Good carbs, bad carbs

The fat factor

So why do people say pasta and bread make you fat? For the same reason you can scarf a large bag of potato chips and still feel like snacking right after. Our bodies convert some carbohydrates to glucose at lightning speed, particularly the white starchy or sugary ones. That means you'll feel a quick blast of energy, but the hit doesn't last. Within 30 minutes your body senses that energy levels are declining, which will leave you hungry again and running to the fridge, even if you've just eaten loads of calories.

The quick spike in blood sugar also poses another problem. When you eat white bread or a handful of red licorice, your insulin levels skyrocket in order to convert these carbs to energy. Since insulin's other job is to tell your body to store fat, higher insulin levels in your blood make you more likely to convert your food to body fat rather than usable energy. This is why experts use a rating system called the glycemic index (GI) to rate carbs according to how fast they stimulate our insulin response and are converted into blood sugar and, ultimately, fat. The theory, according to diets based on the glycemic index, is this: eat low-GI carbs that your body must digest slowly—foods such as whole grains and green vegetables that are full of slow-to-digest fibre—and you'll stay slim. While most low-carb diets base some of their ideas on similar science, they won't help you keep off the pounds for good. But that doesn't mean you can't steal a trick or two from them.