Research

Aerobic fitness does not modulate protein metabolism in response to increased exercise: a controlled trial

Tracey J Smith¹ , Matthew A Pikosky¹ , Ann Grediagin¹ , Carmen Castaneda-Sceppa² , Lauri O Byerley³ , Ellen L Glickman¹ and Andrew J Young¹

¹U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts, USA

²Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, Boston, Massachusetts, USA

³University of North Carolina at Charlotte, Charlotte, North Carolina, USA

author email corresponding author email

Nutrition & Metabolism 2009, 6:28doi:10.1186/1743-7075-6-28

Published:	16 June 2009

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% VO_2peak). 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.

Conclusion

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.