Tolerable amounts of amino acids for human supplementation: summary and lessons from published peer-reviewed studies.

Amino acid supplementation may be indicated to correct for insufficient amino acid intake in healthy individuals, and in specific physiological or pathophysiological situations. However, there is a concern to not supplement beyond the tolerable upper intake level (UL) by determining parameters of no-observed-adverse-effect level (NOAEL) or lowest-observed-adverse-effect level (LOAEL) for each amino acid. Since the NOAEL and LOAEL values are at least one order of magnitude different when comparing the values obtained in rats and humans, the aim of this review is to evaluate to what extent the amino acid UL measured in the rat model, when referenced to the dietary usual consumption (UC) and dietary requirement (RQ) for indispensable amino acids, may be used as an approximation of the UL in humans. This review then compares the ratios of the NOAEL or LOAEL over UC and RQ in the rat model with the same ratios calculated in humans for the nine amino acids (arginine, serine, glycine, histidine, leucine, lysine, methionine, phenylalanine, and tryptophan) for which this comparison can be done. From the calculations made, it appears that for these 9 amino acids, the calculated ratios for rats and humans, although rather different for several amino acids, remains for all of them in the same order of magnitude. For tryptophan, tyrosine, and valine, the ratios calculated in rats are markedly different according to the sex of animals, raising the view that it may be also the case in humans.

Subchronic Tolerance Trials of Graded Oral Supplementation with Phenylalanine or Serine in Healthy Adults.

Phenylalanine and serine are amino acids used in dietary supplements and nutritional products consumed by healthy consumers; however, the safe level of phenylalanine or serine supplementation is unknown. The objective of this study was to conduct two 4-week clinical trials to evaluate the safety and tolerability of graded dosages of oral phenylalanine and oral serine. Healthy male adults (n = 60, 38.2 ± 1.8y) completed graded dosages of either phenylalanine or serine supplement (3, 6, 9 and 12 g/d) for 4 weeks with 2-week wash-out periods in between. Primary outcomes included vitals, a broad spectrum of circulating biochemical analytes, body weight, sleep quality and mental self-assessment. At low dosages, minor changes in serum electrolytes and plasma non-essential amino acids glutamine and aspartic acid concentrations were observed. Serine increased its plasma concentrations at high supplemental dosages (9 and 12 g/day), and phenylalanine increased plasma tyrosine concentrations at 12 g/day, but those changes were not considered toxicologically relevant. No other changes in measured parameters were observed, and study subjects tolerated 4-week-long oral supplementation of phenylalanine or serine without treatment-related adverse events. A clinical, no-observed-adverse-effect-level (NOAEL) of phenylalanine and serine supplementation in healthy adult males was determined to be 12 g/day.

Proposals for Upper Limits of Safe Intake for Methionine, Histidine, and Lysine in Healthy Humans.

Based on research presented during the 10th Amino Acid Assessment Workshop, no observed adverse effect levels (NOAELs) for supplemental methionine at 46 mg/(kg·d) (∼3.2 g/d), for supplemental histidine at 8.0 g/d, and for supplemental lysine at 6.0 g/d have been proposed. These NOAELs are relevant to healthy adults and are applicable only to high-purity amino acids administered in fortified foods or dietary supplements. Because individuals are exposed to the above supplemental amino acids in the context of complex combinations of essential amino acids or individually in dietary supplements for various physiologic benefits, such as body fat reduction, skin conditioning, mental energy increase, or herpes simplex treatments, the above safety recommendations will make an important contribution to regulatory and nutritional practices.

Reduction in dietary lysine increases muscle free amino acids through changes in protein metabolism in chickens.

Taste is crucial to meat quality, and free Glu is an important taste-active component in meat. Our recent study showed that the short-term feeding of a low-Lys diet increases the concentration of free Glu and other free amino acids in chicken muscle and improves its taste. Here, we investigated the mechanisms by which the feeding of a low-Lys diet increases free Glu in chicken muscle. Two groups (n = 10 per group) of 28-day-old female Ross strain broiler chickens were fed diets with a graded Lys content of 90% or 100% of the recommended Lys requirement (according to National Research Council [1994] guidelines) for 10 D. Free amino acid concentrations and the mRNA abundance of protein metabolism-related genes were measured in breast muscle, and breast muscle metabolome analysis was conducted. Free Glu in muscle was increased by 51.8% in the Lys 90% group compared with the Lys 100% group (P < 0.01). Free threonine, glutamine, glycine, valine, isoleucine, leucine, tyrosine, phenylalanine, histidine, and 3-methyl-histidine concentrations in breast muscle were also increased in the Lys 90% group (P < 0.05). Metabolome analysis also showed that free amino acids were increased in the Lys 90% group. The mRNA abundance of µ-calpain, caspase-3, and 20S proteasome C2 subunit were increased in the Lys 90% group (P < 0.05). Moreover, the free Glu concentration in muscle was correlated with mRNA abundance of µ-calpain (r = 0.74, P < 0.01), caspase 3 (r = 0.69, P < 0.01), 20S proteasome C2 subunit (r = 0.65, P < 0.01), and cathepsin B (r = 0.52, P < 0.05). Our study suggests that the feeding of a low-Lys diet to chickens increased the free Glu content of breast muscle by promoting protein degradation.

Physiological Multifunctions of Rice Proteins of Endosperm and Bran.

Although it is considered a staple food, rice intake is under serious debate for its physiological usefulness, especially for diabetic patients, because of starch content. However, rice protein, the second major component of rice, has gained attention recently for its newly-discovered functions, which were previously unknown. Rice protein, a plant protein, shows multiple beneficial functions on lipid metabolism and diabetes and its complications, nephropathy, fatty liver and osteoporosis. Rice proteins of endosperm and bran, an ingredient of white rice and an unused product of brown rice, respectively, are valuable components for human health.

A Randomized, Double-Blind, Crossover Pilot Trial of Rice Endosperm Protein Supplementation in Maintenance Hemodialysis Patients.

In maintenance hemodialysis (MHD) patients, low protein intake is associated with protein-energy wasting, a risk factor that affects outcome. However, increased protein intake may lead to hyperphosphatemia and hyperkalemia, which are also mortality risk factors. Here, we evaluated the safety and effects of purified rice endosperm protein (REP), which contains less phosphorus and potassium than soy and casein proteins, as a supplemental protein source for MHD patients. This randomized, double-blind, placebo-controlled, crossover pilot study of REP supplementation (5 g/day × 4 weeks) was carried out in 50 Japanese adult MHD patients (1 dropped out); the primary outcome was the change in the urea kinetic-based normalized protein catabolic rate (nPCR), an indicator of protein intake in MHD patients. Intention-to-treat analyses of 24 patients in the REP-first group and 25 in the placebo-first group showed that REP supplementation increased nPCR significantly by 0.07 g/kg/day (95% confidence interval, 0.03-0.11), whereas changes in serum phosphorus and potassium concentrations were not different from the placebo. REP supplementation did not show a significant effect on other nutritional or metabolic parameters and no specific complications. In conclusion, purified REP with efficient bioavailability may be safe and useful for dietary supplementation in MHD patients.

Effect and proposed mechanism of vitamin C modulating amino acid regulation of autophagic proteolysis.

Autophagy is an intracellular bulk degradation process, induced under nutrient starvation. Failure of autophagy has been recognized as a contributor to aging and multiple age related neurodegenerative diseases. Improving autophagy is a beneficial anti-aging strategy, however very few physiological regulators have been identified. Here, we demonstrate that vitamin C is a nutritional stimulator of autophagy. Supplementation of fresh hepatocytes with vitamin C increased autophagic proteolysis significantly in the presence of amino acids in a dose- and time-dependent manner, although no effect was observed in the absence of amino acids. In addition, inhibitor studies with 3-methyladenine, chloroquine, leupeptin and ß-lactone confirmed that vitamin C is active through the lysosomal autophagy and not the proteasome pathway. Furthermore, the autophagy marker LC3 protein was significantly increased by vitamin C, suggesting its possible site of action is at the formation step. Both the reduced (ascorbic acid, AsA) and oxidized form (dehydroascorbic acid, DHA) of vitamin C exhibited equal enhancing effect, indicating that the effect does not depend on the anti-oxidation functionality of vitamin C. To understand the mechanism, we established that the effective dose (50 µM) was 15× lower than the intracellular content suggesting these would be only a minor influx from the extracellular pool. Moreover, transporter inhibitor studies in an AsA deficient ODS model rat revealed more accurately that the enhancing effect on autophagic proteolysis still existed, even though the intracellular influx of AsA was blocked. Taken together, these results provide evidence that vitamin C can potentially act through extracellular signaling.

Reduction of dietary lysine increases free glutamate content in chicken meat and improves its taste.

Taste is a crucial factor of meat quality, and amino acids are important taste-active components in meat. Here, the effects of dietary lysine (Lys) content on taste-active components in meat, especially free glutamate (Glu), were investigated. Twenty-eight-day-old broilers (Gallus gallus) were fed diets with graded Lys content of 90% or 100% of the recommended Lys requirement, (according to the National Research Council, ) for 10 days. Free amino acid content in meat and sensory scores of meat soup were estimated. Free Glu content, the main taste-active component of meat, was significantly increased by a reduction of dietary Lys. Compared with the Lys 100% group (control), free Glu concentrations of meat were increased by 35.7% in the Lys 90% group (P < 0.05). In addition, free glycine, valine, isoleucine, leucine, histidine and threonine concentrations of meat were significantly increased in the Lys 90% group (P < 0.05). Sensory evaluation of meat soup made from the Lys 100% and 90% groups indicated different meat tastes. Sensory scores of taste intensity, umami and kokumi tastes were significantly higher in the Lys 90% group. These results suggest that a reduction of dietary lysine increased free glutamate content in meat and improved its taste.

Proposals for Upper Limits of Safe Intake for Arginine and Tryptophan in Young Adults and an Upper Limit of Safe Intake for Leucine in the Elderly.

On the basis of research presented during the 9th Amino Acid Assessment Workshop, a No Observed Adverse Effect Level (NOAEL) for diet-added arginine (added mostly in the form of dietary supplements) of 30 g/d and an upper limit of safe intake (ULSI) for diet-added tryptophan (added mostly in the form of dietary supplements) of 4.5 g/d have been proposed. Both recommendations apply to healthy young adults. The total dietary leucine ULSI proposed for elderly individuals is 500 mg · kg-1 · d-1 All 3 recommendations are relevant only to high-quality amino acid-containing products with specifications corresponding to those listed in the US Pharmacopeia Because the above amino acids are extensively utilized as dietary supplements for various real or perceived benefits, such as vasodilation, spermatogenesis, sleep, mood regulation, or muscle recovery, the above safety recommendations will have an important impact on regulatory and nutritional practices.

Rice endosperm protein slows progression of fatty liver and diabetic nephropathy in Zucker diabetic fatty rats.

We previously reported that rice endosperm protein (REP) has renoprotective effects in Goto-Kakizaki rats, a non-obese diabetic model. However, whether these effects occur in obese diabetes remains unclear. This study aimed to clarify the effects of REP on obese diabetes, especially on fatty liver and diabetic nephropathy, using the obese diabetic model Zucker diabetic fatty (ZDF) rats. In total, 7-week-old male ZDF rats were fed diets containing 20 % REP or casein (C) for 8 weeks. Changes in fasting blood glucose levels and urinary markers were monitored during the experimental period. Hepatic lipids and metabolites were measured and renal glomeruli were observed morphologically. HbA1c levels were significantly lower in rats fed REP, compared with C (P<0·05). Compared with C in the liver, REP prevented lipid accumulation (total lipid, TAG and total cholesterol, P<0·01). Liver metabolome analysis indicated that levels of metabolites associated with glycolysis, the pentose phosphate pathway and carnitine metabolism were significantly greater in the REP group than in the C group (P<0·05), suggesting activation of both glucose catabolism and fatty acid oxidation. The metabolite increases promoted by REP may contribute to suppression of liver lipid accumulation. Urinary excretion of albumin and N-acetyl-ß-d-glucosaminidase was significantly reduced in rats fed REP for 8 weeks (P<0·01). In addition, there was a distinct suppression of mesangial matrix expansion and glomerular hypertrophy in response to REP (P<0·01). Thus, REP had preventive effects on obese diabetes, fatty liver and diabetic nephropathy.

Protective effect of composite earthworm powder against diabetic complications via increased fibrinolytic function and improvement of lipid metabolism in ZDF rats.

Thrombosis is the leading cause of mortality globally. It is not only a complication but also a risk factor for progression of diabetes. However, alternative oral therapies and prophylaxis with less adverse effect for thrombosis have not been well studied. In this study, composite powder containing earthworm (CEP) was used and its fibrinolytic activity was measured. CEP was found to have a high urokinase-type plasminogen activator like activity in an in vitro assay. It also had significantly shortened euglobulin clot lysis time (ECLT) at 4 and 24 h after ingestion in Sprague Dawley rats. Zucker Diabetic Fatty rats were used to assess the effect of CEP on diabetes and diabetic nephropathy. After 10 weeks of feeding, CEP significantly shortened ECLT and attenuated HbA1c, hepatic lipid accumulation, and urinary albumin excretion and improved glomerular mesangial matrix score. Therefore, CEP may have beneficial effects on diabetes and diabetic nephropathy.

Rice protein hydrolysates stimulate GLP-1 secretion, reduce GLP-1 degradation, and lower the glycemic response in rats.

Rice has historically been consumed in Asia as a major source of carbohydrates, however, little is known regarding the functional roles of rice proteins as dietary factors. In the present study, we investigated whether peptides derived from rice proteins could stimulate GLP-1 secretion, which results in reducing glycemia via the incretin effect in normal rats. Hydrolysates were prepared from the protein fraction of rice endosperm or rice bran, and the effects of these hydrolysates on GLP-1 secretion were examined in a murine enteroendocrine cell line GLUTag. Plasma was collected after oral administration of the rice protein hydrolysates, under anesthesia, or during glucose tolerance tests in rats. In anesthetized rats, plasma dipeptidyl peptidase-IV (DPP-IV) activity was measured after ileal administration of the rice protein hydrolysates. GLP-1 secretion from GLUTag cells was potently stimulated by the rice protein hydrolysates, especially by the peptic digest of rice endosperm protein (REPH) and that of rice bran protein (RBPH). Oral administration of REPH or RBPH elevated plasma GLP-1 concentrations, which resulted in the reduction of glycemia under the intraperitoneal glucose tolerance test. In addition, the plasma DPP-IV activity was attenuated after ileal administration of REPH or RBPH, which resulted in a higher ratio of intact (active) GLP-1 to total GLP-1 in the plasma. These results demonstrate that rice proteins exert potent stimulatory effects on GLP-1 secretion, which could contribute to the reduction of postprandial glycemia. The inhibitory effect of these peptides on the plasma DPP-IV activity may potentiate the incretin effect of GLP-1.

Effect of dietary histidine on contents of carnosine and anserine in muscles of broilers.

Carnosine (ß-alanyl-L-histidine) and anserine (ß-alanyl-1-methyl-L-histidine) are dipeptides mainly found in skeletal muscle and brain of many vertebrates, and particularly high concentrations are observed in chicken pectoral muscles. It was reported that these peptides have many functions, such as antioxidant activity. In this study, we examined the effect of different levels of dietary histidine on carnosine and anserine contents in broiler muscles. The 14-days-old female Chunky strain broilers were given feeds containing three different levels of histidine; 67% (Low-His), 100% (Control) and 200% (High-His) of histidine requirement according to the NRC (1994). Chicks were fed experimental diets for 10 days. Both dipeptides in muscle were significantly decreased. In particular, carnosine was not detected at all in the Low-His group and was significantly increased in the High-His group. Both dipeptides were not detected in plasma. These results indicated the possibility to produce chicken meat with enhanced amount of these dipeptides by high histidine feeding.

Regulation of free glutamate content in meat by dietary lysine in broilers.

Regulation of taste is important for improving meat quality and glutamate (Glu) is one of the important taste-active components in meat. Here, the effects of dietary lysine (Lys) content on taste-active components in meat, especially free Glu, were investigated. Fourteen-day-old broiler chicks (Gallus gallus) were fed on diets containing 100% or 150% of the recommended Lys content for 10 days. Concentrations of free amino acids in plasma, muscle and liver were measured. The levels of messenger RNAs (mRNAs) for enzymes related to Glu metabolism were determined in muscle and liver. The concentration of muscle metabolites was also determined. The free Glu content in muscle of chicks fed the Lys150% diet was increased by 44.0% compared with that in chicks fed the Lys100% diet (P < 0.01). The mRNA level of lysine α-ketoglutarate reductase, which is involved in Lys degradation and Glu production, was significantly increased (P < 0.05) in the Lys150% group. Metabolome analysis showed that the Lys degradation products, muscular saccharopine, pipecolic acid and α-aminoadipic acid, were increased in the Lys150% group. Our results suggest that free Glu content in muscle is regulated by Lys degradation. These results suggest that a short-term feeding of high-Lys diet could improve the taste of meat.

In vivo digestibility of rice prolamin/protein body-I particle is decreased by cooking.

Rice has storage proteins, e.g., glutelin, globulin and prolamin, in the seeds, which are used as nitrogen sources during germination. Rice prolamin has been reported to be an indigestible protein that decreases the nutritional value of rice. However, the causes for the indigestibility of prolamin are currently not clear. The objective of this study was to determine if prolamin is naturally indigestible or if cooking affects its digestibility. The gastrointestinal (GI) transit of rice 23 kDa glutelin (23G) and 13 kDa prolamin (13P) in Wistar/ST rats fed raw rice (RR) and cooked rice (CR) diets was assessed using Western blot analysis. We also measured the excretion of these proteins in the feces of these rats. Additionally, morphological observation of the structure of type-I protein bodies in the feces was performed using electron microscopy. Assessment of GI transit revealed that 23G rapidly disappeared from the GI contents of both the RR and CR groups, but 13P accumulated in the cecum of the CR group. In the CR group, prolamin, maintaining the structure of PB-I, was fully excreted in the feces. These results indicate that rice prolamin is not indigestible by nature, but is rendered indigestible by cooking.

A quick signal of starvation induced autophagy: transcription versus post-translational modification of LC3.

Autophagy is the major intracellular lysosomal bulk degradation pathway induced by nutrient starvation and contributes to the elimination of damaged organelles and protein aggregates to recycle building block and is essential for cell survival. Microtubule-associated protein 1 light chain 3 (LC3) plays an indispensable role in macroautophagy formation and is a molecular marker for the process. Here, we show that autophagy increased through quick robust signaling from starvation by enhanced levels of LC3, LC3-EGFP (enhanced green fluorescent protein) punctate, and bulk proteolysis in rat hepatoma H4-II-E cells and fresh rat hepatocytes. After the addition of amino acids to the starvation condition, a similar quick signal appeared by significant reduction of the LC3 ratio and bulk proteolysis. Interestingly, we observed that post-translational modification of LC3 conversion occurred even long before the changes happened in the level of LC3-mRNA (messenger RNA) expression. A similar coordinated but diverse effect on LC3 was confirmed by using autophagy and lysosomal inhibitors. These results indicated that during starvation the initial robust signal to the cytoplasm can induce autophagy activity through modification at the protein level, whereas after depleting readily available autophagy proteins the signal goes to the DNA transcription level to maintain the autophagy capacity of cells.

Effect of dietary astaxanthin rich yeast, Phaffia rhodozyma, on meat quality of broiler chickens.

We evaluated effects of dietary supplementation with astaxanthin (Ax)-rich yeast, Phaffia rhodozyma (Xanthophyllomyces dendrorhous), on broiler chicken meat quality. Fourteen-day-old female Ross broilers were divided into three groups: control group, Ax-free diet; Ax 10 group, 10 mg/kg Ax diet; and Ax 20 group, 20 mg/kg Ax diet for 28 days. At 42 days old, chickens were slaughtered, and then growth performance, meat quality and sensory attributes were analyzed. Compared with the control, a* values increased significantly after slaughter and 48 h postmortem for Ax 20 samples (P<0.05) and for b* values in Ax 20 and Ax 10 groups (P<0.05). Cooking loss decreased in the Ax 20 group (P<0.05). After 120 h aging, contents of several free amino acids and total free amino acid content of Ax 20 group were significantly higher than the control (P<0.05). In sensory evaluation, meat texture attributes improved significantly in the Ax 20 group (P<0.01). No significant changes occurred in flavor attribute scores of meat soup from the Ax 20 group compared with the control even though most assessors preferred meat soup from the Ax 20 group. Overall, Ax-rich yeast in the diet improves broiler chicken meat quality.

Diversity of amino acid signaling pathways on autophagy regulation: a novel pathway for arginine.

Autophagy is the intracellular bulk degradation process to eliminate damaged cellular machinery and to recycle building blocks, and is crucial for cell survival and cell death. Amino acids modulate autophagy in response to nutrient starvation and oxidative stress. We investigated the relevance of reactive oxygen species (ROS) production on the regulation of autophagy using amino acids, both as a mixture and individually, in rat hepatoma H4-II-E cells. Nutrient starvation elevated ROS production and stimulated autophagy. Treatment with complete (CAA), regulatory (RegAA) and non-regulatory (NonRegAA) amino acid mixtures showed significant suppression of ROS production, whereas only CAA and RegAA exhibited significant suppression of autophagy, suggesting a dissociation of the two responses. The effects of individual amino acids were examined. Leucine from RegAA decreased ROS production and suppressed autophagy. However, methionine and proline from RegAA and arginine, cystine and glutamic acid from NonRegAA suppressed autophagy with an opposite increase in ROS production. Other amino acids from the NonRegAA group showed stimulating effects on ROS production without an autophagic response. Arginine's effect on autophagy suppression was not blocked by rapamycin, indicating an mTOR-independent pathway. Inhibitor studies on arginine-regulated autophagy may indicate the involvement of NO pathway, which is independent from ROS and mTOR pathways.