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.
The absorption of food takes place from the stomach to a limited extent, to a great degree from the small intestine, and to a lesser degree from the larger intestine. The rectum is capable of absorbing enough predigested food to sustain life for several weeks. (See Food Enemata.) The entire digestive process does not have to be completed before absorption begins. Usually those foods which are first digested, or which are administered in predigested form, are first absorbed.
The rate of absorption depends upon the degree of digestibility of the food, the extent of absorbing surface with which it is brought into contact by the peristaltic movement, the composition of the blood, the relative pressure in the intestine, blood vessels, and lymphatics, and, probably more than anything else, upon the functional activity of the cells covering the intestinal villi. The accompanying table shows the percentage of food swallowed which is actually absorbed:
Weight of Food.
Of 100 parts of solids of mixed diet
" " albumin
" " fats or carbohydrates