Diet, the sum of the food consumed by an organism or group.
Dieting, the deliberate selection of food to control body weight or nutrient intake.
Diet food, foods that aid in dieting

Everybody has a diet, its just that some are dieting because they want/need to lose weight. Most people just have horrible nutrition and get no exercise, so anybody who is exercising and try to eat well looks like they are dieting to fat and lazy people.