In simple terms, carbohydrates are broken down by various enzymes into simple sugars then glucose so they can be absorbed into the blood. Digestive enzymes are like biological scissors - they chop long starch molecules into simpler ones.
According to experts, if we cannot burn all the fat we consume, the remainder is stored as fat tissue. This fat-burning ability is determined by the amount of insulin in our bloodstream. [Note: a major factor in insulin release is the glycemic index (GI) value of the carb-foods or meal consumed.] When insulin levels are low, we burn mainly fat. When they are high, we burn mainly carbs. But a problem arises when insulin levels remain constantly high, as in the case of individuals suffering from insulin insensitivity. In such cases, the constant need to burn carbs reduces our fat-burning ability. Result? More fat is stored as fatty (adipose) tissue.
Generally speaking, the speed of digestion is determined by the chemical nature of the carb itself, and thus how "resistant" it is to the activity of the enzymes. A simple sugar is usually much less resistant than a starch, and is digested or metabilized much faster. Things that slow down digestion include: the presence of acid (from gastric juices or the food itself), and the presence of soluble fiber.
High Insulin Levels May Increase Risk of Obesity
This is why experts are linking high insulin levels, together with a reduced ability to burn fat, with obesity. And as you can see, high insulin levels are typically determined by the type of carbs we eat. High GI foods or meals trigger higher levels of insulin than intermediate or low GI foods. This is why the Glycemic Index is considered to be so important in assessing carb eating habits.
As we have seen, the human body is fuelled by glucose. Therefore all foods must be converted into glucose before they can be used as fuel. Carbohydrates are more easily converted into glucose than protein or fat, and are considered to be the body's "preferred" source of energy, and the brain's essential source of energy.
Simple carbs (excepting fruit sugar) are more easily converted into glucose because their molecular structure breaks down faster in the stomach and small intestine. Therefore these carbs raise glucose levels in the bloodstream quite rapidly (less than 30 minutes). This explains why diabetics, who occasionally suffer from an excessively low blood-glucose level, can quickly restore their balance by eating simple carb-foods, like sweets.