Dietary L-homoserine spares threonine in chicks.

Four chick bioassays were conducted to evaluate the threonine (Thr) replacement value of l-homoserine (HS). Growth rate was increased (P < 0.05) by dietary addition of 800 mg l-HS/kg diet to a purified diet severely deficient in Thr or by the addition of 800 or 1000 mg of l-HS/kg diet to a corn-peanut meal diet distinctly deficient in Thr. The addition of an isomolar level of alpha-ketobutyrate, a catabolic product of both Thr and HS, did not elicit a response. Standard-curve methodology predicted a Thr replacement value of 38 +/- 9% for HS. Interactions (P < 0.01) were observed in assays 2 and 4 between dietary Thr adequacy and 800 or 1000 mg l-HS/kg supplementation. Thus, HS improved growth performance when added to a Thr-deficient diet (0.46 g Thr/100 g diet), but it decreased growth performance when added to the same diet containing surfeit Thr (0.80 g Thr/100 g diet). The results indicate that low levels of HS elicit a growth response in young chicks fed Thr-deficient diets.

Advances in protein-amino acid nutrition of poultry.

The ideal protein concept has allowed progress in defining requirements as well as the limiting order of amino acids in corn, soybean meal, and a corn-soybean meal mixture for growth of young chicks. Recent evidence suggests that glycine (or serine) is a key limiting amino acid in reduced protein [23% crude protein (CP) reduced to 16% CP] corn-soybean meal diets for broiler chicks. Research with sulfur amino acids has revealed that small excesses of cysteine are growth depressing in chicks fed methionine-deficient diets. Moreover, high ratios of cysteine:methionine impair utilization of the hydroxy analog of methionine, but not of methionine itself. A high level of dietary L: -cysteine (2.5% or higher) is lethal for young chicks, but a similar level of DL: -methionine, L: -cystine or N-acetyl-L: -cysteine causes no mortality. A supplemental dietary level of 3.0% L: -cysteine (7x requirement) causes acute metabolic acidosis that is characterized by a striking increase in plasma sulfate and decrease in plasma bicarbonate. S-Methylmethionine, an analog of S-adenosylmethionine, has been shown to have choline-sparing activity, but it only spares methionine when diets are deficient in choline and(or) betaine. Creatine, or its precursor guanidinoacetic acid, can spare dietary arginine in chicks.

Excess dietary L-cysteine causes lethal metabolic acidosis in chicks.

A 72-h time-course study was conducted to elucidate the physiological mechanism underlying cysteine (Cys) toxicity in chicks beginning at 8-d posthatch. Biochemical markers quantified in plasma and liver samples collected from chicks receiving 30 g/kg excess dietary Cys were compared with baseline measurements from chicks receiving an unsupplemented corn-soybean meal diet over a 72-h feeding period. Concomitant with chick mortality were indices of acute metabolic acidosis, including a rapid increase (P < 0.001) in anion gap that resulted from a reduction (P < 0.001) in plasma HCO(3)(-) of approximately 40% and a 2.8-fold increase (P < 0.001) in plasma sulfate in chicks receiving excess Cys. Additionally, provision of 30 g/kg excess Cys resulted in a 1.5-fold increase (P < 0.05) in hepatic oxidized glutathione compared with the 0-h control time-point. Excess dietary Cys did not affect plasma free Met, but plasma free Cys increased (P < 0.05) from 89 to 107 mumol/L at 12 h and remained elevated through 36 h. Strikingly, ingestion of 30 g/kg excess Cys caused more than a doubling (P < 0.001) of plasma free cystine, the oxidized form of Cys, beginning 12 h after initiating the study, and it remained elevated throughout the 72-h feeding period. Taken together, these data suggest that ingestion of 30 g/kg excess l-Cys causes both acute metabolic acidosis and oxidative stress in young chicks when fed a nutritionally adequate, corn-soybean meal diet.

Animal models in nutrition research.

Current knowledge in nutrition is based largely on the use of appropriate animal models together with defined diets. Numerous examples are cited where animal models have been used to solve nutrient x nutrient interactions, to evaluate bioavailability of nutrients and nutrient precursors, and to test for nutrient tolerances and toxicities. Advantages, disadvantages, and idiosyncrasies of various animal species are discussed.

Betaine can partially spare choline in chicks but only when added to diets containing a minimal level of choline.

The ability of betaine to serve as a methyl donor in chicks was assessed in 3 bioassays using a choline-free purified diet that contained adequate methionine (Met). In assay 1, choline and betaine were each supplemented at 300 mg/kg in a 2 x 2 factorial arrangement of diets. Supplemental choline improved (P < 0.05) growth performance over the 9-d growth period, whereas betaine alone had no effect. In assay 2, graded supplements of choline produced a linear increase (P < 0.05) in growth performance criteria over a 9-d growth period. Additionally, hepatic betaine-homocysteine (Hcy) methyltransferase (BHMT) activity decreased linearly (P < 0.05), whereas plasma total Hcy remained unchanged. Addition of 260 or 600 mg/kg betaine to the choline-free basal diet did not affect growth performance or BHMT activity, but 600 mg/kg betaine reduced (P < 0.05) plasma total Hcy. Assay 3 was designed to quantify the ability of betaine to spare choline. Minimal supplemental choline requirements of 20.8 +/- 1.50 mg/d (722 mg/kg diet) and 10.5 +/- 1.03 mg/d (412 mg/kg diet) were estimated in the absence and presence of 1000 mg/kg supplemental betaine, respectively. Based on these estimates, 50% of the dietary choline requirement must be supplied as choline per se, but the remaining 50% can be replaced by betaine. Collectively, these data suggest betaine and Met have minimal choline-sparing activity in chicks fed purified diets devoid of preformed choline. However, addition of betaine to diets containing minimal choline allows a marked reduction in the total dietary choline requirement.

2-keto-4-(methylthio)butyric acid (keto analog of methionine) is a safe and efficacious precursor of L-methionine in chicks.

Relative bioefficacy and toxicity of Met precursor compounds were investigated in young chicks. The effectiveness of DL-Met and 2-keto-4-(methylthio)butyric acid (Keto-Met) to serve as L-Met precursors was quantified using Met-deficient diets of differing composition. Efficacy was based on slope-ratio and standard-curve methodology. Using L-Met as a standard Met source added to a purified diet, DL-Met and Keto-Met were assigned relative bioefficacy values of 98.5 and 92.5%, respectively, based on weight gain. Relative bioefficacy values of 98.5 and 89.3% were assigned to DL-Met and Keto-Met, respectively, when chicks were fed a Met-deficient, corn-soybean meal-peanut meal diet. Thus, both DL-Met and Keto-Met are effective Met precursor compounds in chicks. Additionally, growth-depressing effects of L-Met, DL-Met, and Keto-Met were compared using a nutritionally adequate corn-soybean meal diet supplemented with 15 or 30 g/kg of each compound. Similar reductions in weight gain, food intake, and gain:food ratio were observed for each compound. Subjective spleen color scores, indicative of splenic hemosiderosis, increased linearly (P < 0.01) with increasing intakes of each compound, suggesting a similarity in overall toxicity among these compounds. Because conversion of Keto-Met to L-Met in vivo merely requires transamination, Keto-Met may prove to be a useful supplement not only in food animal production, but also as a component of enteral and parenteral formulas for humans suffering from renal insufficiency.

Lysine, arginine, and related amino acids: an introduction to the 6th amino acid assessment workshop.

The focus of the 6th workshop is on lysine, arginine, and related amino acids. Functions, metabolic pathways, clinical uses, and upper tolerance intakes are emphasized in the articles that follow. Lysine is arguably the most deficient amino acid in the food supply of countries where poverty exists, and since the discovery of the nitric oxide synthase pathway, arginine has come into prominence clinically because of the role of nitric oxide in cardiovascular physiology and pathophysiology.

Excess dietary L-cysteine, but not L-cystine, is lethal for chicks but not for rats or pigs.

A comparative species investigation of the relative pharmacologic effects of sulfur amino acids was conducted using young chicks, rats, and pigs. Ingestion of excess Met, Cys, or Cys-Cys supplemented at 2.5-, 5.0-, 7.5-, or 10 times the dietary requirement in a corn-soybean meal diet depressed chick growth to varying degrees. Strikingly, ingestion of excess Cys at 30 g/kg Cys (7.5-times the dietary requirement) caused a chick mortality rate of 50% after only 5 d of feeding. Growth was restored and chick mortality was reduced by supplementing diets containing 25 g/kg excess Cys with KHCO3 at 10 g/kg. Additionally, mortality was prevented by supplementing the drinking water of chicks receiving 25 g/kg supplemental Cys with H2O2 (0.05% final concentration). After young rats and pigs consumed excess Cys or Cys-Cys up to 40 g/kg for 14 d, weight gain was severely depressed, but we observed no mortality. An excess of dietary Cys-Cys>or=48 g/kg caused some mortality in rats. Pigs exhibited rapid recovery from growth-depressing excesses of Cys or Cys-Cys. These results lend credence to the acute toxic effects associated with the ingestion of excess sulfur amino acids and highlight the potential for excess dietary cyst(e)ine to be more pernicious than Met in certain species.

Comparative species utilization and toxicity of sulfur amino acids.

Animal studies have shown that several methionine (Met) and cysteine (Cys) analogs or precursors have L-Met- and L-Cys-sparing activity. Relative oral bioavailability (RBV) values, with the L-isomer of Met and Cys set at 100% (isosulfurous basis), are near 100% for D-Met for animals but only about 30% for humans. Both the OH and keto analogs of Met have high RBV-sparing values, as does N-acetyl-L-Met (the D-isomer of acetylated Met has no bioactivity). L-Homocysteine has an RBV value of about 65% for Met sparing in rats and chicks, but D-homocysteine has little if any Met-sparing activity. S-Methyl-L-Met can partially spare Met, but only when fed under dietary conditions of choline/betaine deficiency. Relative to L-Cys, high RBV values exist for L-cystine, N-acetyl-L-Cys, L-homocysteine, L-Met, and glutathione, but D-cystine, the keto analog of Cys, L-cysteic acid, and taurine have no Cys-sparing activity. l-2-Oxothiazolidine-4-carboxylate has an RBV value of 75%, D-homocysteine 70%, and DL-lanthionine 35% as Cys precursors. Under dietary conditions of Cys deficiency and very low inorganic sulfate (SO4) ingestion, dietary SO4 supplementation has been shown to reduce the Cys requirement of several animal species as well as humans. Excessive ingestion of Met, Cys, or cystine has also been studied extensively in experimental animals, and these sulfur amino acids (SAA) are well established as being among the most toxic of all amino acids that have been studied. Even though Cys and its oxidized product (cystine) are equally efficacious at levels at or below their dietary requirements for maximal growth, Cys is far more toxic than cystine when administered orally in the pharmacologic dosing range. Isosulfurous (excess) levels of cystine, N-acetyl-L-Cys, or glutathione are far less growth depressing than L-Cys when 6 to 10 times the minimally required level of these SAA compounds are fed to chicks.

Comparative nutrition and metabolism: explication of open questions with emphasis on protein and amino acids.

The 20th century saw numerous important discoveries in the nutritional sciences. Nonetheless, many unresolved questions still remain. Fifteen questions dealing with amino acid nutrition and metabolism are posed in this review. The first six deal with the functionality of sulfur amino acids (methionine and cysteine) and related compounds. Other unresolved problems that are discussed include priorities of use for amino acids having multiple functions; interactions among lysine, niacin and tryptophan; amino acid contributions to requirements from gut biosynthesis; the potential for gluconeogenesis to divert amino acids away from protein synthesis; the unique nutritional and metabolic idiosyncrasies of feline species, with emphasis on arginine; controversies surrounding human amino acid requirements; and the potential for maternal diet to influence sex ratio of offspring.

Dietary S-methylmethionine, a component of foods, has choline-sparing activity in chickens.

Acid hydrolysis of dehulled soybean meal (SBM) and corn gluten meal (CGM) followed by chromatographic amino acid analysis (ninhydrin detection) revealed substantial quantities of S-methylmethionine (SMM) in both ingredients (1.65 g SMM/kg SBM; 0.5 g SMM/kg CGM). Young chicks were used to quantify the methionine- (Met) and choline-sparing bioactivity of crystalline L-SMM, relative to L-Met and choline chloride standards in 3 assays. A soy isolate basal diet was developed that could be made markedly deficient in Met, choline, or both. When singly deficient in choline or in both choline and Met, dietary SMM addition produced a significant (P < 0.01) growth response. In Assay 2, dietary SMM did not affect (P > 0.10) growth of chicks fed a Met-deficient, choline-adequate diet. A standard-curve growth assay revealed choline bioactivity values (wt:wt) of 14.2 +/- 0.8 and 25.9 +/- 5.1 g/100 g SMM based on weight gain and gain:food responses, respectively. A fourth assay, using standard-curve procedures, showed choline bioactivity values of 20.1 +/- 1.1 and 22.9 +/- 1.7 g/100 g SMM based on weight gain and gain:food responses, respectively. It is apparent that SMM in foods and feeds has methylation bioactivity, and this has implications for proper assessment of dietary Met and choline requirements as well as their bioavailability in foods and feeds.

Tolerance for branched-chain amino acids in experimental animals and humans.

There is no good evidence for establishing branched-chain amino acid (BCAA) tolerance levels for humans. With pigs, chicks, and rats, data are available concerning excessive intake levels of BCAA, but most of the information is for growing animals instead of for adults. Estimates of maintenance requirements for (high-quality) protein and BCAA in pigs weighing between 43 and 140 kg are 350 mg . kg(-1) . d(-1) for protein and 28.7 mg . kg(-1) . d(-1) for total BCAA. In contrast, human adult maintenance requirement estimates are much higher, i.e., 660 mg . kg(-1) . d(-1) for good quality protein and a range of 68 to 144 mg . kg(-1) . d(-1) for total BCAA. The human maintenance BCAA requirement estimates range from 10.3 to 22% of the maintenance protein requirement. Whole-body protein of 45-kg pigs contains 14.2 g BCAA/100 g protein, but the maintenance requirement (based on nitrogen balance) for total BCAA is only 8.2% of the total maintenance protein requirement. Conversely, sulfur amino acid (methionine + cysteine), threonine, and tryptophan maintenance requirements of pigs as a percentage of the maintenance protein requirement are much higher than whole-body protein levels of these amino acids. This suggests that the efficiency of using absorbed amino acids of dietary origin or of reusing endogenous amino acids arising from body protein catabolism may vary considerably among the indispensable amino acids. Additionally, work with pigs points to the conclusion that whole-body amino acid concentrations are poor predictors of both maintenance requirements and ideal amino acid profiles. Based on studies with young experimental animals, a rather large dietary excess (above requirement) of an individual BCAA is well tolerated when consumed in diets containing surfeit levels of protein and the other 2 BCAA.

Decreased protein accretion in pigs with viral and bacterial pneumonia is associated with increased myostatin expression in muscle.

Chronic respiratory infections reduce growth in pigs but protein accretion (PA) during an ongoing multifactorial respiratory infection has not been determined, and the mechanisms underlying growth inhibition are largely unknown. The objectives of this study were to determine whether viral and bacterial pneumonia in young pigs decrease PA, increase serum IL-1beta and IL-6, and increase myostatin (MSTN) mRNA in biceps femoris and triceps muscles. Mycoplasma hyopneumoniae (Mh) or medium was given intratracheally at 4 wk of age, Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) or medium was given intranasally at 6 wk of age, and pigs were killed 7 or 14 d after PRRSV inoculation for body composition analysis. PRRSV but not Mh induced a marked increase (P < 0.01) in IL-1beta, IL-6, and MSTN mRNA and a decrease (P < 0.01) in food intake, daily weight gain, PA, and lipid accretion. PRRSV also reduced (P < 0.01) myofiber area in the biceps femoris. Food intake, weight gain, PA, and weight of biceps femoris and triceps muscles were negatively correlated (r = -0.4 to -0.8, P < 0.05) with serum IL-1beta and IL-6 and with MSTN mRNA in muscle. These results suggest that the magnitude of increases in inflammatory cytokines during a respiratory infection may be predictive of decreases in PA and growth. They further suggest that during infection growth of skeletal muscle is limited in part by myostatin.

Supplementation of total parenteral nutrition with butyrate acutely increases structural aspects of intestinal adaptation after an 80% jejunoileal resection in neonatal piglets.

BACKGROUND: Supplementation of total parenteral nutrition (TPN) with a mixture of short-chain fatty acids (SCFA) enhances intestinal adaptation in the adult rodent model. However, the ability and timing of SCFA to augment adaptation in the neonatal intestine is unknown. Furthermore, the specific SCFA inducing the intestinotrophic effects and underlying regulatory mechanism(s) are unclear. Therefore, we examined the effect of SCFA supplemented TPN on structural aspects of intestinal adaptation and hypothesized that butyrate is the SCFA responsible for these effects. METHODS: Piglets (n = 120) were randomized to (1) control TPN or TPN supplemented with (2) 60 mmol/L SCFA (36 mmol/L acetate, 15 mmol/L propionate and 9 mmol/L butyrate), (3) 9 mmol/L butyrate, or (4) 60 mmol/L butyrate. Within each group, piglets were further randomized to examine acute (4, 12, or 24 hours) and chronic (3 or 7 days) adaptations. Indices of intestinal adaptation, including crypt-villus architecture, proliferation and apoptosis, and concentration of the intestinotrophic peptide, glucagon-like pepide-2 (GLP-2), were measured. RESULTS: Villus height was increased (p < .029) within 4 hours by supplemented TPN treatments. Supplemented TPN treatments increased (p < .037) proliferating cell nuclear antigen expression along the entire intestine. Indicative of an antiapoptotic profile, jejunal Bax:Bcl-w abundance was decreased (p = .033) by both butyrate-supplemented TPN treatments, and ileal abundance was decreased (p = .0002) by all supplemented TPN treatments, regardless of time. Supplemented TPN treatments increased (p = .016) plasma GLP-2 concentration at all time points. CONCLUSIONS: Butyrate is the SCFA responsible for augmenting structural aspects of intestinal adaptations by increasing proliferation and decreasing apoptosis within 4 hours postresection. The intestinotrophic mechanism(s) underlying butyrate's effects may involve GLP-2. Ultimately, butyrate administration may enable an infant with short-bowel syndrome to successfully transition to enteral feedings by maximizing their absorptive area.

Animal models of human amino acid responses.

The principal differences between experimental animals and humans with regard to amino acid responses are 1) growing animals partition most of their amino acid intake to protein accretion, whereas growing children partition most of their intake to maintenance; 2) invasive assessment procedures are common in animals but very limited in humans; and 3) humans can describe how they feel in response to amino acid levels or balances, whereas animals cannot. New (pharmacologic) uses of amino acids have been and are being discovered (e.g., cysteine, arginine, leucine, glutamine), and this makes it imperative that tolerance limits be established. Work with pigs suggests that excessive intake of methionine and tryptophan present the biggest problems, whereas excessive intake of threonine, glutamate, and the branched-chain amino acids seems to be well tolerated.

Iodine toxicity and its amelioration.

Iodine (I) toxicity is rare in animals and humans, but nuclear explosions that give off radioactive I and excessive stable I ingestion in parts of the world where seaweed is consumed represent specialized I toxicity concerns. Chronic overconsumption of I reduces organic binding of I by the thyroid gland, which results in hypothyroidism and goiter. Bromine can replace I on position 5 of both T(3) and T(4) with no loss of thyroid hormone activity. Avian work has also demonstrated that oral bromide salts can reverse the malaise and growth depressions caused by high doses of I (as KI) added as supplements to the diet. Newborn infants by virtue of having immature thyroid glands are most susceptible to I toxicity, whether of stable or radioactive origin. For the latter, the 1986 Chernobyl nuclear accident in Belarus has provided evidence that KI blockage therapy for exposed individuals 18 years of age and younger is effective in minimizing the development of thyroid cancer. Whether bromide therapy has a place in I toxicity situations remains to be determined.

Oral iodine toxicity in chicks can be reversed by supplemental bromine.

Four chick bioassays were conducted to quantify iodine (I) toxicity and its amelioration in young chicks. A supplemental I level from KI of 600 mg/kg depressed growth in chicks fed methionine-deficient diets but not in those fed methionine-adequate diets. An I dose level >or= 900 mg/kg was required to cause growth depression in chicks fed a methionine-adequate corn-soybean meal diet. Iodine intoxicated chicks also displayed neurological symptoms and extreme malaise, but dose levels up to 1200 mg I/kg had no effect on blood hemoglobin or hematocrit. Supplemental I levels of 1000-1500 mg/kg caused severe growth depressions that could be totally reversed by dietary addition of 50 or 100 mg/kg bromine provided as NaBr. Nuclear accidents or terrorist actions that result in I toxicity and thyroid cancer or goiter may benefit from use of NaBr as a therapeutic agent.

The relative vitamin A value of 9-cis beta-carotene is less and that of 13-cis beta-carotene may be greater than the accepted 50% that of all-trans beta-carotene in gerbils.

The effectiveness of beta-carotene (betaC) as a vitamin A (VA) precursor may be influenced by the proportions of cis isomers of betaC consumed in the diet. Although the metabolic fates of cis isomers of betaC are poorly understood, their retinol equivalency has been assigned a value 50% that of all-trans (at) betaC. A dose-response design was used to estimate the relative VA value (VAV) of atbetaC, 9-cis (9c) betaC and 13-cis (13c) betaC in gerbils using total liver retinol as a measure of VAV. Ten groups of gerbils received a daily dose of oil with or without betaC isomer by gavage for 7 d. Nine groups (n = 5) were divided equally among the three betaC dosing treatments with each isomer provided at 141, 275 and 418 nmol/d. Total liver VA (171-259 nmol) in gerbils administered atbetaC was higher than total liver betaC (25-53 nmol). Stores of VA and betaC in livers from gerbils administered atbetaC were higher than stores of VA and betaC in livers from those given 9cbetaC or 13cbetaC. The relative VAV of cis betaC isomers was estimated by comparing slopes of dose-response lines of all three betaC isomers using atbetaC as a reference. Total liver VA and betaC increased linearly (P < 0.05) with increasing betaC intake in gerbils gavaged with all three betaC isomer oils. The relative VAV of 9cbetaC was less (38%) and 13cbetaC was more (62%) than the assigned value of 50% that of atbetaC. Thus, the proportions of cis isomers of betaC contained in a food could negatively affect the vitamin A value of the diet.