Abnormalities in fat absorption present another problem frequently encountered in clinical nutrition. Bile salts have an important role in the emulsifying system which affects absorption of lipids. Impaired absorption of fat is observed when bile is absent from the intestinal tract, as in obstructive jaundice, although the hydrolysis of fat to fatty acids and glycerol by pancreatic lipase proceeds normally. Fat is poorly absorbed in sprue, idiopathic steatorrhea and the celiac syndrome although bile is available and enzymatic breakdown to fatty acids is unimpaired. It has been postulated that the malabsorption may be due to a defective phosphorylating mechanism as a result of deficiency of one or more vitamins of the B complex. Since administration of folic acid and vitamin B 12 often improve absorption, these vitamins may be involved. Another condition associated with malabsorption of fat is pancreatic fibrosis. In this situation, a lessened amount of pancreatic lipase is responsible for failure to hydrolyze fat.
The best method for measuring fat absorption is the metabolic balance study in which both dietary intake and fecal excretion of fat are determined. In clinical diagnosis, this is seldom feasible. Gross and microscopic examination of the stool for fat gives some information if the previous and current dietary intake of fat can be estimated but this is only a rough qualitative test at best.
A procedure that has been widely used in studying fat absorption is the vitamin A tolerance test. A large dose of vitamin A in an oily medium is administered, for ex-ample, 5000 I.U. per kilogram of body weight, and blood levels are determined before and at 3, 6, 9 and 12 hours after the test dose. In normal subjects, the concentration of vitamin A in blood increases markedly, usually five fold or more. In patients with malabsorption of fat, only small and delayed increases in serum vitamin A are observed.
The administration of Lipiodol® with measurement of iodine excretion in the urine has been suggested as a procedure for estimation of fat absorption but this test has not proved satisfactory.
Estimation of the degree of lipemia after ingestion of a test meal of fat has been used to some extent in studying fat absorption. Since chemical analysis of lipids presents serious technical difficulties, determination of plasma tubidity or chylomicron counts have been advocated to measure lipemia. The latter method is unsatisfactory due to the many variables that influence findings. Estimation of plasma turbidity is a simple and informative test which merits further investigation.
Recently, lipids labelled with I”‘ have become available for studies of intestinal absorption. Triolein and oleic acid labelled with I”‘ are proving most useful in investigating malabsorption syndromes and in differentiating between impairment of digestion and impairment of absorption of fat. A test meal of I”‘ triolein is administered in the fasting state and the radioactivity of blood samples is measured at the 4th, 5th and 6th hours there-after. The radioactivity of the total stool passed during the 48 hour period after the meal may be measured also. The test meal consist of 25 pc J131 triolein, 0.5 cc. peanut oil per kg. body weight, an equal amount of water, 2 cc. of Tween 80® and 20 gm. of barium sulfate. The last ingredient is needed to permit checking the emptying time of the stomach which would influence findings if delayed. Fecal excretion correlates well with fat balance studies. I”‘ levels in whole blood at the 4th, 5th and 6th hours are indicative of absorption in most in-stances. Normal individuals generally excrete less than 2% of the administered dose of radioacivity in the stools in the 48 hour collection. Patients with sprue and celiac disease will have a flat blood curve and high radioactivity in the feces.
Abnormalities of fat digestion may be differentiated from those of absorption by administration of a test meal of I11-labeled oleic acid in addition to the test meal with I11-labeled triolein. Oleic acid, 25 pc, is given in a gelatine capsule together with three capsules of barium sulfate and radioactivity in blood determined in the same manner as described for triolein (56). Most patients who have diseases of the pancreas will have significant impairment of absorption of neutral fat and normal absorption of fatty acid. Subtotal gastric resection has been found to result in impaired neutral fat absorption in approximately half of the paitents studied.