Cycling efficiency is a measure of the ability to convert stored energy into power, and is considered a key determinant of cycling performance. Cycling efficiency has recently been manipulated with various techniques, but most prominently with high intensity training in habitual cyclists and using calorie restriction in sedentary obese participants. It was therefore the primary aim of this thesis to explore the efficacy of utilising a short- and medium-term calorie restriction intervention, to manipulate efficiency with participants accustomed to cycling. A secondary aim was to investigate the validity of measuring efficiency in a field-based environment. Male club level cyclists were recruited for the investigations, which comprised of a moderate -500 kcal.day-1 deficit, utilising portion control and measuring efficiency at both absolute and relative steady-state intensities. Seventeen participants completed the short-term, two-week intervention which utilised a randomised cross-over design. Although a significant reduction in body mass was attained, RMR, gross and net efficiency across all intensities and TT power remained stable. Field and laboratory comparisons indicated that prior to statistical correction absolute efficiency was significantly lower in the field, but after accounting for differences in power, cadence and environmental conditions, no differences were present. Twenty-nine participants conducted the medium-term study and were assigned either to calorie restriction or to no dietary intervention. Following a reduction in mass in the calorie restriction group and an increase in the group given no dietary intervention, a significant interaction between mass and efficiency was found across gross and net efficiency workloads. A six week follow-up period indicated that the process of calorie restriction and not absolute body mass reduction was the main mechanism for altering efficiency. This thesis suggests that efficiency can be manipulated both positively and negatively with calorie manipulation, and that these changes are linked to both laboratory and field based performance.