How a fat can pass through untouched
Ordinary dietary fat is broken into smaller molecules by digestive enzymes, then absorbed through the intestine. Olestra was built to behave differently. It is made by attaching several fatty-acid chains to a sugar molecule. That crowded structure lets it act like fat in food, but digestive enzymes cannot readily break it into pieces the intestine can absorb.
Because the molecule largely travels through the digestive tract intact, the body gets little to no usable energy from it. In a snack, that means it can provide some of the richness and crispness of conventional fat without supplying its calories.
The tradeoff happens in the intestine
The same quality that makes olestra low-calorie also explains its warning. Since it remains in the gut, it can affect digestion in ways absorbed fats do not, including causing cramps or loose stools in some people. It can also dissolve certain fat-soluble vitamins—A, D, E, and K—and carry them out of the body rather than letting them be absorbed. Products containing olestra were therefore associated with a warning about possible digestive effects.
Olestra’s design was a food-engineering trade: make a molecule that performs like fat in the mouth but is difficult for the body to use. Its warning was the visible reminder that a food ingredient can be absent from the calorie count yet still have consequences inside the digestive tract.
