Olestra is a fat substitute, allowing companies to produce snack foods that would normally be high in fat without the associated health risks. However, since it cannot be absorbed by the human body, Olestra produces gastrointestinal side effects and may inhibit the absorption of certain nutrients in the intestines. For these reasons, it was largely abandoned as an ingredient after public outcry in the 1990s.
Olestra is an artificial fat much in the same way aspartame is an artificial sweetener. It is an engineered molecules that behaves like fat does, but cannot be utilized by the body like normal dietary fat. In Olestra's case, the molecule is too large for the body to properly break down, and so it passes through the digestive system intact. This means that foods that use Olestra can have the same taste and mouthfeel as foods produced with traditional fat but is effectively leaner and healthier.
Olestra's action in the intestines inhibit the absorption of fat-soluble vitamins such as A, D, E and K, leading many manufacturers to fortify Olestra products with those vitamins to compensate. Olestra can also cause intestinal cramps and loose stools, memorably described on packaging as anal leakage. For this reason, many consumers shunned Olestra products, hindering its widespread adoption.