As we have said earlier all animals are heterotrophs and require food from the environment. What is this food made up of? If the food of a number of different animals is broken down we find that it consists of proteins, carbohydrates, fats, water, minerals and vitamins. All animals require the above-mentioned broad categories of nutrients although in different amounts.
Some of these nutrients are used mainly as fuel (carbohydrates and fats), while others are required principally as structural and functional components (proteins, minerals and vitamins). However, proteins, carbohydrates and fats can all serve as fuel for the body's energy needs, but no animal can survive on fuels alone. Therefore, a balanced diet is needed to meet all the requirements of the body for energy, growth, maintenance, reproduction and physiological regulation. Now let us discuss the importance of these different classes of food in relation to animal nutrition.