Fifty five to seventy per cent of the required energy in animals is derived from carbohydrates. However, fats and proteins can also be broken down and used for supplying energy. In most animals this happens only when the dietary intake of carbohydrates is low. In contrast, Drosophila uses only carbohydrates as a source of energy for its flight muscles and when the supply is exhausted the insect cannot fly even though it uses stored fat for other metabolic processes. Whereas, locusts are known to use only lipids for their long migratory flights. Most animals, however, use a variety of hexose sugars like glucose, fructose, mannose, and galactose as interchangeable sources of energy.
In this way no particular carbohydrate is really considered essential in a way similar to amino acids. But even if no carbohydrate is considered essential, growth of certain animals will be better on one type of sugar than on another. This can be explained better by the results of the following experiment. Young locusts showed that when dietary sugar was maltose growth was maximum or optimum and growth was minimum when no carbohydrate was given. Other sugars supported sub-optimal growth. What could be the reason for this difference? One of the main causes is the difference in the rate of movement of sugars across the gut wall into the blood. From the above experiment we can conclude that certain insects have a preference for a certain carbohydrate which can be called an essential or preferred nutrient. In the above experiment with locusts, maltose was the preferred nutrient.