Cycling Efficiency during Incremental Cycle Ergometry after 24-hours of Overfeeding or Fasting

OBJECTIVE: To determine whether net cycling efficiency (NET) is altered by 24-hour fasting or overfeeding, and 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 (10–25–50W) was performed the day before and after each dietary intervention. RESULTS: NET did not change following any dietary intervention (all p>0.05 versus 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), i.e., NET increased in lean but decreased in overweight subjects (Δ=0.010±0.010 versus −0.013±0.009, p=0.0003). Increased NET following the low-protein diet was associated with weight gain after six 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.