Question: Last Saturday I was on a long ride and seriously lost energy on the return trip. I knew I was bonking, but instead of taking a gel or energy bar I decided to gut it out. I figured that because all of my available sugar had been consumed and my body was running out of fuel, maybe I would start burning fat for energy and lose some weight. Is there any truth to this theory? — Dave W.
RBR Replies: It’s tempting to think that if you run out of glycogen (carbohydrate-based muscle fuel), you’ll burn lots of fat and end up lighter — and be a better climber. But as the physiologists say, “Fat burns in a glycogen flame.”
In other words, if you don’t have enough glycogen, you won’t be able to go fast enough or far enough to burn significant calories. You’ll be going so slowly that you won’t get many fitness benefits. You’ll feel lousy, too, and a hard bonk can drain you for several days. It’s far better to keep your muscles well stocked with glycogen so you can ride strongly.
Don’t worry, you’ll still burn a lot of fat. True, fat metabolism will be a smaller percentage of your total energy expenditure. But because you burn more total calories thanks to increased speed and distance, the amount of fat calories consumed will be greater, too.
Think of it this way: you can burn about 200 calories per hour of fat as part of your normal exercise. Riding 20 mph burns around 600-700 calories per hour. The math tells you that you can ride on fat alone, but you can’t ride very fast. 10-12 mph has been my “full bonk” experience. That is if I am still conscious (the brain runs on glucose!)