Imagine eating everything delicious you want - with none of the fat. That would be great, wouldn't it? New 'fake fat' products appeared on stone shelves in the United States recently, but not everyone is happy about it. Makers of the products, which contain a compound called olestra, say food manufacturers can now eliminate fat from certain foods. Critics, however, say the new compound can rob the body of essential vitamins and nutrients (营养物) and can also cause unpleasant side effects in some people. So it's up to consumers to decide whether the new fat-free products taste good enough to keep eating. Chemists discovered olestra in the late 1960s, when they were searching for a fat that could be digested by infants more easily. Instead of finding the desired fat, the researchers created a fat that can't be digested at all. Normally, special chemical in the intestines (肠) 'grab' molecules of regular fat and break them down so they can be used by the body. A molecule of regular fat is made up of three molecules of substances called fatty acids. The fatty acids are absorbed by the intestines and bring with them the essential vitamins A, D, E and K. when fat molecules are present in the intestines with any of those vitamins, the vitamins attach to the molecules and are carded into the bloodstream. Olestra, which is made from six to eight molecules of fatty acids, is too large for the intestines to absorb. It just slides through the intestines without being broken down. Manufacturers say it's that ability to slide unchanged through the intestines that makes olestra so valuable as a fat substitute. It provides consumers with the taste of regular fat without any bad effects on the body. But critics say olestra can prevent vitamins A, D, E and K from being absorbed. It can also prevent the absorption of carotenoids (类胡萝卜素), compounds that may reduce the risk of cancer, heart disease, etc. Manufacturers are adding vitamins A, D, E and K as well as carotenoids to their products now. Even so, some nutritionists are still concerned that people might eat unlimited amounts of food mad with the fat substitute without worrying about how many calories they are consuming. We learn from the passage that olestra is a substance that ______.