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I've heard that our body burns carbohydrates when we begin exercising, and after 20 minutes the body will burn fat instead. Is this true?

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With a healthy pancreas, and an eating plan that allows it, your body will start burning fat 6 hours after you have your carbs. If your diet is very high in carbs and your pancreas is having a hard time keeping up, it will never burn fat. I suppose a more complete answer would be in terms of glycemic impact instead of blanket carbs.

I wish I had some reference material to link to. The information I have was from a registered nurse that oversaw the diet plan I was on. The most common time that all of us have with at least 6 hours of no carbs is while we are sleeping. Yet another reason for a good night's rest. NOTE: according to the same source, your body can take up to 12 hours to break down complex carbs and turn it into sugar for the body to use. That's why runners will carb up the night before a race, and not the morning of. They need the boost of energy after the carbs are processed.

As to what your body burns during exercise, it's a function of your heart rate. Zone training is built around different thresholds of what that exercise does for you. The bottom line is that the lower the heart rate the more fat you burn, and the higher the heart rate the more sugar you burn. If your goal is to lose fat, it's better to walk on a treadmill for an hour keeping your heart rate in zone 1 than it is to run a 5k. There are other benefits to the higher heart rate zones, so a balanced cardio plan will have them all covered.

Summary of zones:

  • Zone 1 (healthy heart zone): predominately fat burning. Different sources will have this rated at 100% fat burning to 85% like the link I used.
  • Zone 2 (fitness zone): burns more calories, most sources will have this burning fat at about 80-85%
  • Zone 3 (aerobic zone): aerobic-with oxygen, 50% fat, 50% sugar
  • Zone 4 (anaerobic zone): anaerobic-without oxygen, 15% fat, 85% sugar
  • Zone 5 (power/speed zone): max performance, 10% fat, 90% sugar
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Just a note on the zones: Although the relative amount of fat burning decreases with increasing intensity, the absolute amount of burned calories is increasing. So the absolute amount of burned fat increases with intensity, but again the duration one can keep up that intensity decreases. The highest relative and the lowest absolute fat burning both happen when you are sleeping :) –  Daniel Rikowski Jun 7 '11 at 11:42
    
I'm calling BS on the 'zone training' theory. I have heard theory being parroted around for years with no conclusive evidence to prove it. I don't disagree about the carb intake point, it makes sense that your body would burn readily available stores of glycogen first before fat. I have personally tried the 'zone training' approach with poor results. I'd argue that building more Type II muscle mass and increasing your daily caloric expenditure will have a much greater effect overall (not just while training). –  Evan Plaice Nov 28 '12 at 8:37
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I've heard somewhere that as you're warmed up during the exercises, you could stop and maintain your body temperature so that lipids enter the bloodstream. Then as your system cools, you'd start your light exercises again. Lather, rinse, repeat. All without trying too hard, just being consistent.

Mix that with Berin's advice to do this before consuming carbs in the morning, and you've got yourself a winning formula.

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