Postprandial leucine deficiency failed to alter muscle protein synthesis in growing and adult rats.

OBJECTIVE: This study examined the effect of a specific acute postprandial leucine deficiency on skeletal muscle protein synthesis in growing and adult rats. Because the anabolic action of dietary leucine supplementation is controversial, except during aging, we hypothesized that the maximum leucine effect might be already achieved for a normal postprandial rise of leucine. Preventing this rise during the 1- to 3-h period after feeding may reveal the leucine regulation. METHODS: On the day of the experiment, rats were fasted (postabsorptive, PA group) or fed for 1 h a control meal (postprandial, control, PP group) or a leucine-poor meal (postprandial, PP-Leu group). Muscle protein synthesis was assessed in vivo, over the 1- to 3-h period after meal distribution, using the flooding dose method (L-1-(13)C phenylalanine). RESULTS: As expected, the postprandial increase in plasma free leucine was specifically abolished after feeding the leucine-poor meal, whereas all the other plasma free amino acids were roughly at normal postprandial levels. Plasma insulin increased after feeding in young rats but was constant in adult rats. Plasma insulin was similar whatever dietary leucine levels. Rates of muscle protein synthesis were stimulated by feeding in gastrocnemius and soleus muscles from young rats but only in gastrocnemius muscles from adult rats. The PP-Leu group did not differ from the control PP group regarding muscle protein synthesis. CONCLUSION: The rise in plasma free leucine is not required for the stimulation of muscle protein synthesis during the 1- to 3-h period after feeding young and adult rats, as previously observed in old rats.

Amino acid supplementation prevents the loss of appetite for casein in old Lou/Cjall rats.

A deleterious reduction of casein intake occurring earlier in males than in females had been previously observed in old Lou/Cjall rats. On the contrary, protein intake was observed to be maintained in old males when they were offered whey protein. Present studies were designed to investigate the effect of dietary casein modification on protein decrease. In two lifelong studies, male and female Lou/Cjall rats were tested every four months in order to study protein intake depending on the protein available: casein, whey protein or casein supplemented with an amino acid mixture (SC). In subsequent cross-sectional studies, young, adult, middle-aged and old rats were successively fed with casein, casein supplemented either with leucine or with alanine or with glycine. Supplementing casein with an amino acid mixture both globally increased protein intake and allowed old males to maintain a high rate of protein intake. In cross-sectional experiments, no effect of supplementation was seen in the young group. In older animals, the greatest effect was seen when casein was supplemented with alanine or glycine, independently of sex and age. We therefore, concluded that supplemented casein is more beneficial for old rats than casein alone, probably by increasing amino acid availability. We hypothesize that alanine could act through its effect on gluconeogenesis.

Increased availability of leucine with leucine-rich whey proteins improves postprandial muscle protein synthesis in aging rats.

OBJECTIVE: We previously found that aging was characterized by a decreased sensitivity of muscle protein synthesis to leucine and that a free leucine-supplemented diet corrected this defect in old rats and elderly humans. The present experiment was undertaken to evaluate the efficiency of selected leucine-rich proteins to stimulate postprandial muscle protein synthesis in old rats to optimize nutritional protein support in the elderly. METHODS: Sixty rats (22 mo old) received an experimental meal for the first hour of feeding and a standard diet for the rest of the day for 30 d. Experimental meals contained milk proteins that differed in leucine content: beta-lactoglobulin (14.5% leucine), Prolacta (13.4%), alpha-lactalbumin (10.9%), and casein (10%). As a control, a fifth group was added that received herring flour protein (7.3% leucine). Muscle protein synthesis was determined in vivo in the postprandial state at the end of the 30-d nutritional period using the flooding dose method (1-(13)C phenylalanine). RESULTS: Leucine intake and plasma leucine concentrations were significantly increased in rats fed meals containing the leucine-rich proteins (i.e., beta-lactoglobulin and Prolacta). As previously observed with free leucine-supplemented meals, postprandial muscle protein synthesis was significantly improved in rats fed the meals containing the leucine-rich proteins. Interestingly, the beneficial effect was maintained after the 30-d supplementation. CONCLUSION: The results indicated that leucine-rich proteins were efficient in improving muscle protein synthesis in old rats. Thus, nutritional supplements containing such proteins may be efficient in preventing sarcopenia in the elderly and would represent a safe and optimized nutritional strategy. However, further experiments are necessary to determine the duration of such nutritional support to obtain a significant protein gain in muscle.

Oligosaccharides isolated from goat milk reduce intestinal inflammation in a rat model of dextran sodium sulfate-induced colitis.

BACKGROUND AND AIMS: There is increased interest in the study of manipulation of the flora with pro- and prebiotics regarding inflammatory bowel disease. The aim of this work was to evaluate the effect of oligosaccharides from goat milk in a rat model of dextran sodium sulfate- (DSS-) induced colitis. METHODS: Twenty rats were fed the same diet but with different sources of fiber (5% of the diet): cellulose or a mixture of goat's milk oligosaccharides (GMO) and cellulose. DSS treatment was used to induce a colonic inflammation. Several clinical and inflammatory parameters, as well as intestinal micorbiota and gene expression by DNA microarray technology, were evaluated. RESULTS: DSS induced a decrease in body weight which was not observed in rats fed the GMO (decrease of 21+/-11% in control rats vs increase of 5.2+/-8.6 in GMO rats, P<0.05). DSS also caused an acute colonic inflammatory process which was weaker in rats fed the GMO, as shown by colon myeloperoxidase activity (0.53+/-0.16 vs 0.14+/-0.07U/mg of protein, P<0.05), as well as clinical symptoms measured by a scoring system (1.25+/-1.14 vs 0.4+/-0.07, P<0.05). GMO rats also showed less severe colonic lesions and a more favorable intestinal microbiota. The expression of genes involved in intestinal function, such as mucine-3, was down-regulated in DSS-control rats but returned to normal values in GMO rats. CONCLUSION: GMO reduce intestinal inflammation and contribute to the recovery of damaged colonic mucosa.

Differential effects of insulin and dietary amino acids on muscle protein synthesis in adult and old rats.

The potential roles of insulin and dietary amino acids in the regulation of skeletal muscle protein synthesis were examined in adult and old rats. Animals were fed over 1 h with either a 25% or a 0% amino acid/protein meal. In each nutritional condition, postprandial insulin secretion was either maintained or blocked with diazoxide injections. Protein synthesis in gastrocnemius and soleus muscles was assessed in vivo using the flooding dose method. Insulin suppression decreased protein synthesis in both muscles irrespective of the nutritional condition and age of the rats. Moreover, reduced insulinaemia was associated with 4E-BP1 dephosphorylation, enhanced assembly of the 4E-BP1-eIF4E inactive complex and hypophosphorylation of eIF4E, p70S6k and protein kinase B, key intermediates in the regulation of translation initiation and protein synthesis. Old rats did not differ from adult rats. The lack of amino acids in the meal of insulin-suppressed rats did not result in any additional decrease in protein synthesis. In the presence of insulin secretion, dietary amino acid suppression significantly decreased gastrocnemius protein synthesis in adult but not in old rats. Amino acid suppression was associated with reduced phosphorylation of 4E-BP1 and p70S6k in adults. Along with protein synthesis, only the inhibition of p70S6k phosphorylation was abolished in old rats. We concluded that insulin is required for the regulation of muscle protein synthesis irrespective of age and that the effect of dietary amino acids is blunted in old rats.

Validation of the measurement of glucose appearance rate with [6,6-2H2]glucose in lactating dairy cows.

The aim of this study was to validate the measurement of glucose appearance rate using [6,6-2H2]glucose i.v. infusion in lactating dairy cows. Sample enrichments were analysed by gas chromatography/mass spectrometry. Linearity (enriched solutions) and specificity (enriched plasma) were good: for enrichments ranging between 1.6 and 6.3 mol% excess, the slopes were about 1 and the ordinates at the origin were not different from zero. For a plasma enriched at 3.74 mol% excess, repeatability and long term intralaboratory reproducibility coefficients of variation were 1.31 and 1.90%, respectively. The appearance rates were calculated by two models. The values provided by the steady-state model were not different from those provided by the non-steady-state Steele model. Both models can be used because the treatment effects were similarly discriminated regardless of the model. In our experiments analysing the nutritional effects on Ra in mid-lactating cows, the precision of the method (1.90%) was not the limiting factor to detect a significant difference in Ra compared to the statistical precision obtained with the experimental scheme (4 x 4 and 5 x 5 Latin square design). We conclude that in lactating dairy cows, the measurement of glucose fluxes with this method is relevant and minimally invasive for the animals.

Short-term mild hyperglycemia enhances insulin-stimulated glucose disposal in lactating goats.

This work was designed to study the effect of a 3-day mild hyperglycemia (5.3 vs. 3.3 mM) on the regulation of glucose metabolism in lactating goats. Glucose was intravenously infused at variable rates simultaneously with a constant potassium-amino acid infusion. Diet plus substrate infusion maintained net energy but not protein supply. Milk yield did not change. Skeletal muscle glucose transporter (GLUT-4) was analyzed before and after hyperglycemia. In addition, the acute effect of medium and high insulin doses on glucose turnover was measured in vivo during euglycemic and hyperglycemic hyperinsulinemic clamps under potassium and amino acid replacement. Hyperglycemia reduced the endogenous glucose appearance but increased glucose disposal. It decreased the total membrane-associated GLUT-4 protein in skeletal muscle. In contrast, it improved the acute insulin-stimulated glucose disposal. Both the level and duration (3 days) of hyperglycemia contributed to this improvement. We conclude that short-term mild hyperglycemia has similar effects in lactating goats as those already observed in nonlactating rodents or humans.