A "proprietary" or "patent" food is one which is made from one or more simple articles of diet, and is more or less modified in the process of preparation. These foods may be made from milk or milk products; from carbo-hydrates, with or without a basis of milk; from meat, fish, or eggs; from vegetables, fruits, or nuts; and occasionally from fatty substances. In some cases the food is merely modified, in the relative percentages of its component parts. In others, one or more of the different constituents are separated and altered. In some, partial or complete predigestion of the natural food is carried out. Most of them are subjected to the action of heat. A few are prepared by what is known as "the cold process." Judging by the huge sums of money spent on advertising these food specialties, there must be an enormous sale, and it becomes necessary to investigate the claims which are made on their behalf from the standpoints of nutritive value, economic worth, and aesthetic, mental, and social considerations. The majority of these foods are advertised as specially valuable for the feeding of infants. These will be considered in conjunction with other foods of similar composition and their value then commented on. Some foods are merely altered sufficiently to render them more digestible, palatable and nutritious; or to make them less bulky, for the purposes of travelling.

The value of reduction in bulk is considerable in the case of condensed milk, dried and compressed vegetables, meat powders and pemmican. The claims of meat preparations are generally based on the theory that there is a great concentration of nutriment in little bulk. Many well-known advertisements make an appeal to the imagination and lack of intelligence of the public. It is an appeal which is described in philosophy as the suggestio falsi.

It may be definitely and positively stated that proprietary foods are not necessary, either in sickness or in health. They are neither more nutritious nor more easy to take than the homemade modifications, which can be easily prepared from the common articles of diet, except in the case of some foods which are predigested or contain partially converted starch. Nor can they be regarded as of special value because of their appetising properties. The meat preparations are powerful stimulants of the secretion of gastric juice, but they have no more value in this respect than ordinary beef-tea and clear soups.

Nevertheless, proprietary foods are sometimes useful on account of their high cost, their pleasant flavour and their variety. A patient can generally be induced to take regularly some special oat food, in the form of porridge for breakfast, or some proprietary carbo-hydrate food at bedtime, instead of the ordinary porridge, milk, or gruel. Some of the meat jellies are indirectly useful, when it is advisable to partially starve a patient, for they are regarded by the public as extremely nutritious foods. Partial predigestion of starchy foods is valuable for children and adults, but is of doubtful service when applied to meat foods. The peptonization of protein in the stomach is dependent on pepsin and hydrochloric acid. If the gastric digestion of protein fail, it is due to deficiency of hydrochloric acid and not to lack of pepsin. When the pepsin is deficient it will be found that the secretion of acid is also defective. Apart from this it must be remembered that gastric digestion is not essential and that the stomach may be regarded merely as a receptacle of food. All foods can be digested fully in the intestines. On the other hand, the physiological arguments against the necessity for the predigestion of food are to a certain extent upset by practical experience. Partial predigestion, more especially in the case of milk proteins, is undoubtedly of very great advantage in many diseases. From the economic point of view all these foods are much more expensive than common articles of diet of equal or greater nutritive value. So also the ordinary articles of diet, which are the most appetising and most delightful to the trained palate, are often enormously expensive in proportion to their nutritive value. The price is by no means everything, but is nevertheless a most important consideration for the poor.