This issue comes up every once in a while here on C&A. Namely, "How can I burn more fat by riding?" or some-such derivative. Essentially, they ask about the percentage of fat being burned while exercising versus the percentage of carbohydrates being burned (even if they don't put it into these same words).
I came across this PEZ Cycling article by Matt McNamara (USA Cycling Coach) on fat, carbohydrates, and energy demands. It's good reading if you can understand the technical jargon. (I had to re-read every couple sentences myself to understand. ) The chart below visibly simplifies the concept, so I thought I'd post it here.
In the end, what it comes down to is our bodies are always burning fat and always burning carbohydrates. It's just that during exercise, the proportion (or percentages) of fat:carbohydrate changes. At easier intensity, the body burns more fat and less carbohydrate. At higher intensities, the body switches over to more carbohydrate. Here's a graph:
The red line is the burning of fat stores, while the blue is carbohydrate stores (CHO).
Of interesting note: all athletes, not just us clydes, want to maximize the use of fat stores vs. carbohydrate stores (move that "crossover point" to the right on the graph). The body has only a limited supply Kcals of CHO, but has a large amount of Kcals available in its fat stores. The longer athletes can put off moving to CHO for energy, the longer they can last in the race, period. This might be that concept of "burning matches" we all hear so often about.
Anyway, thought you all might find it interesting. If someone asks about this again, we can point them to the chart & article.
It does send a good reminder my way that I still have some ways to go to get to my target weight.
I always thought that during a work out, you are burning carbs you ate last night. However, increasing the intensity, and/or increasing the duration will eventually force your body to start burning fat stores. It was described to me as "catching your 2nd wind" when this begins. It's like you feel you can just keep on going as you are working out.