There are questions about possible negative health effects of some carbs, such as fructose, which is found in sugar and high-fructose corn syrup, and galactose, which is found in milk. But the question of how carbs affect health is mostly focused on how quickly and efficiently the body can break the molecule down and deliver glucose to the bloodstream.
But you don’t eat carbohydrates; you eat food, so it’s useful to categorize foods by the type of carbohydrates.
Simple-carb foods are those that your body breaks down quickly and easily, such as sweeteners (sugar, honey, maple syrup) and refined grains (white flour, pasta, white rice). These are the carbs that tend to spike blood sugar.
Complex-carb foods, which include whole grains and legumes, have large, complex molecules that are more difficult to digest and don’t cause the same rapid increase in blood sugar.
The simple/complex classification isn’t perfect. Many fruits and vegetables contain both types of carbohydrates: Some get broken down quickly, others more slowly. And it’s not always true that whole foods are digested slowly while refined foods are digested quickly. Potatoes, for example, have lots of carbohydrates in the form of starch, which is broken down quickly.
Carbohydrates in refined grains — bread, white rice, pasta — come packaged with some fiber, some protein and even a few other nutrients, their calories aren’t quite as empty, and the speed with which they’re digested varies. (Refined flour is also fortified with folate, essential to reducing the risk of fetal neural tube defects.)
White bread, for example, lets loose a flood of glucose, so your blood sugar spikes, but pasta, particularly if it’s not overcooked, doesn’t have that effect. Although the ingredients of the two foods are almost identical, pasta has a difficult molecular structure that your body can’t break down as quickly.
There is a measure for how much a particular food increases your blood sugar: the glycemic index, or GI. When carbohydrates in a food get converted quickly, that causes a spike in insulin, which your pancreas releases to prompt cells to absorb the glucose. The hormones that your body releases in response can make you feel hungry.