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.
Buttermilk is the residual milk left after churning and removing the fat. It is wholesome and diuretic, and makes a capital beverage for those patients who fancy its peculiar sour taste. It contains albumin and finely coagulated casein, salts, water, and sugar, which is largely converted into lactic acid. Its fuel value is about that of skimmed milk - 165 calories per pint - and its composition resembles skimmed milk, but it contains a little less protein and carbohydrate and a little more fat. A pint of it contains as much nourishment as two ounces of bread or a large potato (A. C. True). It should be drunk fresh, for it soon decomposes. Some patients can digest it who are unable to take ordinary milk. In those disorders in which the digestion of protein and fats is poor it may be better borne than milk.
It is of especial value in chronic gastric catarrh with atrophy of the gastric glands (Eulenburg). A "buttermilk cure " is sometimes practised after the manner of the "whey cure" (see Dietetic "Cures "), but buttermilk cannot long be depended upon as an exclusive diet.