The last letter had a good point. Often manufacturers just substitute fat for sugar, with about the same amount of calories and a detrimental effect to the teeth. However, fat is not just a threat because of its calorific value - saturated fats are the ones you don't want, polyunsaturated are good but don't eat too much!
I'm a Food Technologist developing foods with artificial sweeteners and we use soluble fibres like Litesse and polyols like Lactitol to partially replace fat and sugar (sorry for the plug - notice I didn't mention my company's name!)