This is somewhat related to the losing weight/gaining muscle question found here, but I would like to find out if there are any studies or knowledge of weight loss impeding the physiological adaptations of endurance exercise training.
There are certain adaptations that occur with endurance training, such as the enhancement of Type IIa and IIb fibers, which includes fiber growth and hypertrophy. Blood vessels grow into the muscles to aid in oxygen transport, the body gets more efficient at using fat and increases the mitochondrial count in the muscles themselves.
This study looks at the physiological and metabolic changes that occur, and would suggest that there was no weight loss in the group (Since the VO2 did not change, weight did not change as weight is central to the VO2 calculation), so it is possible to increase endurance in the short term without weight gain/loss. Also, it is possible to get more fit while losing weight in untrained people.
However, over the long term there are certain physical changes that occur. Specific to cycling, the upper legs and calves can get quite large. You see this most markedly in sprinters, but also in endurance cyclists, just not quite as pronounced. Common sense and the above referenced question would indicate that the muscle growth associated with endurance training might be hampered with weight loss, but would this also impact the changes that occur internally? (Fiber density increases, mitochondrial increases, blood vessel growth). And in the case of untrained people, is there a middle ground where no more fitness/endurance can be gained without gaining weight in the short term? Any information is welcome, but studies are preferable.