In a diet of 2,800 calories per day, if 410 calories be furnished by 100 grammes of protein there remain 2,390 to be supplied by carbo-hydrate or fat, or both. 583 grammes of carbo-hydrate, or 257 of fat, would give this value. The proportion in which the two elements enter into the daily food is usually determined by the amount of fat which can be used. This may depend upon how much fat can be procured, for it is dearer than carbo-hydrate, or, supposing expense is not a matter of importance, upon how much can be borne by the digestive system. In ordinary dietaries we find that the fat varies between 50 and 100 grammes; 100 grammes of fat will supply 930 calories which, with 410 from protein, makes 1,340, leaving 1,460, which would be supplied by 356 of carbo-hydrate. In adults it does not seem of great importance whether there is much fat or not, provided that the heat value of the food is maintained. The most recent work shows that fat and carbo-hydrate are about equal in value, calorie for calorie, as protein sparers. If there is any difference it is in favour of carbo-hydrate. The advantage of a fair quantity of fat is that of its small bulk, since 1 gramme of it is equal in food value to 2.27 grammes of carbo-hydrate. The ease with which a considerable quantity of food can be introduced into the body in such forms as butter, bacon, dripping and cream should always be borne in mind in prescribing a diet for a convalescent patient who is not putting on weight satisfactorily. Under special circumstances much larger quantities of fat than 100 grammes may be taken, for instance, with severe labour 365 grammes (with 1,150 of carbo-hydrate) (see the table on p. 135). In forced feeding, in the diet under the writer's observation mentioned above, an average of 250 grammes of fat per day was taken for 38 days, with a maximum of 311 grammes in one day. In children, fat cannot be entirely replaced by carbo-hydrate without harm resulting. The maldevelopment of rickets is by many believed to be chiefly due to a deficiency of fat in the food. Whether or not this is the main factor, it is certain that great improvement follows the administration of fat in the form of good milk, cod liver oil or dripping.
The table on p. 126 shows the forms in which fat and carbohydrate are taken in ordinary foods.