Abstract
BackgroundA 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.MethodsEleven 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.ResultsA 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.ConclusionThe 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.
Highlights
A sudden increase in exercise and energy expenditure is associated with an increase in protein turnover and nitrogen excretion
In terms of muscle protein metabolism, protein synthesis increases during resting conditions in response to chronic aerobic [7,8] and resistance [9] training, and this increase is attenuated in response to chronic resistance training when measured after an acute exercise bout [9,10]
We hypothesized that whole-body protein turnover would increase in both fit and unfit individuals in response to the increase in exercise energy expenditure, while daily nitrogen balance (NBAL) would be negative initially and rise over time as nitrogen retention improved with adaptation to the increased exercise
Summary
A sudden increase in exercise and energy expenditure is associated with an increase in protein turnover and nitrogen excretion. In terms of muscle protein metabolism, protein synthesis increases during resting conditions in response to chronic aerobic [7,8] and resistance [9] training, and this increase is attenuated in response to chronic resistance training when measured after an acute exercise bout [9,10]. Overall, these findings suggest that fitness training imparts adaptations that down-regulate or spare the use of amino acids as an energy source and imply a more efficient use of protein by the body in fit persons. Whether nitrogen balance and protein utilization responses to a sudden increase in aerobic exercise are modulated by fitness level is not known
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