Most experts describe bad carbohydrates based on their fiber level, or how they rank on the glycemic index. In general terms, carbohydrates with a low amount of fiber or carbohydrates that generate a large blood glucose reaction within the body are described as bad, while carbohydrates with more fiber and a smaller glucose reaction are described as good. Classic examples of bad carbohydrates are things like soft drinks, while good carbohydrates would generally be things like broccoli or whole grain bread. Several modern diets have been focused on restricting bad carbohydrates as a way to encourage weight loss and improve overall health.
The theory behind good and bad carbohydrates comes from the way the body breaks down food during digestion. Theoretically, carbohydrates with higher fiber levels will be broken down more slowly and generate a lessened glycemic reaction within the body. Research has generally shown that when blood-glucose levels get high enough, the body can start storing more fat, so keeping those levels low could potentially be helpful in controlling a person’s weight. Another alleged advantage for good carbohydrates is that they're generally more dense and digest more slowly. These qualities could make the foods more filling for a longer time and result in involuntary caloric reduction.
Another way of looking at bad carbohydrates is through processed versus natural foods. Some people think that any carbohydrates are generally OK as long as they haven’t gone through a lot of modern processing procedures. There are some researchers who worry that preservatives and the inclusion of extra sugars via high-fructose corn syrup and other additives can make any carbohydrate food potentially unhealthy. People who are concerned with the level of processing will generally buy farm-fresh produce and fruits while staying away from pre-packaged products. Some of these dieters will also avoid foods that can’t be digested easily without cooking, like rice or grain products, because of a concern that they may be unnatural for humans to consume.
Studies about good and bad carbohydrates have generally been somewhat inconclusive. There is data to support the idea that eating good carbohydrates is better for weight loss than bad carbohydrates, but the reasons behind this are somewhat murky. Some experts think it’s a question of caloric intake and the level of fullness different foods provide, while others think it has more to do with hormonal reactions within the body. More research will probably be needed to determine the exact mechanisms involved.