The 24-h whole body leucine and urea kinetics at normal and high protein intakes with exercise in healthy adults

Am J Physiol. 1998 Aug;275(2):E310-20. doi: 10.1152/ajpendo.1998.275.2.E310.


In healthy adult men adapted to a diet/exercise regimen for 6 days, the effects of small, frequent meals supplying daily protein intakes of 1 (n = 8) or 2.5 g . kg-1 . day-1 (n = 6) on leucine oxidation, urea production, and whole body protein synthesis (PS) and degradation (PD) have been compared with the use of a 24-h continuous intravenous [13C]leucine and [15N,15N]urea infusion protocol. Two 90-min periods of exercise (approximately 50% maximal O2 consumption) were included during the fasting and the fed periods of the 24-h day. Subjects were determined to be at approximate energy, nitrogen, and leucine balances on both diets. Increased protein intake raised the urea production rate; the absolute rate of urea hydrolysis was the same on both diets. When the first-pass splanchnic uptake of leucine was taken to be 25% of intake, PS was stimulated by feeding (after an overnight fast) at both protein intake levels (P < 0.05 and P < 0.01), whereas PD declined significantly (P < 0.01) at both protein levels. Protein gain at a high protein intake appears to be the result of both a stimulation of PS and a marked decline in PD, whereas at a less generous intake, the gain appears to be a result of a fall in PD with a less evident change in PS. Exercise moderately decreased PS during and/or immediately after exercise at each protein level, and there was a postexercise-induced increase (P < 0.01) in PD, which was more dramatic when feeding was at the higher protein intake level.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adult
  • Analysis of Variance
  • Carbon Isotopes
  • Circadian Rhythm
  • Dietary Proteins*
  • Eating
  • Exercise / physiology*
  • Fasting
  • Humans
  • Kinetics
  • Leucine / metabolism*
  • Male
  • Nitrogen Isotopes
  • Oxidation-Reduction
  • Reference Values
  • Time Factors
  • Urea / metabolism*


  • Carbon Isotopes
  • Dietary Proteins
  • Nitrogen Isotopes
  • Urea
  • Leucine