This is about purification process of the physical body. All impurities at all levels finally manifests in the physical body in the form of a disease. Food must be eaten in its natural or living state. The food we eat must be easily digestible and assimilated into the system without causing an overload to our digestive process.
When food is cooked under high temperature and pressure, it looses it's nutritional value and becomes the cause of many illnesses and sickness due to the longer time it takes for the body to digest the cooked food consumed.
This is avoided, when one eats raw vegetables and fruits in their natural form.
The goodness of the food is easily assimilated into the system when they are consumed in their natural form and so gets digested very fast by the system and does not take more energy from the body for the process of digestion.
The food groups that can provide us with the highest possible biological value falls into four categories - fruits, vegetables, nuts and seeds in sprouted forms. For optimum health, restricting our diet to these four categories is a great help.
NOTE : This post is taken from http://www.ssy.org/annamaya.html with all due credit.