Flexibility. A flexible plan doesn't forbid certain foods or food groups, but instead includes a variety of foods from all the major food groups. A healthy diet includes vegetables and fruits, whole grains, low-fat dairy products, lean protein sources, and nuts and seeds. A flexible plan allows an occasional, reasonable indulgence if you like. It should feature foods you can find in your local grocery store and that you enjoy eating. However, the plan should limit alcohol, sugary drinks and high-sugar sweets because the calories in them don't provide enough nutrients.