Carb blockers also know as starch blockers are made from white kidney beans, which have a natural chemical called amylase. Amylase helps digest starch or complex carbohydrates. When amylase is blocked, those carbs pass through the body undigested, so you don't absorb the calories.
Fat blockers commonly contain chitosan, which is similar to dietary fiber, but is derived from the exoskeletons of shellfish. Like fiber, they pass through the digestive tract without being digested. Chitosan absorbs between 6 and 10 times its weight in fat along the way, all of which is then eliminated in the stool.