Increasingly, over the past ten years, people--especially young people--have become aware of the need to change their eating habits, because much of the food they eat, particularly processed food, is not good for the health. Consequently, there has been a growing interest in natural foods. Foods which do not contain chemical additives (添加剂) and which have not been affected by chemical fertilizers, widely used in farming today. Natural foods, for example, are vegetables, fruit and grain which have been grown in soil that is rich in organic ( 有机的 ) matter. In terms, this means that the soil has been nourished by unused vegetable matter, which provides it with essential vitamins and minerals. This in itself is a natural process compared with the use of chemicals and fertilizers, the main purpose of which is to increase the amount--but not the quality--of foods grown in commercial farming areas. Natural foods also include animals which have been allowed to feed and move freely in healthy pastures(牧场). Compare this with what happens in the mass production of poultry: there are farms, for example, where thousands of chickens live crowded together in one building and are fed on food which is little better than rubbish. Chickens kept in this way are not only tasteless as food, they also lay eggs which lack important vitamins. There are other aspects of healthy eating which are now receiving increasing attention from ex-perts on diet. Take, for example, the question of sugar. This is actually a non-essential food! Al-though a natural alternative, such as honey, can be used to sweeten food if it is necessary, we can in fact do without it. It is not that sugar is harmful in itself. But it does seem to be addictive: the quantity we use has grown steadily over the last two centuries and in Britain today each person con-sumes an average of 200 pounds a year! Yet all it does is to provide us with energy, in the form of calories. There are no vitamins in it, no minerals and no fiber. It is significant that nowadays fiber is considered to be an important part of a healthy di-et. In white bread, for example, the fiber has been removed. But it is present in unrefined flour and of course in vegetables. It is interesting to note that in countries where the national diet con-tains large quantities of unrefined flour and vegetables, certain diseases are comparatively rare. Hence the emphasis is placed on the eating of whole meal bread and more vegetables by modern ex-perts on "healthy eating". Chickens raised in poultry farms are all of the following EXCEPT that__________.