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. 2013 Apr;24(4):693-9.
doi: 10.1016/j.jnutbio.2012.03.021. Epub 2012 Jul 26.

Age and Sex Affect Protein Metabolism at Protein Intakes That Span the Range of Adequacy: Comparison of Leucine Kinetics and Nitrogen Balance Data

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Age and Sex Affect Protein Metabolism at Protein Intakes That Span the Range of Adequacy: Comparison of Leucine Kinetics and Nitrogen Balance Data

Travis B Conley et al. J Nutr Biochem. .
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Abstract

Research suggests that changes in leucine oxidation (leuox) with feeding may reflect adult protein requirements. We evaluated this possibility by assessing the effects of age, sex, and different protein intakes on whole-body leucine kinetics and nitrogen balance. Thirty-four young (n=18, 22-46 years) and old (n=16, 63-81 years) men and women completed three 18-day trials with protein intakes of 0.50, 0.75 and 1.00 g protein·kg body weight(-1)·d(-1). Fasting and fed-state leucine kinetics were quantified on day 12 of each trial using a primed, constant infusion of L-[1-13C]leucine. Protein requirement was estimated using classical nitrogen balance measurements and calculations. Leucine kinetics parameters were influenced by age and sex across all protein intakes. With feeding, leuox increased more in old vs. young adults. Independent of age, fasting and fed-state leuox were lower, and net leucine balance (fasting+fed-state) was higher in women vs. men. Among all subjects and protein intakes, nitrogen balance was correlated with fed-state leuox (r=0.39), fed-state leucine balance (r=0.60), net leucine balance (r=0.49) and the change in leuox from the fasting to fed state (r=0.49) (P<.05 for all results). At the highest protein intake, the change in leuox with feeding was inversely correlated with protein requirement (r=-0.39). These findings indicate that leucine kinetics, especially leuox, reflect nitrogen balance-based estimates of the need for dietary protein and generally support the view that protein requirement is comparable between young and old adults.

Figures

Fig. 1
Fig. 1
Experimental design: priming doses of 7.6 μmol·kg-1 l-[1-13C]leucine and 2.35 μmol·kg−1 NaH13CO2 were immediately followed by a continuous infusion of 7.6 μmol·kg−1·h−1 l-[1-13C]leucine for 8 h. *Beverages contained one twelfth of the subject’s daily protein and energy intakes.
Fig. 2
Fig. 2
Change in leucine oxidation between the fasting and fed states at each protein intake. Old vs. young adults were significantly different (P<.05). Error bars represent the standard error of the change in leucine oxidation from the fasting to the fed states. *Main effect of age (different between young and old, P<.05).The change in leucine oxidation was greater in old vs. young across all three protein intakes. The x-axis represents the change in leucine oxidation at each protein intake (0.5, 0.75 and 1.00 (g protein·kg BW−1·d−1). A three-way repeated-measures ANOVA with a mixed-model approach was used.
Fig. 3
Fig. 3
Net leucine balance at each protein intake. Women vs. men were significantly different (P<.05). Error bars represent the standard error of net leucine balance. Main effect of sex (different between young and old, P<.05). Net leucine balance was greater in women vs. men across all three protein intakes. The x-axis represents the net leucine balance at each protein intake (0.5, 0.75 and 1.00 (g protein·kg BW−1·d−1). A three-way repeated-measures ANOVA with a mixed-model approach was used.
Fig. 4
Fig. 4
Relationship between classical nitrogen balance and isotopically derived nitrogen balance data combining all protein intakes (R2=0.40, P<.05). Partial correlation was used.
Fig. 5
Fig. 5
Relationship between classical nitrogen balance and the change in leucine oxidation from the fasting to the fed states combining all protein intakes. The change in leucine oxidation from the fasted to fed states increased as nitrogen balance increased (R2=0.45, P<.05). Partial correlation was used.
Fig. 6
Fig. 6
Relationship between estimated protein requirements and the change in leucine oxidation from the fasting to the fed states during the HPro treatment. The change in leucine oxidation from the fasted to the fed states decreased at the highest protein intake with increasing protein requirement (estimated from classical nitrogen balance data, (R2=0.16, P<.05). Partial correlation was used.

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