Current issues in determining dietary protein and amino-acid requirements

Pregnancy and the first two years of life are periods of rapid growth and yet the knowledge of requirements for protein and dietary indispensable amino acids is very limited. The development of carbon oxidation methods opens the way to studies that should fill these important gaps in knowledge.

Endogenous proteins in the ileal digesta of adult humans given casein-, enzyme-hydrolyzed casein- or crystalline amino-acid-based diets in an acute feeding study

BACKGROUND/OBJECTIVES

To ascertain if the form of dietary nitrogen (free amino acids (AA), small peptides, or intact protein) affects the endogenous nitrogen containing substances lost from the upper digestive tract of humans.

SUBJECTS/METHODS

Digesta were collected via a naso-ileal tube from the terminal ileum of 16 adult humans in a single parallel study following an acute feeding regimen. Subjects were given an iso-nitrogenous and isocaloric test meal containing 150 g of casein (CAS) (n=6), enzyme-hydrolyzed casein (HCAS) (n=5) or crystalline AA (n=5) dissolved in 550 ml of water, as the sole sources of nitrogen.

RESULTS

The mean concentrations and flows of total nitrogen, protein nitrogen, and soluble protein nitrogen passing the terminal ileum were significantly higher (P <0.01) for the CAS and HCAS test-meal groups compared to the AA meal group. Dietary CAS and HCAS had a considerable influence on digesta mucin concentrations and flows compared to free AA (+41%). Only 3-4% of the total nitrogen remained unidentified.

CONCLUSIONS

The form of dietary nitrogen (protein, small peptides or free AA) had an acute effect upon the secretion or reabsorption of endogenous proteins in the small intestine of healthy humans, as evident from significant differences in both the quantity and composition of the proteins found in digesta at the end of the ileum.

Hydrolyzed collagen improves bone status and prevents bone loss in ovariectomized C3H/HeN mice

SUMMARY

This study evaluates the effect of hydrolyzed collagen (HC) on bone health of ovariectomized mice (OVX) at different ages. Twenty-six weeks after the OVX procedure, HC ingestion was still able to improve significantly bone mineral density (BMD) and some femur biomechanical parameters. Moreover, HC ingestion for 1 month before surgery prevented BMD decrease.

INTRODUCTION

HC can play an important role in preserving BMD before osteoporosis appears. The aim of this study was to evaluate the effect of HC on bone health of ovariectomized mice at different ages.

METHODS

Female C3H mice were either OVX at 3 or 6 months and fed for 6 months (first experiment) or 3 months (second experiment) with diet including 0, 10, or 25 g/kg of HC. In the second experiment, one group received HC 1 month before surgery, and two groups received the supplementation immediately after surgery, one fed ad libitum and the other by gavage. Mice treated with raloxifene were used as a positive control. BMD, femur intrinsic and extrinsic biomechanical properties, and type I collagen C-terminal telopeptide were measured after 12 and 26 weeks. Food intake and spontaneous physical activity were also recorded.

RESULTS

The OVX procedure increased body weight, while food intake decreased, thus suggesting that resting metabolism was decreased. Ingestion of 25 g/kg of HC for 3 or 6 months reduced bone loss significantly in, respectively, 3- and 6-month-old OVX mice. The lowest HC concentration was less efficient. HC ingestion for 3 months is as efficient as raloxifene to protect 3-month-old OVX mice from bone loss. Our results also demonstrated that HC ingestion before surgery prevented the BMD decreases.

CONCLUSION

This study confirms that dietary collagen reduces bone loss in OVX mice by increasing the diameter of the cortical areas of femurs and can have a preventive effect.

Inhibition of food intake induced by acute stress in rats is due to satiation effects

Acute mild stress induces an inhibition of food intake in rats. In most studies, the cumulative daily food intake is measured but this only provides a quantitative assessment of ingestive behavior. The present study was designed to analyze the reduction in food intake induced by acute stress and to understand which behavioral and central mechanisms are responsible for it. Two different stressors, restraint stress (RS) and forced swimming stress (FSS), were applied acutely to male Wistar rats. We first measured corticosterone and ACTH in plasma samples collected immediately after acute RS and FSS in order to validate our stress models. We measured food intake after RS and FSS and determined meal patterns and behavioral satiety sequences. The expressions of CRF, NPY and POMC in the hypothalamus were also determined immediately after acute RS and FSS. The rise in corticosterone and ACTH levels after both acute RS and FSS validated our models. Furthermore, we showed that acute stress induced a reduction in cumulative food intake which lasted the whole day for RS but only for the first hour after FSS. For both stressors, this stress-induced food intake inhibition was explained by a decrease in meal size and duration, but there was no difference in ingestion speed. The behavioral satiety sequence was preserved after RS and FSS but grooming was markedly increased, which thus competed with, and could reduce, other behaviors, including eating. Lastly, we showed that RS induced an increase in hypothalamic POMC expression. These results suggest that acute stress may affect ingestive behavior by increasing satiation and to some extent by enhancing grooming, and this may be due to stimulation of the hypothalamic POMC neurons.

Alterations of lipid metabolism and gene expression in rat adipocytes during chronic olanzapine treatment

Although antipsychotics are established drugs in schizophrenia treatment, they are admittedly known to induce side effects favoring the onset of obesity and worsening its complications. Despite potential involvement of histamine receptor antagonism, or of other neurotransmitter systems, the mechanism by which antipsychotic drugs increase body weight is not elucidated. The aim of the present study was to investigate whether chronic antipsychotic treatments can directly alter the regulation of two main functions of white adipose tissue: lipolysis and glucose utilization. The influence of a classical antipsychotic (haloperidol) was compared to that of two atypical antipsychotics, one known to favor weight gain (olanzapine), the other not (ziprasidone). Cell size, lipolytic capacity and glucose transport activity were determined in white adipocytes of rats subjected to 5-week oral treatment with these antipsychotics. Gene expression of adipocyte proteins involved in glucose transport or fat storage and mobilization, such as glucose transporters (GLUT1 and GLUT4), leptin, matrix metallo-proteinase-9 (MMP9), hormone-sensitive lipase (HSL) and fatty acid synthase (FAS) was also evaluated. Adipocytes from chronic olanzapine-treated rats exhibited decreased lipolytic activity, lowered HSL expression and increased FAS expression. These changes were concomitant to enlarged fat deposition and adipocyte size. Alterations were observed in adipocytes from olanzapine-treated rats whereas the other antipsychotics did not induce any notable disorder. Our results therefore show evidence of an effect of chronic antipsychotic treatment on rat adipocyte metabolism. Thus, impairment of fat cell lipolysis should be considered as a side effect of certain antipsychotics, leading, along with the already documented hyperphagia, to the excessive weight gain observed in patients under prolonged treatment..

Effects of amino acid-derived luminal metabolites on the colonic epithelium and physiopathological consequences

Depending on the amount of alimentary proteins, between 6 and 18 g nitrogenous material per day enter the large intestine lumen through the ileocaecal junction. This material is used as substrates by the flora resulting eventually in the presence of a complex mixture of metabolites including ammonia, hydrogen sulfide, short and branched-chain fatty acids, amines; phenolic, indolic and N-nitroso compounds. The beneficial versus deleterious effects of these compounds on the colonic epithelium depend on parameters such as their luminal concentrations, the duration of the colonic stasis, the detoxication capacity of epithelial cells in response to increase of metabolite concentrations, the cellular metabolic utilization of these metabolites as well as their effects on colonocyte intermediary and oxidative metabolism. Furthermore, the effects of metabolites on electrolyte movements through the colonic epithelium must as well be taken into consideration for such an evaluation. The situation is further complicated by the fact that other non-nitrogenous compounds are believed to interfere with these various phenomenons. Finally, the pathological consequences of the presence of excessive concentrations of these compounds are related to the short- and, most important, long-term effects of these compounds on the rapid colonic epithelium renewing and homeostasis.

Activation of vagal afferents in the rat duodenum by protein digests requires PepT1

Intestinal infusion of protein digests activates a vago-vagal reflex inhibition of gastric motility. Protein digests release cholecystokinin (CCK) from enteroendocrine cells; however, the precise cellular mechanisms leading to vagal afferent activation is unclear. The hypothesis that the oligopeptide transporter PepT1 plays a major role in the initiation of this vago-vagal reflex was tested by recording activation of duodenal vagal afferent activity and inhibition of gastric motility in response to protein hydrolysates in the presence of 4-aminomethylbenzoic acid (4-AMBA), a competitive inhibitor of PepT1, or 4-aminophenylacetic acid (4-APAA), an inactive 4-AMBA analog. Duodenal infusion of the protein hydrolysate increased vagal afferent discharge and inhibited gastric motility; these responses were abolished by concomitant infusion of 4-AMBA, but not 4-APAA. Duodenal infusion with Cefaclor, a substrate of PepT1, increased duodenal vagal afferent activity; Cefaclor and protein hydrolysates selectively activated CCK-responsive vagal afferents. This study demonstrates that products of protein digestion increase spontaneous activity of CCK-sensitive duodenal vagal afferents via a mechanism involving the oligopeptide transporter PepT1.

A tryptophan-rich protein diet efficiently restores sleep after food deprivation in the rat

Sleep depends on the quantity and quality of the diet. Several studies have shown that food deprivation results in a reduction in sleep duration. It has also been demonstrated that in the newborn, the supply of certain essential amino acids improves sleep through their action on the synthesis of specific neurotransmitters. The aim of the present study was to test if the quantity and/or quality of dietary protein could improve the recovery of sleep during re-feeding after caloric deprivation. Sleep parameters were compared in rats fed ad libitum, food restricted during 4 days, or reefed isocalorically after food restriction with three dietary regimens varying in terms of the amount (14% versus 30%) or quality (milk protein or alpha-lactalbumin) of protein. The results showed that sleep recovery, in particular slow-wave sleep, was improved in rats re-fed with alpha-lactalbumin. This result confirms the close relationship between feeding and sleep and suggest that alpha-lactabumin could be used to improve sleep in adult submitted to nutritional disturbances such as food restriction, shift work, Ramadan.

Fat-depleted CLA-treated mice enter torpor after a short period of fasting

Resting energy expenditure (Resting-EE), EE with treadmill exercise, and post-prandial thermogenesis were continuously monitored by indirect calorimetry during a 24 h recording session in control (CT) and CLA-treated (CLA) (1% CLA in the food, by weight) C57Bl/6 male mice. After 15 days of CLA treatment, the fat content of CLA mice had fallen to 20% of that in CT mice. CLA mice were able to face the energy challenge of exercise but used less lipid than CT mice. Resting-EE values fell during the post-exercise period. The thermogenic response to a calibrated test meal given 5 h after the run abolished the differences in EE and substrate oxidation between CT and CLA mice. However, 2.5 h after ingestion of the test meal onward, CT mice gradually increased their lipid oxidation to sustain resting-EE levels. In contrast, CLA mice did not increase their lipid oxidation and their resting-EE levels fell significantly until they entered into torpor. Blood leptin was low but similar in CT and CLA-treated mice suggesting that leptin is not critical to induce torpor. We suggest that the durable inhibition of lipid oxidation in fasting CLA mice was an adaptive behaviour devoted at sparing the residual adipose deposits.

Evidence for the absence of an intestinal adaptive mechanism to compensate for C. parvum-induced amino acid malabsorption in suckling rats

In order to assess the impact of Cryptosporidium parvum on host intestinal physiology, we investigated absorption of the two principal amino acids in dam's milk (leucine, glutamate), using Ussing chambers and RT-PCR analyses. Experiments were performed in both heavily (ileum) and mildly (duodenum) infected segments of the small intestine at the peak of infection [day 8 post-infection (PI)] and after spontaneous clearance of the parasite (day 17 PI). At day 8 PI, amino acid fluxes across the mucosa were decreased throughout the small intestine (P<0.01) and EAAT3 mRNA expression was reduced ( from -49% to -28%). At day 17 PI, leucine and glutamate fluxes were normalized but the decrease in EAAT3 mRNA levels persisted (from -31% to -46%). Our results demonstrate that cryptosporidiosis induces major amino acid malabsorption involving the entire small intestine which is not counterbalanced by any up-regulation, even after spontaneous clearance of the parasite.

A carbohydrate diet rich in sucrose increased insulin and WAT in macronutrient self-selecting rats

In order to evaluate the influence of a carbohydrate (CHO) diet rich in sucrose (37%) on food choice and body composition, Wistar rats received a food selection diet (protein, CHO, fat) from the time of weaning to 13 weeks of age. Three groups of animals were examined: the first received a CHO diet containing 37% sucrose; the second, a diet containing only 10% sucrose; and the third, control group, received a complete standard diet (14% protein, 72% CHO including 10% sucrose). Food intakes and body weight (BW) were recorded four times a week. No differences in total food intake were observed between the two self-selecting groups and the control group, and no differences were observed between the two self-selecting groups in terms of their protein intake (about 45% of the total calorie intake). The latter groups modified their selections during the 10-week period, but the variations were similar. BW gain in the 37% group was lower but the white adipose tissue (WAT)/total BW ratio was significantly higher than those seen in the control and 10% groups. Insulinemia was higher in 37% and control groups. In conclusion, the high preferences for protein and fat were identical, whatever the CHO diet composition. The sucrose level in the diet was an essential factor for the development of hyperinsulinemia, leptin resistance and thus a higher prevalence of obesity. These results confirm the importance of the quality of CHO sources in the diet.

Implication of distinct proteins in cadmium uptake and transport by intestinal cells HT-29

The mechanisms of intestinal absorption have not been clearly elucidated for cadmium, a toxic metal. In this work, we show the implication of distinct proteins in cadmium transport, and the transport step where these proteins are involved. We first validated the HT-29 model by evaluating nontoxic doses of cadmium (ranging from 1 to 20 micromol/L), and by quantifying metal uptake and transepithelial transport. The time-course of 1 micromol/L cadmium uptake at pH 7.5 showed three steps: a rapid one during the first 4 min, probably due to cadmium binding to the membrane; a slower one, characterized by Km of 1.65+/-0.54 micromol/L and Vmax of 3.9+/-0.3 micromol/min per mg protein; and a third, corresponding to slow accumulation that was not equilibrated even after 48 h of cadmium exposure. Intracellular metallothionein content following 1 or 5 micromol/L cadmium exposure showed a significant increase after 6 h of exposure, and was not equilibrated even after 72 h, allowing cadmium accumulation. After 24 h of exposure, metallothionein content was 5-fold, 14-fold, 26-fold, and 50-fold, respectively, for cells grown in the presence of 1, 5, 10, and 20 micromol/L cadmium, compared to control cells. The second step of uptake, characterized by carrier-mediated transport, was markedly increased at pH 5.5, compared to pH 7.5, and strongly inhibited by the metabolic inhibitor dinitrophenol. Moreover Nramp2 transporter cDNA was present in HT-29 cells. These data suggest the involvement of a proton-coupled transporter, which may be the divalent cation transporter Nramp2 (natural resistance-associated macrophage protein 2). Cadmium uptake was also inhibited by copper, zinc, and para-chloromercuribenzenesulfonate (pCMBS), but not by verapamil or ouabain. Taken together, our results indicate that cadmium could enter HT-29 cell by Nramp2 proton-coupled active transport and by diffusion, and accumulates in the cell as long as it binds to metallothionein. Cadmium toxicity could depend partly on the activity of Nramp2, and partly on metallothionein content.