BURGER chains that advertise food cooked in '100 per cent vegetable oil' may be misleading their customers into believing that this is a healthy option, according to a letter in the New England Journal of Medicine. Most fast food chains deep-fry in vegetable oil converted to saturated fat by hydrogenation, says Dr Dirk Elston of the Brooke Army Medical Center, Houston, Texas. Such fat is often used in commercial foods because it is cheap and has a long shelf life, but has been shown to raise cholesterol levels and to contribute to cardiovascular disease. Fast food chains should use monosaturated and polyunsaturated oils to help lower cholesterol levels, he argues.