Bioavailability Of Tryptophan Research Articles

Dietary tryptophan-rich protein hydrolysate can acutely impact physiological and psychological measures of mood and stress in healthy adults

ABSTRACT Background Tryptophan is the precursor to the mood regulating neurotransmitter serotonin. Its brain bioavailability from food can be dependent on the dietary source. Egg protein hydrolysate (EPH), a dietary supplement rich in tryptophan, has previously shown to acutely impact cognition, mood and stress benefits at 2 g dose. No data exist on the acute effects of lower doses in a food matrix. Methods This exploratory study tested the acute effects of low-doses EPH (0.5, 1 g) in a food matrix on cognition, mood and stress. The study employed a double-blinded randomized controlled parallel design in 45 participants with three arms. The effects of the interventions were measured after a multi-task cognitive stressor on blood biomarkers, self-reported mood states, performances of attention, autonomic parameters and, emotional reactivity responses from electroencephalographic recording. Results As compared to the reference, the 1 g EPH dose increased tryptophan bioavailability from baseline, and, both doses improved heart rate variability parameters related to parasympathetic activation while showing differences in the late neural response to negative versus neutral emotions. Post-hoc analyses indicated a gender difference in the baseline tryptophan bioavailability and further examination suggested the change in mood rating depends on the interaction between gender and change from baseline of tryptophan bioavailability. Conclusions Overall, this study suggests that low levels of tryptophan rich EPH in a food matrix positively impact mood or stress in acute settings and adds to the body of evidence linking tryptophan and dietary sources thereof with these benefits. Confirmatory randomized controlled trials are needed to confirm these findings. Trial registration number: CER-VD N°2019-00218

Indoleamine 2,3-dioxygenase (IDO)-1 and IDO-2 activity and severe course of COVID-19.

COVID‐19 is a pandemic with high morbidity and mortality. In an autopsy cohort of COVID‐19 patients, we found extensive accumulation of the tryptophan degradation products 3‐hydroxy‐anthranilic acid and quinolinic acid in the lungs, heart, and brain. This was not related to the expression of the tryptophan‐catabolizing indoleamine 2,3‐dioxygenase (IDO)‐1, but rather to that of its isoform IDO‐2, which otherwise is expressed rarely. Bioavailability of tryptophan is an absolute requirement for proper cell functioning and synthesis of hormones, whereas its degradation products can cause cell death. Markers of apoptosis and severe cellular stress were associated with IDO‐2 expression in large areas of lung and heart tissue, whereas affected areas in brain were more restricted. Analyses of tissue, cerebrospinal fluid, and sequential plasma samples indicate early initiation of the kynurenine/aryl‐hydrocarbon receptor/IDO‐2 axis as a positive feedback loop, potentially leading to severe COVID‐19 pathology. © 2021 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd on behalf of The Pathological Society of Great Britain and Ireland.

The gut microbiota metabolite indole increases emotional responses and adrenal medulla activity in chronically stressed male mice

Background and aimsThe gut microbiota produces metabolites that are an integral part of the metabolome and, as such, of the host physiology. Changes in gut microbiota metabolism could therefore contribute to pathophysiological processes. We showed previously that a chronic and moderate overproduction of indole from tryptophan in male individuals of the highly stress-sensitive F344 rat strain induced anxiety-like and helplessness behaviors. The aim of the present study was to extend the scope of these findings by investigating whether emotional behaviors of male mice that are moderately stress-sensitive but chronically exposed to environmental stressors would also be affected by indole. MethodsWe colonized germ-free male C3H/HeN mice with a wild-type indole-producing Escherichia coli strain, or with the non-indole producing mutant. Gnotobiotic mice were subjected to an unpredictable chronic mild stress procedure, then to a set of tests aimed at assessing anxiety-like (novelty and elevated plus maze tests) and depression-like behaviors (coat state, splash, nesting, tail suspension and sucrose tests). Results of the individual tests were aggregated into a common z-score to estimate the overall emotional response to chronic mild stress and chronic indole production. We also carried out biochemical and molecular analyses in gut mucosa, plasma, brain hippocampus and striatum, and adrenal glands, to examine biological correlates that are usually associated with stress, anxiety and depression. ResultsChronic mild stress caused coat state degradation and anhedonia in both indole-producing and non-indole producing mice, but it did not influence behaviors in the other tests. Chronic indole production did not influence mice behavior when tests were considered individually, but it increased the overall emotionality z-score, specifically in mice under chronic mild stress. Interestingly, in the same mice, indole induced a dramatic increase of the expression of the adrenomedullary Pnmt gene, which is involved in catecholamine biosynthesis. By contrast, systemic tryptophan bioavailability, brain serotonin and dopamine levels and turnover, as well as expression of gut and brain genes involved in cytokine production and tryptophan metabolism along the serotonin and kynurenine pathways, remained similar in all mice. ConclusionsChronic indole production by the gut microbiota increased the vulnerability of male mice to the adverse effects of chronic mild stress on emotional behaviors. It also targeted catecholamine biosynthetic pathway of the adrenal medulla, which plays a pivotal role in body’s physiological adaptation to stressful events. Future studies will aim to investigate the action mechanisms responsible for these effects.

Variation of Genes Encoding Tryptophan Catabolites Pathway Enzymes in Stroke.

The abnormal activation of the tryptophan catabolites pathway (TRYCATs) is observed in patients suffering from cerebrovascular disease, including stroke. A previous study confirmed that lower bioavailability of tryptophan for serotonin synthesis was characterized in the patients during the acute stroke phase. Interestingly, according to various studies, polymorphisms of the genes involved in the TRYCATs pathway may modulate the risk of stroke occurrence. Therefore, this study aimed to investigate the association between the occurrence of TPH1, TPH2, KAT1, KAT2 and IDO1 polymorphisms and the risk of stroke development.The following 10 polymorphisms of the genes encoding enzymes of the TRYCATs pathway were selected: c.804-7C > A (rs10488682), c.-1668T > A (rs623580), c.803+221C > A (rs1800532), c.-173A > T (rs1799913) – TPH1, c.-1449C > A (rs7963803), and c.-844G > T (rs4570625) – TPH2. c.*456G > A of KAT1 (rs10988134), c.975-7T > C of KAT2 (rs1480544), c.-1849C > A (rs3824259) and c. -1493G > C (rs10089084) of IDO1. The study was carried out on DNA isolated from the peripheral blood taken from 107 patients after a stroke and 107 healthy volunteers. All DNA samples were genotyped using TaqMan probes. The genotypes of eight studied polymorphisms modulated the risk of stroke. No significant difference in genotype and allele frequencies of the c.804-7C > A –TPH1 (rs10488682) and c.*456G > A – KAT1 (rs10988134) polymorphisms were found between patients and controls. Having performed haplotype and gen-gen analyses, it was possible to determine that patients after a stroke and controls differed in terms of the frequency of selected genotypes and haplotypes. Among the studied polymorphisms, eight SNPs were linked with stroke risk modulation. The results obtained confirmed our hypothesis regarding the involvement of the TRYCATs pathway in the pathogenesis of stroke.

Chronic Mild Stress Alters Kynurenine Pathways Changing the Glutamate Neurotransmission in Frontal Cortex of Rats.

Immune stimulation might be involved in the pathophysiology of major depressive disorder (MDD). This stimulation induces indoleamine 2,3-dioxygenase (IDO), an enzyme that reduces the tryptophan bioavailability to synthesize serotonin. IDO products, kynurenine metabolites, exert neurotoxic/neuroprotective actions through glutamate receptors. Thus, we study elements of these pathways linked to kynurenine metabolite activity examining whether antidepressants (ADs) can modulate them. Male Wistar rats were exposed to chronic mild stress (CMS), and some of them were treated with ADs. The expression of elements of the IDO pathway, including kynurenine metabolites, and their possible modulation by ADs was studied in the frontal cortex (FC). CMS increased IDO expression in FC compared to control group, and ADs restored the IDO expression levels to control values. CMS-induced IDO expression led to increased levels of the excitotoxic quinolinic acid (QUINA) compared to control, and ADs prevented the rise in such levels. Neither CMS nor ADs changed significantly the antiexcitotoxic kynurenic acid (KYNA) levels. The QUINA/KYNA ratio, calculated as excitotoxicity risk indicator, increased after CMS and ADs prevented this increase. CMS lowered excitatory amino acid transporter (EAAT)-1 and EAAT-4 expression, and some ADs restored their expression levels. Furthermore, CMS decreased N-methyl-D-aspartate receptor (NMDAR)-2A and 2B protein expression, and ADs mitigated this decrease. Our research examines the link between CMS-induced pro-inflammatory cytokines and the kynurenine pathway; it shows that CMS alters the kynurenine pathway in rat FC. Importantly, it also reveals the ability of classic ADs to prevent potentially harmful situations related to the brain scenario caused by CMS.

Increased sensitivity to tryptophan bioavailability is a positive adaptation by the human strains of Chlamydia pneumoniae

One of the most significant activities induced by interferon-gamma against intracellular pathogens is the induction of IDO (indoleamine 2,3-dioxygenase) expression, which subsequently results in the depletion of tryptophan. We tested the hypothesis that human strains of Chlamydia pneumoniae are more sensitive to tryptophan limitation than animal C. pneumoniae strains. The human strains were significantly more sensitive to IFN-γ than the animal strains in a lung epithelia cell model (BEAS-2B), with exposure to 1 U ml(-1) IFN-γ resulting in complete loss of infectious yield of human strains, compared to the animal strains where reductions in infectious progeny were around 3.5-4.0 log. Strikingly, the IFN-γ induced loss of ability to form infectious progeny production was completely rescued by removal of the IFN-γ and addition of exogenous tryptophan for the human strains, but not the animal strains. In fact, a human heart strain was more capable of entering a non-infectious, viable persistent stage when exposed to IFN-γ and was also more effectively rescued, compared to a human respiratory strain. Exquisite susceptibility to IFN-γ, specifically due to tryptophan availability appears to be a core adaptation of the human C. pneumoniae strains, which may reflect the chronic nature of their infections in this host.

Activation of the Kynurenine Pathway in the Acute Phase of Stroke and its Role in Fatigue and Depression Following Stroke

Many stroke survivors suffer from poststroke fatigue (PSF) and poststroke depression (PSD), indicating the importance of increasing the base of knowledge about the mechanisms underlying these sequelae. The primary aim of this study was to determine whether activation of the kynurenine (KYN) pathway predicts subsequent fatigue or depression in acute ischemic stroke (AIS) patients. Acute serum levels of 5-hydroxytryptamine (5-HT), tryptophan (TRP) catabolites (TRYCATs), and competing amino acids, as well as subsequent fatigue and depression, were measured in 45 stroke patients. TRP index [=100 × TRP / (tyrosine + valine + phenylalanine + leucine + isoleucine)] was significantly lower in patients with a Fatigue Severity Scale (FSS) score of ≥4 at 12 months than in those with an FSS score of <4 (p = 0.039). Furthermore, the serum level of kynurenic acid in the acute stroke phase was significantly higher in patients with an FSS of score ≥4 at 18 months than in those with an FSS score of <4 (p = 0.026). These findings indicate that stroke patients with PSF have a lower bioavailability of TRP for 5-HT synthesis in the brain in the acute stroke phase. However, they also appear to have greater neuroprotective potential in that phase. In contrast to PSF, no predictors of PSD were found. These findings together with those of previous studies suggest that the immune response and indoleamine 2,3-dioxygenase activation that follows AIS can predict PSF but not PSD.

Effect of diet on serotonergic neurotransmission in depression

Depression is characterized by sadness, purposelessness, irritability, and impaired body functions. Depression causes severe symptoms for several weeks, and dysthymia, which may cause chronic, low-grade symptoms. Treatment of depression involves psychotherapy, medications, or phototherapy. Clinical and experimental evidence indicates that an appropriate diet can reduce symptoms of depression. The neurotransmitter, serotonin (5-HT), synthesized in the brain, plays an important role in mood alleviation, satiety, and sleep regulation. Although certain fruits and vegetables are rich in 5-HT, it is not easily accessible to the CNS due to blood brain barrier. However the serotonin precursor, tryptophan, can readily pass through the blood brain barrier. Tryptophan is converted to 5-HT by tryptophan hydroxylase and 5-HTP decarboxylase, respectively, in the presence of pyridoxal phosphate, derived from vitamin B6. Hence diets poor in tryptophan may induce depression as this essential amino acid is not naturally abundant even in protein-rich foods. Tryptophan-rich diet is important in patients susceptible to depression such as certain females during pre and postmenstrual phase, post-traumatic stress disorder, chronic pain, cancer, epilepsy, Parkinson’s disease, Alzheimer’s disease, schizophrenia, and drug addiction. Carbohydrate-rich diet triggers insulin response to enhance the bioavailability of tryptophan in the CNS which is responsible for increased craving of carbohydrate diets. Although serotonin reuptake inhibitors (SSRIs) are prescribed to obese patients with depressive symptoms, these agents are incapable of precisely regulating the CNS serotonin and may cause life-threatening adverse effects in the presence of monoamine oxidase inhibitors. However, CNS serotonin synthesis can be controlled by proper intake of tryptophan-rich diet. This report highlights the clinical significance of tryptophan-rich diet and vitamin B6 to boost serotonergic neurotransmission in depression observed in various neurodegenerative diseases. However pharmacological interventions to modulate serotonergic neurotransmission in depression, remains clinically significant. Depression may involve several other molecular mechanisms as discussed briefly in this report.

Liquid Concentrates Are Lower in Bioavailable Tryptophan than Powdered Infant Formulas, and Tryptophan Supplementation of Formulas Increases Brain Tryptophan and Serotonin in Rats

The bioavailability of tryptophan in powdered and/or liquid concentrate forms of milk-based infant formulas was determined by studying rat growth response by using a slope ratio method (food conversion efficiency: weight gain/food consumed vs. tryptophan consumed). A gelatin basal diet formulated to be adequate in all nutrients, except tryptophan (0.03%), for rat growth was supplemented with graded levels of crystalline L-tryptophan (0.02, 0.04, 0.06, 0.08, 0.10, 0.12 and 0.14%, standard diets) or infant formulas providing 0.04 and 0.08% supplemental tryptophan (test diets). These diets were fed to weanling rats for 2 wk. Tryptophan bioavailabilities of various formulas varied from 83 to 95%, with some of the liquid concentrates having the lowest values. The levels of bioavailable tryptophan in the liquid concentrate forms (9.7–12.6 mg/g protein) and the powdered forms (11.1–13.1 mg/g protein) were considerably lower than those of human milk (17–19 mg/g protein). Supplementation of the liquid concentrates with graded levels of L-tryptophan (0.1, 0.5 and 1.0%) had no effect on protein quality indices, based on rat growth, but resulted in a dose-related increase in the concentrations of tryptophan in the plasma and brain and of serotonin and 5-hydroxyindole-3-acetic acid in the brains of rats. This study supports further research to investigate the influence of tryptophan supplementation of infant formulas, to more closely simulate tryptophan composition of human milk, on tryptophan metabolites and their potential related effects on sleep latency and neurobehavioral developments in infants.

Bioavailability of tryptophan in soybean meal and tryptophan retention in the carcasses of four-week-old ducks

The objectives of the experiment were to determine the bioavailability of tryptophan in soybean meal for 4-wk-old ducks using the slope-ratio bioassay with L-tryptophan as the reference diet and estimate the efficiency of dietary tryptophan retention in the carcasses of 4- to 7-wk-old ducks. An initial representative group of 12 ducks with an average body weight of 1,951 g (SD = 69) were killed for carcass compositional analysis. A basal diet formulated to contain 1.13 g of tryptophan/kg and adequate in all other amino acids, was supplemented with 0.2, 0.4, or 0.6 g tryptophan/kg from L-tryptophan, or 0.2 or 0.4 g tryptophan/kg from soybean meal. Four pens of three 4-wk-old ducks received each of the six diets for 21 d. Ducks fed the basal diet and the basal diet supplemented with 0.2 or 0.4 g of L-tryptophan/kg for 21 d were used as the final slaughter groups for carcass compositional analysis. Dietary supplemental tryptophan from either L-tryptophan or soybean meal linearly increased final weight (P < 0.05), weight gain (P < 0.10), and gain:feed ratio (P < 0.05). Common-intercept, multiple linear regressions in slope-ratio methodology performed using weight gain and gain:feed ratio as dependent variables and grams of supplemental tryptophan/kg of diet as the independent variable gave 94 and 92% tryptophan bioavailability in soybean meal relative to L-tryptophan, respectively. Linear regression of tryptophan deposition in the carcass on tryptophan intake resulted in a 21% efficiency of carcass tryptophan retention above maintenance.

Bioavailability of tryptophan in soybean meal for 10-kg pigs using slope-ratio assay.

A basal diet formulated to contain 1.1 (analyzed to contain 1.03) g of tryptophan/kg diet as-fed, but adequate in all other amino acids, was supplemented with .2, .4, or .6 g tryptophan/kg from L-tryptophan or .2 or .4 g tryptophan/kg from soybean meal. Each of the six diets was fed for 28 d to six barrows and six gilts (crossbred; Hampshire x Duroc sires by Yorkshire x Landrace dams) in individual pens and with an average initial weight of 9.8 kg. Dietary supplemental tryptophan from either L-tryptophan or soybean meal increased (P < .05) daily weight gain, feed intake, and gain:feed ratio. Plasma concentrations of tryptophan increased and urea decreased with increasing dietary L-tryptophan supplementation (P < .05). Common-intercept, multiple linear regression in a slope-ratio methodology were performed using nonpartitioned and partitioned daily weight gain, gain:feed ratio, or plasma tryptophan: urea ratio as a dependent variable and supplemental tryptophan level (g/kg), daily total tryptophan intake (g/d), or daily supplemental tryptophan intake (g/d) as independent variables. For each of the three dependent variables, estimates of tryptophan bioavailability were > 100% when daily supplemental tryptophan intake was used as the independent variable. Estimates of TRP bioavailability based on nonpartitioned ADG were 90, 99, and 110% when supplemental TRP level, daily total TRP intake, and daily supplemental TRP intake were used as the independent variables, respectively.

Inflammation-associated changes in the cellular availability of tryptophan and kynurenine in renal transplant recipients

In the course of evaluating the hypothesis that tryptophan or tryptophan metabolites mediate some of the physiological or pathological aspects of the inflammatory response, we assessed the bioavailability of tryptophan and kynurenine in renal allograft recipients during periods of stable graft function, acute rejection and OKT3 therapy. In normal controls and patients with stable function, approximately 8% of the tryptophan and less than 5% of the kynurenine in serum were present in the freely diffusable form. The free tryptophan concentration was significantly increased during acute rejection, while free tryptophan as well as total and free kynurenine concentrations were significantly increased during OKT3 therapy. In each case the ratio of free indole to the sum of the plasma concentrations of large neutral amino acids was also increased. In vitro studies of indole binding to human serum proteins demonstrated the parallel displacement of bound tryptophan and kynurenine by physiological changes in pH, serum albumin concentration and free fatty acid concentration. The results suggest that inflammation associated increases in the oxidative metabolism of tryptophan are accompanied by the increased availability of serum indoles for intracellular metabolism in the tissues.

Bioavailable Level and Source of Cysteine Determine Protein Quality of a Commercial Enteral Product: Adequacy of Tryptophan but Deficiency of Cysteine for Rats Fed an Enteral Product Prepared Fresh or Stored Beyond Shelf Life

Young rats were used in bioassays designed to assess the protein quality and tryptophan as well as cystine adequacy of the enteral product TwoCal-HN that was either freshly prepared or had been stored (nonrefrigerated) in a warehouse for 10 mo (i.e., beyond shelf life). Based upon supplementation studies, cystine was observed to be the first-limiting amino acid in both fresh and expired TwoCal-HN, and tryptophan was not second limiting. Protein efficiency ratio (PER) of expired, but not fresh, TwoCal-HN was lower than that of the casein control diet, but with cystine supplementation, PER of the TwoCal-HN products was equal to or greater than the PER of the casein control. With a diet containing 10 g protein/100 g that also contained energy-furnishing ingredients simulating TwoCal-HN, maximal growth enhancement occurred with a supplement of 1 g cystine/kg diet. Both glutathione and N-acetyl-L-cysteine were observed to be equivalent to an isomolar level of L-cystine in stimulating growth. Using a chemically defined amino acid diet that was singly deficient in tryptophan, bioavailability of tryptophan was determined for casein, fresh TwoCal-HN, expired TwoCal-HN and D-tryptophan. Slope-ratio bioefficacy values relative to L-tryptophan (weight gain regressed on supplemental tryptophan intake) indicated that none of the experimental sources of tryptophan had bioavailabilities different from 100%. The results indicated that tryptophan did not deteriorate, as measured analytically or biologically, as a result of storing TwoCal-HN beyond shelf life.(ABSTRACT TRUNCATED AT 250 WORDS)

Urinary 6-hydroxymelatonin sulfate as a measure of melatonin secretion during acute tryptophan depletion

The essential amino acid tryptophan (TRP) is the precursor for serotonin (5-HT), which is in turn an intermediary product in the synthesis of melatonin. Acute TRP depletion has recently been shown to decrease nocturnal plasma levels of melatonin in humans. Melatonin is metabolized to 6-hydroxymelatonin sulfate (6-SM), a highly stable end-product which is excreted into urine. To determine the effects of TRP bioavailability on 6-SM, 11 healthy volunteers underwent active and sham TRP depletion in a randomized, double-blind fashion. Samples of plasma free and total TRP, plasma melatonin, and urinary 6-SM were obtained before and after the depletion. Acute TRP depletion decreased free and total plasma tryptophan levels by more than 80% from baseline levels. Nocturnal 6-SM excretion was significantly decreased and highly correlated with decreases in plasma melatonin. These results suggest that nocturnal urinary 6-SM excretion is a valid measure of melatonin secretion under conditions of decreased 5-HT function. Collection of urine for 6-SM is considerably easier than nocturnal plasma sampling for melatonin. Further studies are needed to clarify the relationship between 6-SM excretion and other measures of 5-HT function in neuropsychiatric disorders.

Protein Quality and Metabolizable Energy of Corn Gluten Feed

Four experiments were conducted using chicks to evaluate the protein quality of four corn gluten feed (CGF) samples (CGF1 through 4). In addition, a 48-h excreta-collection assay was conducted using cecectomized (CEC) and conventional (CONV) cockerels to determine amino acid digestibility and TMEn. The protein efficiency and net protein ratios, estimated from feeding chicks a N-free diet or diets containing 9% CP supplied solely by CGF, were significantly higher for CGF3 (2.9 and 3.3, respectively) relative to the other three CGF diets (χ̄ = 2.2 and 2.7, respectively). The order of amino acid limitation in CGF was determined with a 14% CP, semipurified diet supplemented with various combinations of essential amino acids. The results indicated that Trp and Lys were equally first-limiting in CGF protein, with Arg being the third-limiting amino acid. Lysine bioavailability, determined by a chick growth assay, was 83, 63, 87, and 57% for CGF1 through 4, respectively. The values did not differ significantly among CGF sources, but the Lys bioavailability values for CGF2 and 4 were significantly lower man that for crystalline L-Lys·HCl (100%). Tryptophan bioavailability in one CGF from two chick growth assays averaged 68%.Lysine digestibility values for CEC birds used in the excreta-collection assay were 69, 60, 72, and 79%; Trp digestibility values were 80, 65, 80, and 86% for CGF1 through 4, respectively. Digestibility values from CEC birds were significantly lower than those from CONV birds for most of the amino acids in CGF2 but not the other CGF. The TMEn values for CEC birds averaged 7.5% lower than those for CONV birds, with the differences being significant for CGF2. The TMEn values measured in CONV birds were 3.179, 2.754, 2.776, and 2.785 kcal per g of DM for CGF1 through 4, respectively.

Bioavailability of tryptophan in selected foods by rat growth assay.

Tryptophan bioavailabilities were estimated in 16 protein sources using 10 day rat growth assays with casein as the reference protein. Growth responses of rats fed test food diets were compared to growth responses of rats fed basal diets with graded levels of tryptophan ranging from 50 to 100 mg of tryptophan/100 g diet. Estimates of tryptophan availabilities were 85-100% for all products except whole wheat cereal (73%) and pinto beans (59%). Results of a previous study on lysine availability indicated that poor response to pinto beans was due either to poor digestibility or to the presence of some unidentified growth inhibitor.

Oxidation rate of free and protein‐bound tryptophan by hydrogen peroxide and the bioavailability of the oxidation products

AbstractThe oxidation of free and protein‐bound tryptophan by hydrogen peroxide (H2O2) has been used to evaluate the potential risk of oxidative damage of tryptophan during processing of foods. In the presence of excess H2O2 the first order rate constant for the oxidation of free tryptophan was 0.362 h−1 (at pH 8.4 and 50°C). This was 5 and 15 times faster than the rate of oxidation of tryptophan bound in casein or lactalbumin, respectively. To determine to what extent the oxidation products are utilisable in vivo, three levels of tryptophan oxidation were tested in a metabolic study. L[3‐14C]‐tryptophan was treated with H2O2 to give 0, 54 and 100% oxidation of the amino acid. The products were then given to rats fed ad libitum on a diet limiting in tryptophan. Radioactivity measurements of liver protein showed that oxidation products were not utilised for protein synthesis. For unreacted tryptophan only 4.6%±0.4 of the ingested dose was recovered in urine and 6.5%±0.8 in faeces (42 h period following ingestion). For completely oxidised tryptophan the recovered proportions increased to 32.6%±1.8 and 21.1%±1.5, respectively, which again indicates a reduced ability of the rat to use the oxidation products for anabolic purposes. Finally, H2O2‐treated casein was fed to rats in a growth assay which showed no significant decrease in tryptophan bioavailability. This supports the results obtained chemically. Even under the relatively strong oxidising conditions used in this study, the results indicate that protein‐bound tryptophan is not very sensitive to oxidation and, therefore, loss of tryptophan would not be an important limiting factor in food processing.