Cycling Efficiency During Incremental Cycle Ergometry After 24 Hours of Overfeeding or Fasting

Objective: The objective of this study was to determine whether net cycling efficiency (NET) is altered by 24-hour fasting or overfeeding and whether it correlates with dietary-related energy expenditure (EE) and future weight change. Methods: In a crossover design, healthy subjects fasted or were overfed for 24 hours while in a whole-room calorimeter using five diets with doubled energy needs: standard, high-carbohydrate (75%), high-fat (60%), high-protein (30%), and low-protein (3%) diets. Graded cycling exercise at low power outputs (10-25-50 W) was performed the day before and after each dietary intervention. Results: NET did not change following any dietary intervention (all P > 0.05 vs. 0). Individual changes in NET did not correlate with EE responses to dietary interventions. However, the change in NET after low-protein overfeeding was inversely correlated with baseline body fat (r = −0.60, P = 0.01); that is, NET increased in lean but decreased in overweight subjects (Δ = 0.010 ± 0.010 vs. −0.013 ± 0.009, P = 0.0003). Increased NET following the low-protein diet was associated with weight gain after 6 months (r = 0.60, P = 0.05). Conclusions: Despite no substantial effect of acute overfeeding or fasting on NET, the change in NET following low-protein overfeeding depends on adiposity and may influence weight change, suggesting that increased efficiency in a setting of protein scarcity is an adaptive response that may ultimately lead to weight gain.