The foods which tend to produce fat in the body are chiefly sugars and starches. Eating fat in excess does not necessarily cause fat to accumulate in the system, for it may be completely oxidised.

To increase the albuminous constituents of the body without the accession of fat, a diet should be ordered in which proteid food predominates, with a moderate allowance of carbohydrates. To increase the body fat, however, the proportion of carbohydrate should considerably exceed the proteid food and a little fat should be added.

In seeking to remedy excessive leanness by dietetic treatment it is obviously necessary to first ascertain, if possible, its cause. It may be due to the use of improper food, to erroneous habits of eating, bad cooking, maldigestion or malassimilation, overwork and nervous exhaustion, disease of the various organs connected especially with nutrition, and besides these and other causes there are those in whom leanness seems to be constitutional or hereditary. They may enjoy excellent health, but are always so thin as to be the subject of comment. No diet seems to have much effect in increasing their weight. Another class of persons are those whose weight is constantly fluctuating and whose annual variation is as much as ten or even twenty pounds. In winter, in town life, when overworked, worried, or oppressed with mental strain, they lose weight rapidly, and in summer, in a brief holiday in the country, with little to do but eat and sleep, they gain at the rate of two or three pounds a week.

This is true more often of those whose general tendency is towards obesity rather than towards leanness.

It is almost hopeless to attempt to remedy obstinate leanness by diet unless other favouring conditions can be secured. First among these is entire freedom from mental strain, and of almost equal importance is abundant and regular sleep. A warm climate and inactive life favour increase in weight. In most respects the diet for leanness must be the reverse of that for obesity, as would be naturally expected, and in prescribing starchy foods it must be remembered that the leanness is often caused solely by an entire inability to digest amylaceous or saccharine material owing to "nervous dyspepsia," gastric catarrh, etc. These conditions should receive careful treatment on the lines recommended on pp. 535 and 537. By a little care it will often be possible to discover some forms of starches which can be digested. It is a good rule in such cases to give the cereals or vegetables at one meal and meat at another, so that articles involving different rates and organs of digestion do not interfere with each other. All bread should be stale or toasted. Crackers and zwieback may be allowed.

As a rule such patients can be made to digest starches before they can sugars - oatmeal will be better digested without sugar, and a cup of sweetened coffee with a meal of starchy foods may produce flatulency which lasts for hours.

It is desirable to increase the body weight (a) after serious acute disease, (b) in chronic wasting disease (especially tuberculosis), (c) in conditions of nervous exhaustion (neurasthenia). To increase the body fat alone the carbohydrates especially should be increased (perhaps doubled), and the fats also should be moderately increased in the dietary; but if the body proteid is to be increased (i. e., the volume and tone of muscle), proteid food should be increased, together with carbohydrates (as proteid sparers), and muscular exercise and oxidation should receive attention.

When the digestive organs admit, the following articles may be prescribed in the diet for leanness:

Abundant fat meats, butter, cream, milk, cocoa, and chocolate. Bread, potatoes, legumes, well-cooked cereals, especially oatmeal and cornmeal, farinaceous puddings with sugar and cream, cake, sweets, sirup, honey, sweet wines, port, porter, stout, ales, and beer. Malt preparations are also useful. Sweet fruits may be eaten. To be avoided are pickles, acids, condiments, much bulk of green vegetables, and strong liquors.

The following regimen is employed by William S. Ely in the treatment of the underfed, or rather ill-nourished, neurasthenic patient. The patient is kept in bed in order to reserve as much nerve force for digestion as possible, and dietetic treatment is begun by giving, every two hours by day and every three hours by night, four to eight ounces of milk, or chocolate made with milk, or two raw eggs, these foods being alternated. At the end of a week the daily ration may be increased to two quarts of milk, two of chocolate with milk, and a dozen raw eggs. The patient receives daily baths, massage, passive movements, and electricity.

This regimen emphasises the fundamental principle that muscular as well as mental rest must be enjoined if any dietetic treatment for leanness is to be successful. Excessive leanness may not be incompatible with the enjoyment of perfect health, and, upon the whole, it is less uncomfortable than excessive stoutness; yet in the very lean, bodily resistance against certain forms of disease, especially chronic ailments, is diminished, the subject of leanness is apt to suffer from cold and often from digestive disorders, muscular fatigue may be easily induced, and in youths who have grown rapidly to extreme height, disproportionate leanness leads to stunted development in other directions, especially that of normal chest expansion. Other persons, more especially young women, complain of their leanness upon aesthetic grounds.

The important principle to impress upon the subject of leanness is that he must learn to make a storage battery of himself; that if he expends his energies too fast in any direction, the food which is their ultimate source will never be stored as a source of latent energy. The difficulty may be hereditary, constitutional, or acquired, but it can only be overcome by appreciating the full significance of this principle.