If calories are not burned, our body converts them into fat. Often, people feel they can eat all the fat-free foods they want. Most of the students, including me, actually preferred the lowfat one. The oil in the full-fat vinaigrette masked the flavors and left a filmy coating in the mouth. The fat-free was too tart and too thin.
Moderation is the key.
You need the fat, but just enough to enhance other flavors, not as the flavor itself. St. Andrew's uses vegetable stock thickened by a slurry to replace some of the oil in vinaigrettes. A slurry is nothing more than arrowroot or cornstarch mixed with a liquid. Slurries are used in Asian cooking to make sauces from the juices of a stir-fry. To thicken vegetable stock, make a slurry with arrowroot and just enough vegetable stock to make it the consistency of heavy cream.