The nature and quality of the diet appropriate for adults must depend somewhat upon individual constitution, occupation, habits of life, and to some extent upon the climate in which they live.

The full adult weight is not usually attained before the twenty-fifth year, although in some cases it is reached earlier. In many instances adults after the age of forty or forty-five years increase considerably in weight. In men the bones continue to solidify until about the thirtieth year; in women this condition is usually reached a few years earlier. When the final growth of the body and development of the vital organs is completed the use of food consists simply in maintaining the proper equilibrium of the tissues by replacing waste with new material and in furnishing fuel for the development of force, the food being no longer required for growth. Many persons eat more than is required for these purposes.

As men advance in life, become prosperous, and acquire more and more luxurious habits of living or become absorbed in various occupations, they are apt to take less exercise, while the number of luxuries of the table which their means enable them to command constantly tempt them to indulge in food which is not needed and which is imperfectly consumed in the body.

Sir H. Thompson writes (Diet in Relation to Age and Activity):

"More than one half of the disease which embitters the middle and latter part of life among the middle and upper classes, of the population is due to avoidable errors in diet".

It is often difficult for a man between the ages of twenty and thirty years who has led an exceptionally active and vigorous or perhaps even an athletic life to realise, as he grows older and passes on towards forty-five or fifty years of age, that he requires less food to maintain his organic equilibrium than formerly, and his habits of eating acquired at a more impressionable period of life cling with great pertinacity. It may prove an advantage sometimes in such cases to lessen the appetite for dinner by a late lunch, or to try other simple means of restriction.

These observations fail of application to the poor, whose means do not allow them to gratify their appetites, and in the case of those whose diet is established for them. There is quite as much, if not more, ill arising from deficient feeding as from overfeeding; in either case the difficulty is only made serious by long continuance, and occasional great indulgence in overeating as in overdrinking is less productive of harm than habitual indiscretions of a lesser degree.

So much is due to habit in the different organs of the body in 22 relation to food that the stomach and intestine will usually reject undigested the contents of an occasional excessive meal, whereas with habitual overeating they become accustomed to appropriating very large quantities of food, which are absorbed, but which are not eliminated without straining the excretory organs, or which may be stored in the body in the form of incompletely oxidised material, which accumulates until the system rebels and a violent bilious attack by vomiting and purging relieves the overburdened body. There are persons who have so little restraint over their appetites that they persistently indulge themselves in this way.