Food habits, like all habits, save work when they are such as help, and make work when they are such as hinder. They may nourish or they may prevent nutrition. As the body must not be habitually overburdened with food or overworked by it, so it must not be undersupplied or underexercised in using food.

In excessive meat diet extractives overstimulate the body; in excessive vegetable diet vegetable fiber overirritates the digestive tract. Excess of building food overworks the kidneys; excess of energy food overweights the body with fat that may make it idle instead of active.

A diet of food deficient in the food-constituents needed leaves the body undernourished. This happens no matter how much food is eaten, if it is not of the kinds that together make the food-combination needed. An undernourished body is without energy or health; hence the importance to human life of knowing and using in living what science has learned about:

(1) Which food-constituents different foods contain.

(2) How much of the different food-constituents is present in a given amount (as 1 lb.) of any food.

(3) What amounts of each food-constituent the body needs in a given time, as a day or week.

Two of the five food-constituents (protein, carbohydrate, fat, mineral salts, water) do not need constant consideration if a mixed diet of wholesome natural food is eaten; these constituents are mineral salts and water. A mixed diet gives the mineral salts needed in health. When an excess of mineral salts is needed, as in growth and some types of illness (as bone-deterioration, such as rickets in children), milk and eggs should be eaten in larger quantities; these provide the additional salts then required. Water is needed by the body in relatively large quantity. It is also present in most foods in large proportion.