Following both vitamin E treatments in the current study, α-T was the major vitamin E detected in circulating plasma and lipoproteins. All vitamin E isomers from dietary sources (including supplements) are absorbed and delivered to the liver, although only α-T is preferentially recognized by the α-tocopherol transfer protein (α-TTP) for incorporation into circulating plasma . Other T (γ-T, δ-T) and T3 isomers (α-T3, γ-T3, δ-T3) are not preferentially utilized and are mostly excreted from circulation . This is the main reason why α-T is the only vitamin E isomer that is currently used as the standard to estimate human vitamin E requirements . However it is increasingly acknowledged that T3 and T serve different biological functions and bench marking only α-T to estimate human vitamin E requirements may no longer be the most accurate measure [3, 5, 8].
In the current study, supplementation with α-T or TRF resulted in significantly increased plasma α-T concentration compared to the baseline value. Furthermore, plasma total circulating α-T for the 24 h postprandial duration (described as AUC) was significantly higher after α-T treatment. This observation was however anticipated, due to the higher content of α-T administered (537 mg of α-T) from the α-T treatment compared to that of TRF (only 167 mg α-T).
In most human clinical and bioavailability studies of vitamin E, only the plasma and lipoprotein concentrations of α-T have been reported . It would therefore be ideal if the concentrations of individual T and T3 are measured to gain new insight into the physiological roles of these vitamin E isomers in humans . Evaluation of the metabolic response following T3 supplementation through plasma or serum concentration of T3 and α-T is advocated. Our present study shows that all T3 isomers (α-T3, γ-T3, δ-T3) were detected in plasma and lipoproteins following supplementation of TRF, although their concentration was significantly lower compared to that of α-T. These findings are in agreement with our previous postprandial observation  and several other human studies that examined the bio-kinetics [2, 13, 16, 17, 20] or physiological effects [9–12, 14, 15, 21] of T3 supplementation.
Detection of individual vitamin E isomers in plasma, following postprandial challenge could assist in elucidating their preferential absorption into circulating blood. However, this may not be true for α-T, since the liver actively secretes α-T into circulating plasma and impacts final plasma concentration of this vitamin E isomer. α-T was detected in both fasted and postprandial states in the current study. T3 isomers, including α-T3, the major T3 isomer in TRF, however was not detected in the fasted state. Their occurrence throughout the postprandial state was apparent, only in significantly lower levels compared to α-T. Despite these observations, we note that T3 have been demonstrated to have biological functions well below plasma concentrations noted in this study (5, 8). Among the T3 isomers, the absorption rates appear in the order α-T3 > γ-T3 > δ-T3. These findings might explain the possibility of bio-discrimination between T and T3 isomers in humans. Such bio-discrimination has also been demonstrated in several animal studies. Ikeda et al.  demonstrated that α-T3 is preferentially absorbed into the lymphatic circulation compared to γ-T3 and δ-T3. Similar observations were found by Yap et al.  who investigated the influence of route of administration on the absorption and disposition of α-T3, γ-T3 and δ-T3 in rats. Of the 3 isomers, α-T3 achieved the highest concentration and AUC after an oral ingestion of T3. This was followed by γ-T3 and δ-T3. In humans, plasma concentrations of α-T3 were 2-fold higher than that of γ-T3, and almost 10 times higher than δ-T3 after supplementation with the same dose of T3 preparations . Similar observations were also demonstrated in hypercholesterolemic subjects who received a high γ-T3 supplements that contained ≈4-fold concentration of γ-T3 than α-T3 .
Distribution of T3 isomers in lipoproteins also provides a better explanation of T3 absorption and transport in circulating plasma. In agreement with our previous observation , T3 were transported in TRP (chylomicrons + VLDL), LDL and HDL. Several mechanisms have been postulated to explain this observations  including the selectivity and affinity of hepatic α-TTP , the function of a specific protein carrier in transporting α-T3 in the intestinal cells , and differences in the methyl groups in the chromanol rings of T3  that influenced the absorption rate of each T3 isomers . Following its hepatic uptake, it would be intriguing to know whether nascent VLDL or HDL generated from the liver, is readily enriched in T3 from the liver itself. The role of HDL in transporting vitamin E has recently been identified as one of the primary mechanisms in vitamin E absorption in the fasted states .
The competitive uptake between isomers is only initiated following the hepatic uptake of vitamin E from chylomicrons, where the selectivity role of α-TTP is significant in transferring vitamin E into circulating VLDL . The relative affinity of vitamin E isomers towards α-TTP has been demonstrated to be in the order of α-T (100%) > α-T3 (12%) > γ-T (9%) > δ-T (2%) . This mechanism explains the occurrence of α-T as the major vitamin E isomer detected in TRP, LDL, and HDL, and the rapid disappearance of α-T3, γ-T3 and δ-T3 from circulating plasma and lipoproteins. Other physiological factors such as bile, urinary and fecal excretion that may influence the rapid disappearance of T3 has also been postulated [19, 32]. The exchange of T3 between circulating chylomicron, VLDL, LDL, and HDL has also been suggested to explain their distribution in the lipoproteins [19, 20].
There is no bio-discrimination between T and T3 during intestinal absorption after dietary intake of vitamin E [26, 33–35]. However, the rapid disappearance of T3 may be associated with its preferential utilization in humans (8, 25, 34). In the current and previous [19, 20] studies, the amount of T3 absorbed into TRP was very low. This observation may indicate the possibility of bio-discrimination of T3, prior to the intestinal absorption. Although mechanism for the preferential absorption of T3 is difficult to describe, it has been suggested that the complexity of T3 absorption is probably due to the difference in their micellar solubility, affinity for intestinal brush border membranes, transport in enterocytes, incorporation into chylomicrons, or a combination of these processes . Besides, there might be variability in the mucosal handling of vitamin E that could affect their intestinal absorption . Although we did not separate chylomicrons and VLDL from TRP fraction to differentiate the T3 uptake from intestine by chylomicrons and from liver by VLDL, recent findings from Abuasal et al.  demonstrated that there was an inverse relationship between intestinal uptake of γ-T3 and their concentration in the intestinal lumen. Therefore, any elevation of γ-T3 concentration in the lumen would likely reduce the amount of γ-T3 transported into the enterocytes. However, no investigations on other T3 isomer were carried out. The intestinal absorption of T3, as well as T still merits further investigations, since their mechanism has not been fully described . In rats, dietary vitamin E including T3 are converted to their metabolite by CYP-dependent pathway in the intestine during absorption. This could likely regulate T3 concentration in plasma and tissue . Yet, excess intake of T3 has been observed to lead excretion of α-T3 and γ-T3 into bile, before both T3 isomers were metabolized into α- and γ-CEHC derivatives .
The postprandial dose response effect of vitamin E in humans has basically been evaluated from the plasma and lipoproteins profiles of α-T and γ-T [39–41]. Surprisingly, no such evidence exists for T3, although T3 always positively imaged as a superior antioxidant compared to T [2, 8]. In the previous study , we investigated the postprandial response after 1011 mg TRF supplementation. In fact, this dose used was higher than the Tolerable Upper Limit Intake (UL) for vitamin E . One of the rationale of conducting the current study was to investigate whether supplementation with 526 mg TRF would resulted a similar postprandial response, in comparison to the dose used in the previous study , since concentrations of vitamin E in plasma can only be raised maximally two to three-fold after supplementation . Plasma α-T3, γ-T3 and δ-T3 response after TRF treatment in the current study were not significantly different from the previous study. Additionally, α-T and γ-T concentrations in plasma, TRP, LDL and HDL were not apparent between both TRF treatments. However, observations in lipoprotein fractions still remains to be elucidated. In HDL, starting from 4 h to 6 h postprandial, α-T3 concentration after 526 mg TRF treatment were significantly lower compared to the 1011 mg TRF treatment. These observations merits further investigation since the transportation of vitamin E by HDL may possibly be influenced by supplementation dose and was not affected by amount of dietary fat intake . In both postprandial studies, the amount of dietary fat in the test breakfast consumed before TRF supplementation was standardized.
Several studies have suggested the effectiveness of T3 as a hypocholesterolemic agent in lowering plasma or serum total cholesterol in humans [15, 42]. Nevertheless, it is questionable why the effectiveness of T3 in lowering plasma total cholesterol has not been compared with α-T, since α-T has been recognized as the only form utilized to estimate human vitamin E requirements. Furthermore, the effectiveness of T3 in humans was only compared with a placebo treatment in most studies [10, 15, 18, 21, 42, 43]. Although in several studies, physiological effects of T3 was compared with α-T, the concentration of α-T in the control preparations or supplements was very low [9, 11, 44] Unlike our previous observation  where supplementation with 1011 mg palm TRF or 1074 mg α-T resulted in significant lowering of plasma postprandial total cholesterol, supplementation with 526 mg palm TRF or 537 mg α-T in the current study did not demonstrated any hypocholesterolemic effect. Several postulations have been discussed to explain the inability of T3 to lower plasma or serum cholesterol in humans such as the higher content of T in the T3 supplements, in vivo bio-conversion of T3 to α-T, and very low concentration of T3 that did not reach the pharmacologically effective level in plasma .