Terminology
In addition to diet other words or phrases are used to identify and describe these foods including light or lite, low calorie, low fat, no fat, fat free, no sugar, sugar free, and zero calorie. In some areas use of these terms may be regulated by law. For example in the U.S. a product labeled low fat must not contain more than 3 grams of fat per serving; and to be labeled fat free it must contain less than 0.5 grams of fat per serving.
Process
The process of making a diet version of a food usually requires finding an acceptable low calorie substitute for some high calorie ingredient. This can be as simple as replacing some or all of the food's sugar with a sugar substitute as is common with diet soft drinks such as Coca-Cola (for example Diet Coke). In some snacks, the food may be baked instead of fried thus reducing the calories. In other cases, low fat ingredients may be used as replacements.
In whole grain foods, the higher fiber content effectively displaces some of the starch component of the flour. Since certain fibers have no calories, this results in a modest caloric reduction. Another technique relies on the intentional addition of other reduced-calorie ingredients, such as resistant starch or dietary fiber, to replace part of the flour and achieve a more significant caloric reduction.
Controversy
In diet foods which replace the sugar with lower-calorie substitutes, there is some controversy based around the possibility that the sugar substitutes used to replace sugar are themselves harmful. Even if this question is satisfactorily resolved (which remains unlikely at this time[, the question still remains as to whether the benefits of caloric reduction would outweigh the potential loss.
In many low-fat and fat-free foods the fat is replaced with sugar, flour, or other full-calorie ingredients, and the reduction in caloric value is small, if any | 
