The bottom line on the topic really is portion control and moderation.
Quite frankly, most people know exactly what they need to do. Most folks will go to the doctor and tell their doc that all they ever eat is whole grain cereal, turkey sandwich on wheat bread, and salad without dressing and only drink water. The patient and the doctor are then left with the medical mystery of how that patient managed to gain 30 lbs over the last year on that diet.