Sugar is an essential nutrient and belongs to the group of carbohydrates, and they can be simple or complex. Simple sugars include refined sugar, pastries, and sugary drinks, and should be avoided. Nevertheless, simple sugars that come from fruit, or complex sugars coming from cereal, pasta or bread are highly recommended during cancer treatment because they produce energy and help the body function properly. Carbohydrates are part of a balanced diet and help to maintain an adequate nutritional state during cancer treatment. In the Mediterranean diet, 50-60% of daily energy should come from this food source.