Submitted to: Nutrition and Metabolism
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/8/2009
Publication Date: 6/16/2009
Citation: Smith, T.J., Pikosky, M.A., Grediagin, A., Casteneda-Sceppa, C., Byerley, L.O., Glickman, E.L., Young, A.J. 2009. Aerobic fitness does not modulate protein metabolism in response to increased exercise: a controlled trial. Nutrition and Metabolism. 6:16-28. Interpretive Summary: Fundamental physiological adaptations develop as sedentary individuals become physically trained. For example, research implies that fit people use protein more efficiently compared to sedentary people. We hypothesized that physically fit people could conserve protein better than sedentary people when they are faced with a sudden increase in exercise (i.e. an additional 1000 calories per day beyond their habitual energy output) for 5 days. This knowledge could be applied to scholastic athletes during pre-season training or Army Soldiers during intense combat missions and lead to better understanding of the protein needs in these populations. In order to determine how much protein is used by the body, we collected nitrogen (i.e. a waste product of protein) from urine, sweat and feces. We also used isotope tracer methodology to determine how the body uses protein (i.e. special amino acids – building blocks of protein- are injected in the arm followed by blood draws to measure the blood content of these amino acids). Fit people used protein more efficiently over the course of the entire study, which is not surprising based on previously published data. However, there were no differences in the way that sedentary and fit people used protein in response to the 5 days of increased exercise. Our findings indicate that aerobic fitness level does not have an impact on protein metabolism in response to an increase in exercise when energy intake matches energy expenditure.
Technical Abstract: Purpose: This study examined how a sudden increase in exercise energy expenditure affected whole body protein turnover and nitrogen balance in people of differing aerobic fitness. We hypothesized that whole-body protein turnover would be attenuated, and nitrogen balance would be preserved, in aerobically fit vs. lower fit individuals. Methods: Eleven men, categorized as either having a lower (LOW-FIT; n = 5) or higher (FIT; n =6) aerobic fitness level, completed a 4- day (D1-4) baseline period (BL) of an energy balance diet while maintaining usual physical activity level, followed by a seven day (D 5-11) intervention consisting of 1,000 kcal·day-1 increased energy expenditure via exercise (50-65% VO2 peak). All volunteers consumed 0.9 g protein·kg- 1·d-1 and total energy intake was adjusted to maintain energy balance throughout the 11-day study. Mean nitrogen balance (NBAL), calculated per kg fat free mass (FFM), 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 infusions 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. Additionally, neither the effect of time nor a group-by-time interaction was significant for Phe flux, conversion rate of Phe to Tyr or derived whole-body protein synthesis. Conclusion: The absence of significant differences 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.