If you have a good program designed to built strength, and eat good food, you will get stronger. Protein is important, and quality carbs and enough fat. Trans-fats and saturated fats are not good for you, but other kinds of fat in moderation will help you gain strength. In any case, you don't need to be on a "diet" to gain strength, you just need to ingest enough quality food, and train hard and consistently. Getting enough rest is important also. Really, it is not all that complicated.