(1) Six nutrients include protein, sugar, oil, cellulose, inorganic salts and water. Rice mainly contains sugar, stewed potatoes mainly contain sugar and vitamins and a small amount of inorganic salts and water, fried cabbage mainly contains vitamins and a small amount of inorganic salts and water, radish soup mainly contains vitamins and a small amount of inorganic salts and water, and fried peppers mainly contain vitamins and a small amount of oil.

(2) According to the principle of reasonable diet, students should have balanced nutrition. They need more protein, but there is a lack of food containing protein in their diet. Therefore, it is suggested to increase meat (or eggs or beans and products).

So, the answer is: (1) sugar; (2) Meat (or eggs).