Composition, particle size and role in dietary fat transport of two different lipoproteins of the intestinal lymph

Absorption, transport and metabolism of vitamin E

Vitamin E includes eight naturally occurring fat-soluble nutrients called tocopherols and dietary intake of vitamin E activity is essential in many species. alpha-Tocopherol has the highest biological activity and the highest molar concentration of lipid soluble antioxidant in man. Deficiency of vitamin E may cause neurological dysfunction, myopathies and diminished erythrocyte life span. alpha-Tocopherol is absorbed via the lymphatic pathway and transported in association with chylomicrons. In plasma alpha-tocopherol is found in all lipoprotein fractions, but mostly associated with apo B-containing lipoproteins in man. In rats approximately 50% of alpha-tocopherol is bound to high density lipoproteins (HDL). After intestinal absorption and transport with chylomicrons alpha-tocopherol is mostly transferred to parenchymal cells of the liver were most of the fat-soluble vitamin is stored. Little vitamin E is stored in the non-parenchymal cells (endothelial, stellate and Kupffer cells). alpha-Tocopherol is secreted in association with very low density lipoprotein (VLDL) from the liver. In the rat about 90% of total body mass of alpha-tocopherol is recovered in the liver, skeletal muscle and adipose tissue. Most alpha-tocopherol is located in the mitochondrial fractions and in the endoplasmic reticulum, whereas little is found in cytosol and peroxisomes. Clinical evidence from heavy drinkers and from experimental work in rats suggests that alcohol may increase oxidation of alpha-tocopherol, causing reduced tissue concentrations of alpha-tocopherol. Increased demand for vitamin E has also been observed in premature babies and patients with malabsorption, but there is little evidence that the well balanced diet of the healthy population would be improved by supplementation with vitamin E.

Influence of dietary fat saturation on lipid absorption in the rat

Flux (mumole/h) of triglycerides, phospholipids and cholesterol into the thoracic duct lymph was measured in rats receiving a constant intraduodenal infusion of a cholesterol-free oil rich in either polyunsaturated (P/S = 4.8) or saturated (P/S = 0.2) fatty acids. Rats had ad libitum access to a fat-free semi-synthetic diet throughout the experiment. Oils were infused at a rate equivalent to a 10% (w/w) fat-containing diet for at least 18 h prior to collection of lymph; both oils were compared in each animal. Although absorption of the infused oils approximated 100%, triglyceride flux was significantly lower during infusion of the saturated compared to the polyunsaturated oil. Phospholipid and total cholesterol fluxes were not significantly affected by the type of oil, but the percent of lymph total cholesterol which was esterified was slightly but significantly lower during infusion of the unsaturated oil. Using the molar phospholipid/triglyceride ratio as a index of lymph lipoprotein size, it was found that absorption of the oil rich in polyunsaturated fatty acids resulted in an increase in the mean size of lymph lipoproteins. The potential significance of an influence of dietary fat saturation on lymph lipoprotein size and cholesterol esterification for the ultimate metabolic fate of absorptive lipoprotein constituents is discussed.

The effect of various dietary factors on the size distribution of lymph fat particles in rat

By means of cellulose acetate electrophoresis and measurement of radio-activity, lipoproteins (d less than or equal to 1,006) of lymph collected from rats fed a diet containing fats added to labeled fatty acids for 20-24 hr, have been separated into two kinds differing in their size. Fats with different saturated and unsaturated fatty acid content were tested: corn oil, lard, mutton tallow, tripalmitin, corn oil-lard mixture, and randomized palmitic-linoleic triglycerides. Factors studied were: (a) increased amounts of these fats given alone or included in a test meal; (b) added amounts of nutrients (other than fats) in the test meal; (c) adaptation to a diet containing 20% fat for 10 days. The major part of the labeled lymph lipid was always transported by the smaller particles. Size distribution of lymph fat particles was influenced by some of the factors studied. Generally, unsaturated fats produced higher amounts of larger particles.

Effect of acute ethanol ingestion on fat absorption

A test meal (300 mg casein, 600 mg sucrose, 100 mg corn oil, tracer dose of 9.10(3)H oleic acid) was given to fasting adult rats with intestinal lymph fistulas. One group received an acute oral dose of ethanol (3.2 g/kg body weight) simultaneously with the test meal. Controls received 2.5 ml of water instead of ethanol. Ingestion of ethanol temporarily delayed the removal of lipid radioactivity from the stomachs. More than 25% of radioactivity fed remained 8 hr after feeding whereas with control rats less than 10% of lipid radioactivity fed remained 6 hr after feeding. In controls and ethanol-treated rats, the amounts of exogenous lipids in the intestinal lumen and mucosa were low and similar enough. Quantities of endogenous and exogenous lipids found in the lymph collected during 24 hr after feeding were similar in the two groups, but the fat absorption peak was found after 6 hr in alcoholic rats and before 6 hr in controls. This delay was probably due to the retention of lipids in the stomach. More of the exogenous lipid was always transported by small particles moving in the region of alpha1 globulins in cellulose acetate electrophoresis than by larger particles remaining at the origin. This proportion was enhanced in the ethanol-treated animals. The larger fat particles were richer in endogenous fatty acids in alcohol-treated rats than in controls.