Early researchers investigated the effect of dietary fat on the intestinal absorption of tocopherol (Duncan et al., 1960). Vitamin E absorption is related to fat digestion and is facilitated by bile and pancreatic lipase (Sitrin et al., 1987). Jensen et al. (1997) found that the ileal digestibilities of tocopherols vary based on the dietary fat source. Whether presented as free alcohol or as esters, most vitamin E is absorbed as the alcohol. Esters are largely hydrolyzed in the gut wall, and the free alcohol enters the intestinal lacteals and is transported via lymph to the general circulation. Balance studies indicate that much less vitamin E is absorbed, or at least retained, in the body than is vitamin A. Vitamin E recovered in feces from a test dose was found to range from 65% to 80% in the human, rabbit and hen, although in chicks it was reported at about 25%.
The naturally occurring tocopherol form is subject to destruction in the digestive tract to some extent, whereas the acetate ester is not. Much of the acetate is readily split off in the intestinal wall and the alcohol is reformed and absorbed, thereby permitting the vitamin to function as a biological antioxidant. Any acetate form absorbed or injected into the body is converted to the alcohol form.
Vitamin E in plasma is attached mainly to lipoproteins in the globulin fraction. Rates and amounts of absorption of the various tocopherols and tocotrienols are in the same general order of magnitude as their biological potencies. Alpha-tocopherol is absorbed best, with gamma-tocopherol absorption 85% that of alpha- forms and with a more rapid excretion. One can generally assume that most of the vitamin E activity within plasma and other animal tissues is alpha-tocopherol (Ullrey, 1981). Lindberg (1973) reported that average values for tocopherol in plasma from pigs are much lower than those measured in plasma from dogs and that tocopherol levels are higher in pigs approaching slaughter weight than in newly weaned pigs. Froseth (1979) reviewed a variety of studies designed to evaluate effects of dietary additions of selenium, vitamin E and various inorganic elements on selenium and vitamin E metabolism, including serum tocopherol levels and tissue selenium. The diet of the sow influences stores in the young at birth and the amount obtained from mother’s milk. However, vitamin E is not highly effective in passing through swine placental membranes (Mahan, 1990); thus, vitamin E reserves of pigs at farrowing are low (Urbanova and Toulova, 1975). Likewise, less than 2% of dietary vitamin E is transferred from feed to milk.
Vitamin E is stored throughout all body tissues. Data regarding vitamin E storage in porcine liver offer conflicting conclusions. In some cases, it would appear that the liver contains the greatest stored vitamin E in comparison to other tissues (Jensen et al., 1990). However, Hoppe et al. (1993) indicated that adipose tissue contained the highest concentrations of vitamin E followed by liver, cardiac muscle and M. longissimus in decreasing order. Hoppe et al. (1993) suggested the discrepancy might be due to whether or not the pigs were sacrificed with or without fasting. The authors noted that in their study the pigs were slaughtered after 24 hours of fasting, which likely caused partial depletion of liver vitamin E. The results by Jensen et al., (1990) would support this explanation. In this work, the liver depletion of vitamin E was relatively rapid, while vitamin E levels in fat and muscle remained unchanged during a one-week depletion period. Liver contains only a small fraction of total body stores, in contrast to vitamin A, for which about 95% of the body reserves are in the liver. Grela and Jakobsen (1994) reported that 0.05% dl-alpha-tocopherol resulted in a greater content of vitamin E in depot fat than did 6.1 mg alpha-tocopherol acetate per 100 g of feed when either was supplemented to a standard diet with 10% soybean oil. Small amounts of vitamin E will persist tenaciously in the body for a long time. However, stores are exhausted rapidly by polyunsaturated fatty acids (PUFA) in the tissues, the rate of disappearance being proportional to the intake of PUFA. A major excretory route of absorbed vitamin E is bile, in which tocopherol appears mostly in the free form.