How many American adults go on a diet each year?

Could you please tell me the approximate number or percentage of American adults who go on a diet each year? Thank you.
According to NPD Group’s (market research company) “National Eating Trends” 2008, 70% of Americans are eating lower fat foods and over 50% are eating “reduced calorie, whole grain or fortified foods”. Increased food choices are “diet, low cholesterol, reduced sodium, caffeine-free, sugar-free, fortified, organic, and low carb” foods. The good news is that fewer Americans are “dieting” and more are interested in making “healthier food choices”. More consumers are looking to add “whole grains, dietary fiber, omega-3 fats, antioxidants, and probiotics” according to NPD. Rather than restricting foods, most Americans are more interested in adding healthy foods which trump saving money.