No-no on some diets is a term used to describe carbohydrates, which are a type of macronutrient found in many foods. Carbohydrates provide energy for the body and are found in grains, fruits, vegetables, and dairy products. Some diets, such as low-carb or ketogenic diets, restrict the amount of carbohydrates consumed in order to achieve certain health goals. Therefore, carbohydrates are considered a no-no on some diets.