Gross efficiency is a measure of the ability to convert stored energy into power and is considered a key determinant of cycling performance. Gross efficiency has been reported to improve by 9% in sedentary participants following four months of calorie restriction combined with exercise (Amati et al, 2008). It is yet to be established if improvements in gross efficiency (GE) and net efficiency (NE) can be achieved with only short-term fat mass reduction in habitual cyclists and the effect on time-trial (TT) performance.
Seventeen male cyclists; age 42 ± 9 yrs, BMI 25.6 ± 3.1 kg.m2, body fat 22.3 ± 5 % were recruited from local cycling clubs. Participants completed four pre intervention baseline visits one week apart and were instructed to maintain their body mass. Resting energy expenditure (REE) and 4-site skinfold were conducted prior to a 16.1km TT on an SRM® cycle ergometer. Gases were measured using the Oxycon® Pro metabolic cart. The intervention consisted of a randomised crossover design where participants either maintained their usual calorie intake or reduced their intake by 500 kcal.day-1 for a 14 day period. Tests were repeated after each of the two week periods.
There was a significant reduction in body mass (1.24kg), body fat (0.64%) and fat mass (0.81kg) when comparing immediately pre to post intervention (p<0.05, in all cases), with no significant reduction in lean mass (p=0.12). There was also no significant difference in REE (p=0.65), blood urea nitrogen (p=0.55), TT power (p=0.58) or TT power expressed relative to body mass (W.kg-1.s-1), (p=0.28). There was a significant increase in TT GE (2.9%) (p=0.008) and NE (2.4%) (p=0.017) following the 14 day intervention.
This study found a significant improvement in both GE and NE following short term calorie restriction, equivalent to approximately 60% of the improvement in GE reported by Amati et al. (2008) in their four month calorie restriction period. Although, a reduction in REE has been reported during more severe calorie restriction and over longer periods (Dulloo & Jacquet, 1998), this study failed to find a significant reduction. Increases in GE and NE did not result in improved 16.1km TT performance, and despite the reduction in body mass the cyclists TT power to weight ratio was also not significantly altered. This data suggests that following short term energy restriction, TT exercise capacity is not compromised in club cyclists. However a reduction in energy expenditure during the performance task is apparent; this has potential implications for the estimation of energy expenditure following reductions in body mass.