How does my body decide where to draw energy from?

Factual Questions
1

I’ve started running again this summer, and it got me thinking.

I know that exercise burns calories. If I run for three miles, I’ll burn a few hundred calories or so. I have sugar (in my blood?), fat, um, unprocessed, complex carbohydrates sitting around somewhere (in my digestive tract still?) . . . does the body draw on all of these at once at differing ratios? If I had a breakfast an hour before running of fruit and an english muffin, will I use the energy from the fruit (simple sugars) but not the english muffin, because my body is still processing it?

What is the Straight Dope about how the body stores and uses energy?

2

Undigested food come last. The body burns glycogen(stored glucose) and fat in varying ratios according to exercise intensity.

Any ingested sugars/starches must be converted to glucose before they can be used.

Also, a small amount of muscle tissue(protein) is burned but even that must be converted to glucose.

3

To expand a little:

Energy for running comes from 3 main sources:

  1. Available blood glucose
  2. Glycogen in liver
  3. Fat

If you’re running at a good clip, your blood has enough glucose in it for a few minutes (IIRC, it’s been a while). When your blood glucose starts dropping, you body will make more glucose from from glycogen stores, which are a sort of intermediate storage form between free glucose and fat. It takes approx. 15-20 min (again, IIRC) to exhaust your glycogen stores and at this point, fat starts being converted to glucose.

As to stuff you’ve eaten, digestion slows down during periods of intense activity, as digestion requires blood for metabolism and now this blood has been redirected to running. Things containing simple sugars cross into the bloodstream more rapidly than things that require digestion. So, what you ate, how long ago and how hard you’re working out would determine how rapidly stuff you’ve eaten recently would cross into the bloodstream. I should thing that your body is mostly pulling energy from blood glucose/glycogen/fat. Your blood glucose may be supplemented a little bit from what you’ve very recently eaten, but not by much. If you try to exert yourself too hard with a full stomach, you’ll get cramps, which is your body’s way of saying “DIVERT BLOOD TO RUNNING: DENIED. NEEDED FOR DIGESTION NOW. SLOW DOWN, IDIOT”

4

Normal stores of glycogen are enough for approximately 20 miles(more or less, several factors), hence “the wall” in marathons.

Training, genetics and feedings of carbohydrates can extend that until muscle fatigue becomes a dominant factor.

Up until the lactate turnpoint (appx. 85%MHR), glycogen and fats are burned together with an increasing % of glycogen as intensity increases.

5

Is that only running? What if one was walking and it took, say, five hours or so?

6

Almost entirely fat metabolism.

The 20 miles of running is for running speeds/fitness levels of average runners.

Faster marathon runners have trained their bodies to burn fat faster thus be able to run at their marathon race pace without hitting the wall.
Note that even a world class runner can run too fast and deplete their glycogen before the finish.

7

There is no simple straight forward answer. Virtually anything someone tells you is loaded with assumptions too numerous to count.

Here is an article that details some of the myths, along with some degree of scientific approach to finding the answer.

Here is another article discussing when the body burns fat vs carbohydrates.

A somewhat simple explanation of the difference between glucose and glycogen is here.

In endurance sports (activities lasting > about 90 minutes), the use of things like gatorade are extremely helpful, not just for the water and electrolytes, but because of the simple sugars found within. They are readily absorbed and used by muscles. Some drinks like accelerade also include protein, to help replenish the protein lost that runner_pat eluded to.

Ultimately, though - a calorie is a calorie. If you burn a calorie worth of glucose, then some glycogen/fat/whathaveyou must be burned to replace it after exercise is over. So long as you don’t stuff your face with food after exercise, the net result is some lost weight.
Even though its anecdotal evidence, I can tell you that after one of my long training days of anywhere from 5-7 hours of cardio (4-5 hours on the bike, followed by 1-2 hours of running) - the weight loss registered on the scale when I get home is 95% water. By the end of the day, after I’ve rehydrated - the scale hasn’t changed much. It is on days 2-4 after the workout that I notice the scale showing a lighter weight, and thus the more real weight loss.