I have never tried a low-carb diet, but maybe my experience will be helpful.

I have lost a total of 35 lbs since Jan 2009, 23 of them since starting cycling in the summer. In July I was only riding 3 miles to the store, and gradually increased my time and distances. I'm using weightwatchers, which uses "points" - based on calories, fiber and fat but basically it's about 60 calories per point. For my weight, I'm allowed 18 points a day, plus 35 extra points per week. Cycling or other exercise adds in "activity points". One activity point is worth about 100 calories burned during exercise. I don't worry about carbs, protein etc but I have cut back on sweets, and eat more things like oatmeal, whole grain pasta, lean protein, fruit and veggies.

Currently, I'm riding 50-100 miles per week (about 4-8 hours), depending on my schedule & the weather. If I ride 45-90 minutes, I might allow myself 2-4 extra points for the day. If I ride 2-3 hours, I will allow myself as much as 8-10 extra points, but I almost never dip into the activity points by the end of the week. I keep my ride snacks small, and most of the extra points come from extra snacks after my ride and slightly bigger lunch or dinner. I try to think of the ride as burning more calories - not as an excuse to eat more.

It's working for me, but the essential ingredients have been careful tracking of everything I eat and adding in the regular exercise.