Kerry's got it right. A few additional thoughts...
Sugar and starch are essentially the same thing but sugar breaks down easier. Many of our favorite plant foods convert sugar, a simple carbohydrate, into starch, a somewhat more complex carb. Peas, carrots, and corn are all good examples. When young and tender they are very sweet as much of the carbohydrate in them is still in the form of sugars. As they mature, the sugar converts to starch which is why they don't taste nearly as good when overripe (or overgrown, in the case of carrots). Our bodies do the reverse and break down the starches back into a type of sugar known as glucose.
Carbohydrates are mistakenly referred to as the preferred fuel of our bodies. They are not. Our bodies function best when burning primarily fat except during strenuous exercise...like extended heavy portaging or running a marathon. During moderate exercise, such as paddling all day and intermittant portaging, your body will perform just as well by burning fat IF that is what it is accustomed to burning on a regular basis. The reason carbs are mistakenly viewed as the "preferred" fuel of the body is that the body will burn them before it burns fat. Your body does this not because carbs are its preferred fuel, but because that's your body's first line of defense against elevated blood sugars, which if they remain elevated for prolonged periods, are destructive to the body.
Insulin also plays a role in keeping your blood sugar at acceptable levels. It also is the primary hormone which regulates the storage of fat in your body. Insulin actually activates our fat cells to store fat. The more insulin your body produces, the more fat your fat cells will store. The sad thing is that much of the fat stored is synthesized by your liver from carbs. Insulin also decreases the ability of muscle cells to burn fat. As long as insulin levels are elevated in the body, your cells are blocked from using much of the fuel already available in your body in the form of fat.
Furthermore, the more insulin you secrete, the more likely it is that your cells will become more resistant to insulin, theus the more insulin you'll need to secrete to keep blood sugar levels under control. Thhis is because too much glucose is toxic to your cells so they increase resistance to insulin to keep the glucose out. So your body produces even more insulin to overcome the resistance. It can become a vicious cycle. And the more insulin you produce, the more your body will attempt to store fat rather than use it for fuel.
If you are engaged in a constant struggle to control or maintain your weight, odds are you have a tendency to be resistant to insulin more than those who seem to effortlessly maintain their weight. Most diets don't work well...or for long...because they don't address the insulin problem. They just try to restrict calories and that doesn't work because the problem is not that you are consuming too many calories but rather that your body is unable to convert the calories you consume to energy because the excessive insulin is telling your body to store it as fat instead.
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