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:
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.