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Body's Choice for Fuel

Old 03-30-09, 11:09 PM
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Body's Choice for Fuel

I think I asked this somewhere but prolly 2-yrs ago and I have a terrible memory.

When we exercise, the body initially gets its needed energy from blood glycogen, correct?
And the body usually has enough to last about 2-hours (depending on intensity, etc...), correct?
After the blood glycogen is exhausted, the body then goes to fat stores and/or muscle glycogen?

Or do I have it backwards somewhere?
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Old 03-31-09, 06:09 AM
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Originally Posted by mkadam68
I think I asked this somewhere but prolly 2-yrs ago and I have a terrible memory.

When we exercise, the body initially gets its needed energy from blood glycogen, correct?
There is glucose in the bloodstream that can be utilized. Glycogen is the stored form of glucose in the human body. The stores are in the muscles and the liver.

Originally Posted by mkadam68
And the body usually has enough to last about 2-hours (depending on intensity, etc...), correct?
Typically two hours for the average trained individual. It could be less for an untrained persons. The two hour time frame refers to what is available in total--blood glucose, liver glycogen, muscle glycogen.


Originally Posted by mkadam68
After the blood glycogen is exhausted, the body then goes to fat stores and/or muscle glycogen?

Or do I have it backwards somewhere?
There is no switch. In other words, the preferred substrate is always a mixture with the ratio changing to favor one over the other depending on intensity (and other factors like temperature and hydration status).

In the extreme, if the glycogen stores are used up and blood sugar level is very low, the body will attempt to make glucose from the protein and fat. This process is degrees of applitude slower than what the body needs if the intensity is kept at a level where glucose is preferred. This deficit is what causes the extreme energy substrate bonk you see in athletes who need to work over great periods of time and who were careless about their feeds (i.e., marathoners and ultra-marathoners, triathletes--especially in the ironman, long distance swimmers, ...)

Anyway, that's what I remember from my readings.
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Old 03-31-09, 06:22 AM
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Fat burns in a carbohydrate flame.
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Old 03-31-09, 08:10 AM
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So weight loss comes from the body utilizing the glycogen stores in fat/liver? I would think only minimal weight loss occurs from the consumption of blood glucose.

Blood glucose is replaced by the body's efforts to maintain homeostasis by converting/transferring the fat/liver stores of glycogen into blood glucose?
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Old 04-06-09, 07:19 AM
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If you are working aerobically the primary fuel is fat which we all have LOTS of... but it needs a small carbohydrate element to convert it to usable energy. It is the lack of carbohydrate that causes the bonk after two hours or so.

Intense anaerobic work shifts strait to carbohydrate and can't be maintained for very long.

Both types of exercise can help with weight loss but most experts recommend a big element of aerobics.
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