with this that didn t happen. reporter: the plan is broken up in four phases. the very first 17 days are the toughest. that s when the first big chunk comes off. kind of a cleansing. some have lost more than 15 pounds almost a pound a day. but is it too fast? is that safe? we ve heard that rapid weight loss can be unhealthy. it can be if you don t do it in a healthy way. your body needs food. the appropriate portions in terms of proteins, carbs and fats. reporter: we ve all experienced the yo-yo of dieting in fact most of it gain it all back. the 17-day diet is about eating healthy and exercising. haven t we heard this before? what s different this time? we re not depriving you or taking away all your calories. it s like getting a tuneup for your car. it allows your body to process foods in a different way. reporter: first, you start the plan walking 17 minutes a day. next up? more exercise, certain food