In health the same person under the same conditions of living needs the same food-constituents and in the same quantity, but needs to obtain these from a variety of foods. Starchy vegetables are of many kinds, as are also green; so are fruits, grains, dairy-products, and animal foods. Though no two foods are exactly alike, a class of foods serves in general the same food-purpose. How foods differ from one another and how the classes of foods differ is shown on pp. 190-193.

Adults can usually digest all kinds of food and all the foods of each kind. That they may be able to do so it is, however, necessary that as they mature they learn to eat every common food. For diet-restrictions in childhood, see pp. 202-205.

The kinds of food a family has eaten, it usually prefers. The kinds that have prevailed in a locality are usually preferred there. Sometimes an earlier need for a kind of diet passes, but leaves that diet as the food habit of the district. It is often found that where it was originally hard to grow or get food a greater variety is not desired even when it becomes possible. Often new foods cannot be easily introduced even when they are desirable and obtainable; this is most frequent where for a long time few foods have been eaten.

Where much physical activity, especially in the open air, has been usual, as in pioneer times or in agricultural districts, an energy-giving diet containing much starch, sugar, and fat is needed. If the conditions of life change and the food habits are not adjusted to the change, the former diet may cause illness. The breakfast of colonial days in New England would menace the health of any one not doing hard work out-of-doors. The need to change diet increases with travel and variation in occupation; the ability to do so comes with the habit of eating many kinds of food.