absolutely not. a healthy REGULAR diet (not an invasive diet) and healthy active lifestyle will prevent MOST people from getting too large, overweight, fat, whatever you like to call it. there are a select few people who cannot control it, even with extreme diet and exercise (i'm related to one), but most people shouldnt ever have to go on a hardcore die,t because they should have eaten healthier and been more active to begin with.
it's kinda funny that these days, living an active lifestyle and having a healthy diet is considered "being on a diet", instead of being the norm.