title: Aerobic fitness does not modulate protein metabolism in response to increased exercise: a controlled trial.
abstract: BACKGROUND
  A sudden increase in exercise and energy expenditure is associated with an increase in protein turnover and nitrogen excretion. This study examined how a sudden increase in exercise-induced energy expenditure affected whole body protein metabolism and nitrogen balance in people of differing levels of aerobic fitness. We hypothesized that alterations in whole-body protein turnover would be attenuated, and nitrogen balance would be preserved, in individual with higher levels of aerobic fitness.
  
  
  METHODS
  Eleven men, categorized as either having a lower (LOW-FIT; n = 5) or higher (FIT; n = 6) aerobic fitness level, completed a 4-d baseline period (BL) of an energy balance diet while maintaining usual physical activity level, followed by a 7-d intervention consisting of 1,000 kcal·d⁻¹ increased energy expenditure via exercise (50-65% VO₂peak). All volunteers consumed 0.9 g protein.kg⁻¹·d⁻¹ and total energy intake was adjusted to maintain energy balance throughout the 11-d study. Mean nitrogen balance (NBAL) was determined for BL, days 5-8 (EX1), and days 9-11 (EX2). Whole-body protein turnover was derived from phenylalanine and tyrosine kinetics assessed while fasting at rest on days 4, 7, and 12 using a priming dose of L-[ring-¹⁵N]tyrosine and a 4-h primed, continuous infusion of L-[¹⁵N]phenylalanine and L-[ring-²H₄]tyrosine.
  
  
  RESULTS
  A significant main effect of time indicated that NBAL increased over the course of the intervention; however, a group-by-time interaction was not observed. Although FIT demonstrated a lower net protein oxidation and higher net protein balance compared to LOW-FIT, neither the effect of time nor a group-by-time interaction was significant for Phe flux, net protein oxidation, or derived whole-body protein synthesis and net protein balance.
  
  
  CONCLUSIONS
  The absence of significant group-by-time interactions in protein metabolism (i.e., NBAL and whole-body protein turnover) between LOW-FIT and FIT males suggest that aerobic fitness level does not modulate protein "sparing" in response to an unaccustomed increase in energy expenditure.
authors.affiliation: U,S, Army Research Institute of Environmental Medicine, Natick, Massachusetts, USA. Tracey.j.Smith@us.army.mil.
authors.forename: Tracey J
authors.initials: TJ
authors.lastname: Smith
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Pikosky
authors.forename: Ann
authors.initials: A
authors.lastname: Grediagin
authors.forename: Carmen
authors.initials: C
authors.lastname: Castaneda-Sceppa
authors.forename: Lauri O
authors.initials: LO
authors.lastname: Byerley
authors.forename: Ellen L
authors.initials: EL
authors.lastname: Glickman
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Young