Protein turnover and energy metabolism of elderly women fed a low-protein diet
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Protein turnover
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Low-protein diet
Summary: Protein turnover was studied in eight premature infants of conceptual age 26-37 weeks. A stochastic model based upon [15N]urea or [15N]ammonia excretion following a single injection of [15N]glycine was used to estimate rates of whole body protein synthesis and catabolism. The urinary 3-methylhistidine/creatinine ratio was determined to differentiate skeletal muscle protein breakdown from total protein catabolism. The rates of whole body protein synthesis ranged from 5.2 to 13.2 g·kg−1·day−1 and protein catabolism ranged from 4.1 to 12.4 g·kg−1·day−1. Linear regression analyses of conceptual age versus (a) whole body nitrogen flux, (b) protein synthesis, and (c) protein catabolism showed significant inverse relationships. A similar relationship obtained between conceptual age and the urine 3-methylhistidine/creatinine ratio. Muscle protein breakdown did not vary with conceptual age, but the fraction of whole body protein breakdown derived from muscle protein breakdown increased significantly with advancing maturation. The ratio net tissue protein gain/total body protein synthesis increased significantly with increasing body weight. Net tissue protein gain appeared to be directly related to protein and caloric intake. The ratio of the rate of whole body protein synthesis and protein intake was greatest in the youngest infants and declined with maturation. A similar relationship was not found between the ratio of protein synthesis and caloric intake and the degree of maturity. More than 90% of nitrogen entering the metabolic pool was used for protein synthesis and more than 50% of calories administered were similarly utilized. We conclude that: (a) protein turnover in premature infants is far more rapid than in term infants, children, or adults and is inversely related to conceptual age; (b) muscle protein turnover constitutes a greater fraction of overall turnover with advancing maturity; (c) energy and protein intake affect net tissue protein gain significantly in rapidly growing infants; and (d) the efficiency of protein synthesis as a function of protein intake is higher in the most immature infants.
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Muscle protein turnover and amino acid (AA) exchange were studied in 4 patients with chronic renal failure (CRF) and in 5 controls in the postabsorptive state by using the forearm perfusion method together with the systemic infusion of 3H-Phe. In CRF patients muscle protein breakdown is increased and is associated with a parallel increase in protein synthesis. Protein breakdown is inversely related to arterial bicarbonate. Net proteolysis is unchanged. The release of total AA, glutamine and alanine is not different from controls, whereas the release of valine and leucine is reduced and serine uptake tends to be decreased. In conclusion, in postabsorptive patients with CRF, well before the uremic stage, an increased protein breakdown associated with metabolic acidosis takes place; net proteolysis is unaffected. Alterations in BCAA metabolism suggest the occurrence of increased BCAA degradation proceeding beyond the transamination step.
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Protein turnover
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Our studies have focused on the regulation of whole body and skeletal muscle protein metabolism in premature infants. Net deposition of protein is the result of a positive balance between protein synthesis and breakdown. To measure protein metabolism we have employed end-product studies with [ 15 N]glycine and 13 [C]leucine. Myofibrillar protein degradation was estimated by measuring the excretion of N-t-methylhistidine in urine. Energy expenditure and substrate utilization were also measured. Premature infants have high rates of protein synthesis (12 g∙kg −1 ∙d −1 ), twice those measured in children and four times those found in adults. Intrauterine malnourished babies have increased rates of protein turnover. Very low birth weight infants (< 1500 g) have higher myofibrillar protein turnover than larger babies. Intravenous feeding decreases whole body protein turnover, and we estimate visceral protein synthesis to be approximately 4 g∙kg −1 ∙d −1 . Suboptimal energy intake worsens nitrogen utilization by reducing the reutilization of endogenous amino acids for protein synthesis. We have also examined the effects of varying the source of nonprotein energy (i.e., glucose only versus glucose plus lipid) at requirement levels and have shown there is no effect on protein metabolism. Recent improvements in technology have opened the way to detailed study of individual amino acid metabolism in neonates in the future.
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Adaptation to a low-protein diet (LPD) involves a reduction in the rate of amino acid (AA) flux and oxidation, leading to more efficient use of dietary AA and reduced ureagenesis. Of note, the concept of 'adaptation' to low-protein intakes has been separated from the concept of 'accommodation', the latter term implying a decrease in protein synthesis, with development of wasting, when dietary protein intake becomes inadequate, i.e. beyond the limits of the adaptive mechanisms. Acidosis, insulin resistance and inflammation are recognized mechanisms that can increase protein degradation and can impair the ability to activate an adaptive response when an LPD is prescribed in a chronic kidney disease (CKD) patient. Current evidence shows that, in the short term, clinically stable patients with CKD Stages 3-5 can efficiently adapt their muscle protein turnover to an LPD containing 0.55-0.6 g protein/kg or a supplemented very-low-protein diet (VLPD) by decreasing muscle protein degradation and increasing the efficiency of muscle protein turnover. Recent long-term randomized clinical trials on supplemented VLPDs in patients with CKD have shown a very good safety profile, suggesting that observations shown by short-term studies on muscle protein turnover can be extrapolated to the long-term period.
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This study was conducted to understand further the mechanisms underlying the loss of body nitrogen after trauma. Six patients who underwent abdominal surgery and six for control were studied. The measurement of whole body protein turnover was made on the 3rd and 10th postoperative day during total parenteral nutrition with constant infusion of [ 15 N]glycine according to Picou and Taylor‐Roberts. The measurement was also made on six control patients during total parenteral nutrition in the nonstressed state. The rates of whole body protein turnover, synthesis, and breakdown were calculated from the plateau 15 N enrichment of urinary total N, which was analyzed with a mass spectrometer. The values were compared with control by Student's t‐test, and the changes in the individual patients were examined by a paired t‐test. Immediately after the operation, whole body protein turnover and breakdown were significantly elevated (p < 0.05 and <0.02, respectively), and decreased with the improvement of N balance after recovery from stress by 0.95 ± 0.21 and 0.61 ± 0.13 g. protein/kg.day, respectively. The changes in whole body protein turnover and breakdown were statistically significant (p < 0.005 and <0.005, respectively). However, no tendency of alteration in whole body protein synthesis was found throughout the study. It is concluded that protein turnover rate increases in surgical stress, and that the increased protein catabolism rather than the alteration in synthesis could account for the postoperative nitrogen losses. (Journal of Parenteral and Enteral Nutrition 9 :452–455, 1985)
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Convalescence
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