I’m the poster of the original question. I thought several of the answers below made good points, especially @Fred the Magic Wonderdog and @Eric Gunnerson. But none quite put it all together the way I would have liked. However, by pulling together and organizing the many good points made by different people, I’ve come up with the following answer.

1. Unlike in a race where you are burning almost exclusively carbs, on a long distance ride you are burning between 25% - 50% body fat and 50% - 75% carbs. The body fat is unlimited, so you only have to replace the carbs portion.

2. If your pace is such that you can carry on a conversation, you are burning 75% of your calories from body fat. You don’t have to replace that. The remaining 25% of calories is from carbs which you do have to replace.

3. If your pace is faster, so you find it difficult to carry on a conversation, you are burning 50% of your calories from body fat, with the remaining 50% of calories coming from carbs. You only have to replace the carbs portion.

4. When I ride I generally find it difficult to talk, so I guess I should be replacing 50% of calories burned. In my case, that’s 400 cals/hr.

5. However, since the body can only digest maximum 300 cals/hr, there will be a deficit of 100 cals/hr.

6. This deficit will draw on the carb reserve of 1,500 cals which you should be coming into the ride with.

7. You should be carb loading the night before and the morning of the race, so you come into the race with this reserve. The carb reserve is essential for making up the difference in the burned carbs you need to replace and the maximum 300 cals/hr limit your digestive system can process.

8. At a deficit of 100 cals/hr, your carb reserve should more than cover a 6 hour ride. In fact, in theory, your reserve should allow you to ride this pace for 15 hours without a break (an unlikely scenario for anyone)

So, the answer to the original question is, yes, it is realistic to fuel a long ride with NO sugar. Thanks everyone.

I’m the poster of the original question. I thought several of the answers below made good points, especially @Fred the Magic Wonderdog and @Eric Gunnerson. But none quite put it all together the way I would have liked. However, by pulling together and organizing the many good points made by different people, I’ve come up with the following answer.

1. Unlike in a race where you are burning almost exclusively carbs, on a long distance ride you are burning a combination of body fat and carbs. The body fat is unlimited, so you only have to replace the carbs portion. The proportion of body fat burned, depends on how hard you are pushing yourself. Pushing less hard means more body fat and less carbs are being burned, pushing harder means less body fat and more carbs are burned.

2. If your pace is such that you can carry on a conversation, you are burning 75% of your calories from body fat. You don’t have to replace that. The remaining 25% of calories is from carbs which you do have to replace.

3. If your pace is faster, so you find it difficult to carry on a conversation, you are burning 50% of your calories from body fat, with the remaining 50% of calories coming from carbs. You only have to replace the carbs portion.

4. When I ride I generally find it difficult to talk, so I guess I should be replacing 50% of calories burned. In my case, that’s 400 cals/hr.

5. However, since the body can only digest maximum 300 cals/hr, there will be a deficit of 100 cals/hr.

6. This deficit will draw on the carb reserve of 1,500 cals which you should be coming into the ride with.

7. You should be carb loading the night before and the morning of the race, so you come into the race with this reserve. The carb reserve is essential for making up the difference in the burned carbs you need to replace and the maximum 300 cals/hr limit your digestive system can process.

8. At a deficit of 100 cals/hr, your carb reserve should more than cover a 6 hour ride. In fact, in theory, your reserve should allow you to ride this pace for 15 hours without a break (an unlikely scenario for anyone)

So, the answer to the original question is, yes, it is realistic to fuel a long ride with NO sugar. Thanks everyone.