From Ergo Log
If it’s fat you want to lose, you’re better off running a couple of times a week than cycling, sports scientists at the British University of Birmingham discovered in 2003. At the same level of intensity, you burn a little more fat by running rather than cycling.
The researchers got 12 reasonably well trained men to cycle on a cyclometer and run on a treadmill, and measured the amount of fat the men burned for each. The researchers gradually increased the intensity of the subjects’ exertions.
You measure the intensity of endurance efforts by recording oxygen expenditure. If you exercise at 60 percent of the maximal amount of oxygen you can take in [your VO2max], you can keep going for a long time, and carry out a conversation at the same time. This intensity is also the level at which your fat burning is optimal. At a higher intensity your body starts to burn more sugars and proteins, and less fat.
The figure below shows that, at similar levels of intensity, the test subjects burned more fat by running than by cycling.
When the subjects cycled, their concentration of lactic acid rose faster than when they ran. The upper figure of the two below shows the figures for cycling, and the lower figure for the subjects when they ran.
The researchers suspect that running uses more muscles and muscle fibres than cycling does. That’s why you burn more fat if you run. On the other hand, cycling is more demanding of the muscle fibres that you do use for this activity than running is. That’s why your muscle fibres burn more glucose when you cycle and why your lactic acid concentration rises. Lactic acid is really a kind of half-burned glucose.
Metabolism. 2003 Jun; 52(6): 747-52.