During the race the concentration of sugar in the athletes blood decreases

I would think this is true.

This may be caused by cellular respiration, precisely, the glycolysis stage of anaerobic respiration (because an athletes in a race would need immediate energy). Glucose breaks down at this stage to produce lactic acid, so it would probably make sense that the concentration of sugar decreases.

This is only what I think, and may not all be truth, so I suggest that you wait for a more 'advanced' response : D