C(2)]glycine. Compared with VLED group, the starvation group showed greater increases (at a 5% weight loss) in whole body protein oxidation (P < 0.05); fractional synthetic rates of HDL-apoA1 (25.3 vs. -1.52%; P = 0.003) and retinol binding protein (30.6 vs. 7.1%; P = 0.007); absolute synthetic rates of HDL-apoA1 (7.1 vs. -3.8 mg.kg(-1).day(-1); P = 0.003) and alpha(1)-AT (17.8 vs. 3.6 mg.kg(-1).day(-1); P = 0.02); and plasma concentration of alpha(1)-AT (P = 0.025). Relationships between synthetic rates and plasma concentrations varied between the secreted proteins. It is concluded that synthetic rates of hepatic secreted proteins in lean men are more closely related to the rate than the magnitude of weight loss. Changes in concentration of these secreted proteins can occur independently of changes in synthetic rates, and vice versa.     

The effects of dietary protein:energy ratio and amino acid pattern on nitrogen utilization and excretion of rainbow trout Oncorhynchus mykiss (Walbaum)

This study was undertaken to investigate: (1) the effects of both deficiencies and excesses in essential amino acids (EAAs) from an estimated optimum dietary EAA pattern on nitrogen (N) utilization and excretion of rainbow trout Oncorhynchus mykiss, (2) the effects of dietary digestible protein (P_D): digestible energy (E_D) ratio (P_D:E_D) on N utilization and excretion of O. mykiss and (3) the potential interaction of these two factors. A 3 × 3 factorial experiment was conducted, with the two factors EAA pattern and P_D:E_D ratio. The three levels of EAA pattern were: (1) optimum EAA pattern, (2) 60% deficiencies in the three amino acids arginine, histidine and lysine, and (3) 60% excesses in the three amino acids arginine, histidine and leucine. The three levels of P_D:E_D ratio were 18, 21 and 24 g MJ^?1. Amino acid deficiencies from an optimum amino acid pattern caused reductions in mean N retention of 29 to 37%, with the greatest reduction associated with the lowest P_D:E_D ratio, and similar substantial increases in total N and ammonia‐N excretion at all of the dietary P_D:E_D ratios investigated. Amino acid excesses, however, did not negatively affect N retention or excretion. Increasing P_D:E_D ratio was associated with decreasing N retention and increasing N excretion over the range of dietary protein and lipid levels tested. Results of this study showed that a diet with optimum dietary amino acid pattern and lowest P_D:E_D ratio produced the highest N retention (47% of ingested N) and the lowest total N and ammonia‐N excretion of O. mykiss.     

Influence of a high protein diet on the urinary and fecal excretion of calcium in albino rats

Albino rats (Sprague-Dawley) of mean weight 100 g were divided into four groups and given for 7 days a balanced diet. They were then placed in metabolic cages for fifteen days and fed diets containing different quantities of casein: 18% (D18), 36% (D36), 50% (D50) and 72% (D72). The levels of total calcium, inorganic phosphorus, alkaline phosphatase activity, total proteins and urea were determined. The urinary and fecal excretion of calcium were determined on specimens of urine and stool collected every two days. The metabolic balance of nitrogen was also estimated. The results show there is not a linear relationship between a high protein diet and plasma protein levels, but a progressive body calcium loss was observed with the increase of casein in the diet, which confirms what other workers have already suggested.     

The effect of dietary water soluble carbohydrate to nitrogen ratio on nitrogen partitioning and isotopic fractionation of lactating goats offered a high-nitrogen diet

The objective of this study was to investigate the relationship between nitrogen (N) partitioning and isotopic fractionation in lactating goats consuming diets with a constant high concentration of N and increasing levels of water soluble carbohydrate (WSC). Eight lactating goats were offered four different ratios of WSC : N in the diet. A two-period incomplete cross-over design was used, with two goats assigned to each treatment in each period. N balance measurements were conducted, with measurement of feed N intake and total output of N in milk, faeces and urine. Treatment, period and infusion effects were tested using general ANOVA; the relationships between variables were analysed by linear regression. Dietary treatment and period had significant effects on dry matter (DM) intake (g/day). DM digestibility (g/kg DM) and N digestibility (g/kg N) increased as the ratio of WSC : N increased in the diet. No treatment effect was observed on milk urea N concentration (g/l) or urinary excretion of purine derivatives (mM/day). Although dietary treatment and period had significant effects on N intake, the change of N intake was small; no effect was observed for N partitioning among faeces, milk and urine. Milk, plasma and faeces were enriched in ¹⁵N compared with feed, whilst urine was depleted in ¹⁵N relative to feed. No significant relationship was established between N partitioning and isotopic fractionation. This study failed to confirm the potential to use N isotopic fractionation as an indicator of N partitioning in dairy goats when diets provided N in excess to requirements, most likely because the range of milk N output/N intake and urinary N output/N intake were narrow.     

Wartime discoveries on amino acids: functions in protein structure and as a dietary nitrogen source

In 1943, 10?years after leaving Germany as a refugee, Albert Neuberger published two papers in the Biochemical Journal which became citation classics of their time. They concerned very different aspects of amino acid biochemistry. One, co-authored with John Crammer, described the discovery that the reactivity of the tyrosine side chains in ovalbumin differed considerably from that observed with the free amino acid, but changed substantially when ovalbumin was denatured. This provided some of the earliest evidence that the folded structure of proteins involves interactions between amino acid side chains. The other, with Fred Sanger, focused on the utility of different lysine derivatives as components of the diet. This extended Sanger and Neuberger's previous investigations into the nutritional value of the potato as a source of nitrogen, a line of work that they had initiated in response to the need to carry out research of greater relevance to the war effort. In the present article, I describe these two citation classics and place them in the context of my father's establishment of his career in the U.K.     

The effect of dietary protein quality on nitrogen isotope discrimination in mammals and birds

We tested the competing hypotheses that (1) nitrogen discrimination in mammals and birds increases with dietary nitrogen concentration or decreasing C:N ratios and, therefore, discrimination will increase with trophic level as carnivores ingest more protein than herbivores and omnivores or (2) nitrogen discrimination increases as dietary protein quality decreases and, therefore, discrimination will decrease with trophic level as carnivores ingest higher quality protein than do herbivores. Discrimination factors were summarized for five major diet groupings and 21 different species of birds and mammals. Discrimination did not differ between mammals and birds and decreased as protein quality (expressed as biological value) increased with trophic level (i.e., herbivores to carnivores). Relationships between discrimination factors and dietary nitrogen concentration or C:N ratios were either the opposite of what was hypothesized or non-significant. Dietary protein quality accounted for 72% of the variation in discrimination factors across diet groupings. We concluded that protein quality established the baseline for discrimination between dietary groupings, while other variables, such as dietary protein intake relative to animal requirements, created within-group variation. We caution about the care needed in developing studies to understand variation in discrimination and subsequently applying those discrimination factors to estimate assimilated diets of wild animals.     

Effect of niacin supplementation on digestibility,nitrogen utilisation and milk and blood variables in lactating dairy cows fed a diet with a negative rumen nitrogen balance

The aim of the present experiment was to determine if a niacin supplementation of 6 g/d to lactating dairy cow diets can compensate negative effects of a rumen nitrogen balance (RNB) deficit. A total of nine ruminally and duodenally fistulated lactating multiparous German Holstein cows were successively assigned to one of three diets consisting of 10 kg maize silage (dry matter [DM] basis) and7 kg DM concentrate: Diet RNB– (n = 6) with energy and utilisable crude protein at the duodenum (uCP) according to the average requirement of the animals but with a negative RNB (–0.41 g N/MJ metabolisable energy [ME]); Diet RNB0 (n = 7) with energy, uCP and a RNB (0.08 g N/MJ ME) according to the average requirement of the animals and, finally, Diet NA (n = 5), which was the same diet as RNB–, but supplemented with 6 g niacin/d. Samples of milk were taken on two consecutive days, blood samples were taken on one day pre- and post-feeding and faeces and urine were collected completely over five consecutive days. The negative RNB reduced milk and blood urea content and apparent total tract digestibility of DM, organic matter (OM) and neutral detergent fibre (NDF). Also N excretion with urine, the total N excreted with urine and faeces and the N balance were reduced when the RNB was negative. Supplementation of niacin elevated plasma glucose concentration after feeding and the N balance increased. Supplementing the diet with a negative RNB with niacin led to a more efficient use of dietary N thereby avoiding the negative effects of the negative RNB on the digestibility of DM, OM and NDF.     

The effects of a low protein diet on amino acids and enzymes in the serine synthesis pathway in mice

Branched-chain amino acids, especially leucine, exert regulatory influences on protein and carbohydrate metabolism, ribosome biogenesis and gene expression. This study investigated the effects of leucine in fibroblastic cells analysing viability, proliferation, morphology, proteolysis enzymes activities and protein turnover. After exposure to culture medium enriched with 25 or 50 μM leucine for 24, 48 and 72 h, Vero cells have no alterations on viability and morphology. Leucine-treated cells showed increase on alkaline phosphatase activity and proliferation. The protein synthesis was slightly increased, whereas the protein degradation showed a deep reduction after leucine incubation. The chymotrypsin-like, cathepsin B and H and calpain activities were decreased in leucine-treated cells. In conclusion, the proteolytic pathways and the total protein degradation were modulated by leucine in Vero cells. Our observations support the concept that Vero cells may represent a new model for protein turnover study.     

Effect of niacin supplementation on digestibility, nitrogen utilisation and milk and blood variables in lactating dairy cows fed a diet with a negative rumen nitrogen balance

The aim of the present experiment was to determine if a niacin supplementation of 6 g/d to lactating dairy cow diets can compensate negative effects of a rumen nitrogen balance (RNB) deficit. A total of nine ruminally and duodenally fistulated lactating multiparous German Holstein cows were successively assigned to one of three diets consisting of 10 kg maize silage (dry matter [DM] basis) and 7 kg DM concentrate: Diet RNB- (n = 6) with energy and utilisable crude protein at the duodenum (uCP) according to the average requirement of the animals but with a negative RNB (-0.41 g N/MJ metabolisable energy [ME]); Diet RNB0 (n = 7) with energy, uCP and a RNB (0.08 g N/MJ ME) according to the average requirement of the animals and, finally, Diet NA (n = 5), which was the same diet as RNB-, but supplemented with 6 g niacin/d. Samples of milk were taken on two consecutive days, blood samples were taken on one day pre- and post-feeding and faeces and urine were collected completely over five consecutive days. The negative RNB reduced milk and blood urea content and apparent total tract digestibility of DM, organic matter (OM) and neutral detergent fibre (NDF). Also N excretion with urine, the total N excreted with urine and faeces and the N balance were reduced when the RNB was negative. Supplementation of niacin elevated plasma glucose concentration after feeding and the N balance increased. Supplementing the diet with a negative RNB with niacin led to a more efficient use of dietary N thereby avoiding the negative effects of the negative RNB on the digestibility of DM, OM and NDF.     

Influence of incrementally substituting dietary soya bean meal for rapeseed meal on nutrient digestibility, nitrogen excretion, growth performance and ammonia emissions from growing-finishing pigs

A completely randomised design experiment was performed to examine the effects of replacing different levels of soya bean meal (SBM) with rapeseed meal (RSM) on the growth performance, carcass characteristics, apparent nutrient digestibility, nitrogen (N) balance and manure ammonia emissions of growing-finishing pigs. Pigs (n = 336; mean live weight 42.1 kg) were assigned to one of four dietary treatments containing per kg diet: 210 g SBM; 140 g SBM and 70 g RSM; 70 g SBM and 140 g RSM; and 210 g RSM. All diets were formulated on an ileal digestible amino acid, net energy and available phosphorus basis. There was no significant treatment effect on average daily gain, feed intake, feed conversion ratio and carcass characteristics. There was a linear decrease in gross energy digestibility (p < 0.01) as RSM increased at the expense of SBM in the diet. There was a linear decrease in urinary N excretion (p < 0.01), N digestibility (p < 0.05), total N excretion (p < 0.05) and N retention (p < 0.05) with increasing levels of RSM. There was no effect of dietary treatment on manure ammonia emissions. The results of this study indicate that RSM can be used as a direct replacement for SBM with no associated depression in performance, when formulated on an ileal digestible amino acid and net energy basis. Consumption of diets containing incremental levels of RSM linearly decreased urinary N and total N excretion, reflecting the associated decrease in crude protein concentrations.