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.
Albuminuria may be produced by alterations in the composition of the blood and by structural or functional changes in the kidney itself, or by both. The albumin is derived from the blood serum, which in turn comes from the protein of the food. Under ordinary circumstances proteid matter in passing through the body undergoes several transformations which affect its degree of solubility and its ability to osmose or go through animal membranes such as line the alimentary canal, the blood vessels, and the tubules of the kidney. Most protein taken into the body as food is not readily absorbed until converted into some form of peptone or albumose. These substances diffuse very easily, and their presence in the blood would render them liable to constant osmosis from the vessels into the lymph spaces or tissues almost immediately after absorption, but in their transit through the intestinal villi they are reconverted into serum albumin, which does not osmose easily, and which therefore remains in the vascular system. A certain amount of albuminous material, however, must pass by osmosis from the blood vessels into the lymphatic channels, and thence into the tissues which surround them or which they penetrate.
For serum albumin to pass into the urine it must osmose through two layers of cells - namely, the capillary wall of the blood vessel and the epithelial lining of the tubules of the kidney. Variations in the density of these walls, and probably also alterations in the functional activity of the epithelial cells which compose them, will modify the rate of osmosis so that it may be completely checked, or it may be allowed to take place unhindered. The conditions of osmosis also depend upon the relative density of the fluids on either side of the membrane, hence alterations in the composition of the blood plasma may allow of the osmosis of serum albumin into the tubules of the kidney. It is important to observe that the presence of serum albumin in the urine in excess not only indicates a functional or structural weakness in the kidneys themselves, or in the composition of the blood, but it represents actual loss of substance from the body - i. e., of material which is not yet converted into waste matter, but which is capable of furnishing nutrition to the tissues - in other words, there is a leak of nutrient matter from the kidneys.
From the foregoing statements it might seem feasible to disregard the leakage and increase the amount of nitrogenous food sufficiently to counterbalance the loss sustained. The same plan might theoretically be thought to be of benefit in cases of diabetes, where it would seem quite possible to counterbalance the loss of saccharine material from the kidneys by ingestion of much larger quantities of starches and sugars; but this is a fallacious comparison, for excess of sugar is really a foreign body in the blood, while albumin is not (Granger Stewart). Clinically, however, it has been found that when either of these conditions of loss of substance from the blood exists in the kidneys it is better to withhold as much as possible the particular variety of food which is leaking through them and reduce the work of these organs, hoping by rest, combined with other appropriate means of relief, to cure the faulty condition.