That's good question and someone will hopefully correct me if I am wrong, if your not fueling your body, your body slows down it's consumption of stored energy to preserve itself, and will always burn this energy AFTER it has burned everything in your stomach. Its a built in protection system. We all have it.
When you go to sleep your body enters the fasting phase. It just happens. When you wake and eat you "break" the fast (get it?). Your body begins burning the calories you just ate. If you excercise on top if this, it needs more energy. The first place it looks is your stomach, oops empty because it burned it all off to process and store it. Where is it going to look next? Body fat. Which is high in vitamins and nutrients so burns more slowly. Which is why it is hard to loose weight.
A person with a liberal amount of body fat is no different from a person with very little body fat. Our bodies work the same way (for the most part). The difference is, a person with very little body fat usually burns most or all of thier caloric intake during the day (night, whichever). A person with a high percentage of body fat usually doesn't burn off thier caloric intake, so thier bodies store the extra calories and we gain weight. Which is why you are constantly reminded to eat "low calorie" food.
Remember, it takes fuel to burn fuel. You can't start a fire without some kind fuel right? So your body is burning its fat to to burn whats in your stomach. Eating the right kinds of food constantly throughout the day keeps this process happening. Your continuously burning fuel. When your intake exceeds your output, you gain. When your output exceeds your intake you loose. The trick is, getting your body to think that there is no famine and you are not starving to death so it doesn't hold onto every calorie it can.
Grapefruits, cellery, lettuce etc., all have very few calories and it takes more to burn them than there is in them. You should be "grazing" on these types of food throughout the day to keep your system running
. My problem is, I got lazy and didn't care. As long as it tasted good, it was going down the hatch, and since I didn't have to excercise anymore, I didn't
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So if any of this wrong, anybody, chime in and correct me.