This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
In the previous part of this work the writer has incorporated a number of tables of the so-called "nutrient values" of foods as computed by physiological chemists. Such analyses are of unquestioned scientific interest, but it is necessary to caution against their too rigorous application in practical feeding, either in health or disease. For example, as pointed out by Halliburton, pork is the most highly nitrogenous of meats by analysis, yet its muscular fibres are so associated with fat that much of this food fails to be acted upon by the digestive agents of the body, and passes out as waste. Similarly whole wheat bread is chemically more nutritious than the bread of refined flour, yet so much of the proteid-bearing bran is undigested, that bulk for bulk, the whole-wheat breads are much less nutritious. As Halliburton states, "a common error, which it seems impossible to dislodge from the medical mind, is that 'nutritive 'and 'nitrogenous' are synonymous terms".
Another example is found in the application of analyses of red versus white meats to the feeding of patients with enfeebled digestion. The differences in proteid content and extractives may be very slight, yet the variation in toughness of fibre and consequent digestibility may be considerable.
For these reasons it is wholly impracticable to prescribe an invalid dietary on a basis of "calories " as representing energy, or heat, or a matter of storage of body substance, as one would definitely prescribe a medicine. Moreover, for obvious reasons, nearly all the calorifacient food experiments made with man have been conducted upon those in health, not in disease. Even a healthy man inclosed in a calorimeter for a few days' experimentation is in reality under highly artificial conditions as compared with a healthy man under stress of mental work, physical labour out of doors, or subjected to any of the manifold changing conditions of daily life.
In the ensuing chapters, therefore, the conditions especially affecting food digestion and the true nutrient value of food in disease will be considered from practical and clinical standards, without placing undue emphasis upon analyses and "calories".
Patients often conceive erroneous ideas regarding the digestibility of special foods. The food supposed to "disagree" may have done so because it has been improperly prepared or cooked, or because it has been eaten with other foods forming improper combinations. For example, a simple milk diet may be well tolerated when the milk taken with other foods may produce dyspepsia or biliousness. Moreover, it is quite possible that a food which at one time proved indigestible, may have done so because of transient abnormal conditions in the digestive apparatus, rather than from any injurious quality in the food itself.