Bioavailability of Vitamin E

52.1

Introduction

The term vitamin E (VE) is the generic descriptor for all tocol and tocotrienol derivatives exhibiting qualitatively the biological activity of α-tocopherol. It refers to a group of eight naturally occurring fat-soluble micronutrients: four tocopherols (α, β, γ, and δ) and four tocotrienols (α, β, γ, and δ) ( Fig. 52.1 ). VE is composed of a substituted chromanol ring linked to a C16 isoprenoid side chain. Tocotrienols differ from tocopherols by the presence of three double bonds in their side chain. Tocopherol and tocotrienol isomers differ in the location of the methyl groups on the chromanol ring. Due to the presence of the asymmetric carbons in their side chains, tocopherols can form eight stereoisomers ( RRR , RRS , RSR , RSS , SRR , SRS , SSR , SSS ). Naturally occurring tocopherols exist as RRR stereoisomers (formerly known as d-tocopherol) and are not esterified. However, when supplemented to the diet, tocopherol is usually a racemic mixture of the eight possible stereoisomers (all- rac -tocopherol, formerly known as dl-tocopherol) and is esterified to protect the phenol group against oxidation (e.g., α-tocopheryl acetate) ( Fig. 52.1 ).

α- and γ-Tocopherol are the most abundant VE forms in Western diets and are the forms found at the highest concentrations in human blood and tissues. VE is found at relatively high concentrations in vegetable oils and nuts but it is also present in other food matrices such as wheat germs and salad. In the United States, γ-tocopherol represents ≈ 70% of VE intake, due to the high consumption of food sources rich in γ-tocopherol in the typical diet (e.g., soybean oil, corn oil). The respective contribution of α- and γ-tocopherol to VE intake in Europe is not precisely known. The current US recommended daily allowance (RDA) for healthy adults is 15 mg/day but it is estimated that > 90% of men and > 96% women in the United States do not consume the estimated average requirements (EARs). Recent data point at similar inadequacies in several European countries. A recent systematic review of global α-tocopherol status has pointed at a relatively high prevalence of VE deficiency with 13% of the subjects exhibiting serum α-tocopherol concentration < 12 μmol/L, which has been proposed as a criterion for VE deficiency, and only 21% of the subjects reaching serum α-tocopherol concentrations > 30 μmol/L, which has been proposed as a criterion for VE adequacy.

VE is quantitatively the main lipid-soluble antioxidant in mammalian tissues and blood. It acts as a chain-breaking antioxidant, especially against peroxyl radicals, and is thus essential in maintaining the integrity of long-chain polyunsaturated fatty acids found in cell membranes. Recently, it has been shown to also exert nonantioxidant activities : modulation of gene expression, inhibition of cell proliferation and regulation of bone mass. Since oxidative stress has been implicated in the etiology of several diseases, for example, cardiovascular diseases and cancers, numerous epidemiological studies have investigated the association between VE dietary intake or status (usually evaluated as the fasting blood α-tocopherol concentration) and the incidence of these diseases and reported negative associations. However, most randomized controlled trials have failed to show a benefit of VE supplementation on the incidence of these diseases. Several explanations have been put forward, such as the absence of effect of VE supplementation on these diseases, a negative effect of α-tocopherol supplementation on the bioavailability of other VE isomers and the absence of population stratification by VE status or oxidative stress in these negative studies. Recently, it has also been suggested that the high interindividual variability of α-tocopherol bioavailability may have interfered with the effects of VE supplementation and that benefit from VE supplementation depends on a subjecťs genotype.

VE is a lipid and it shares common transport mechanisms with other lipids: in the lumen of the human upper gastrointestinal tract, it is found in structures allowing the solubilization of lipids, that is, micelles and possibly vesicles. It is transported in the blood via lipoproteins and it is found in membranes and lipid droplets in cells. Its bioavailability, that is, the proportion of VE that is available for use or storage by the organism, has been reported to range between 10% and 81%. Numerous factors affect its bioavailability: dietary factors, such as composition of the food matrix or the presence of fat, and also host-related factors, such as pathologies or genetic factors (see Section 52.4 for an overview thereof). Its fate in the gastrointestinal tract includes emulsification, incorporation into mixed micelles, transport through the unstirred water layer, uptake by the enterocyte, incorporation into intestinal lipoproteins, and secretion from the intestinal cell into the lymph or into the portal vein. This chapter will present an overview of what is known/unknown about the fate of VE in the human upper gastrointestinal tract and then list some of the factors hypothesized to affect VE bioavailability.

52.2

VE Fate in the Lumen of the Upper Gastrointestinal Tract During Digestion

Digestion of foods, or supplements, that contain VE, starts in the mouth where the diet is subjected to the action of saliva coupled with mechanical processing, leading to a partial disruption of the food matrix and an increase in the surface area. In the stomach, the food matrix is submitted to an acidic pH (between 2 and 6 during digestion) and to the action of gastric secretions, which contain several enzymes (pepsin, amylase, gastric lipase, etc.) able to further degrade the food matrix. VE, if not naturally incorporated in vegetable oils, is assumed to transfer, at least partly, from its food matrix to the oil phase of the meal. The extent of this phenomenon depends most likely on the food matrix in which VE is incorporated and on the quantity and the nature of lipids present at the same time as VE in its food matrix. It has been shown that there is no significant degradation or metabolism of α-tocopherol in the stomach during digestion. There is a lack of data on the role the gastric lipase could play on the hydrolysis of VE esters (e.g., tocopheryl acetate). This process could be significant when VE is consumed as a supplement and when pancreatic secretions are impaired (e.g., in newborns or in patients with cystic fibrosis).

In the duodenum, digestive enzyme (lipases, proteases, amylases, etc.) participate in the release of VE from the food matrix by degrading it further. The importance of the contribution of pancreatic lipase, which is responsible for the intestinal hydrolysis of triglycerides, to VE bioavailability has been recently emphasized by the association of single nucleotide polymorphisms (SNPs) in its encoding gene with the postprandial chylomicron VE concentration, an acknowledged marker of VE bioavailability, following consumption of a VE-rich meal. It is widely accepted that, to some extent, VE transfers to the lipid phase, if not naturally incorporated in vegetable oils, and then to mixed micelles. However, the possibility that some VE could be transferred to other structures that solubilize lipids in the duodenum, that is, vesicles, cannot be ruled out. Only one in vitro study has investigated VE (as α-tocopheryl acetate) distribution between the different structures present in the lumen of the small intestine during digestion, that is, mixed micelles, vesicles, oil droplets, and nonsolubilized food debris: most VE was found in matrices where its hydrolysis and its uptake by a model of human enterocytes was less efficient than in mixed micelles, suggesting this distribution could affect VE absorption efficiency. Its location within these structures, that is, at the surface or in the core of lipid droplets, or across or inside phospholipid bilayers, are not currently known and likely depend on VE interaction with the different components of these structures. Nevertheless, a computational study has suggested that, at 37°C, α-tocopherol remains in one leaflet of phospholipid bilayers, with its hydroxyl group located between the third and the fifth carbon atom in the sn-2 acyl chains of the lipids. As it is accepted that only free VE is taken up by enterocytes, VE esters need to undergo hydrolysis to be absorbed. This hydrolysis is carried out, at least partly, by cholesteryl ester hydrolase (CEH; also known as carboxyl ester lipase, CEL; bile salt-dependent lipase, BSDL; bile salt-stimulated lipase, BSSL), secreted by the exocrine pancreas and whose activity requires the presence of bile salts. Contrarily to what is observed with retinyl esters, we have shown that pancreatic lipase and pancreatic lipase-related protein 2 are not involved in this process. Nonetheless, other luminal candidate enzymes exist, for example, phospholipase B, and the participation of brush border enzyme has been suggested as well. Actually, Nagy et al. have recently shown that α-tocopherol and α-tocopheryl acetate exhibited the same bioavailability in the absence of digestive enzymes and bile salts in healthy subjects, even at supplemental doses. This study thus stressed the high potential contribution of brush border enzymes, enzymes released after cellular damage, or enzymes on the exterior of shed cells to the cleavage of α-tocopheryl acetate. These processes are summarized in Fig. 52.2 . Interestingly, a recent study has revealed the existence of a VE-ω-hydroxylase activity in human intestinal mucosa with the following rank order of activity on tocopherol isomers: γ = δ ≫ α. The ω-hydroxylase activity in human intestinal mucosa was 20%–30% of that found in the liver. In mice, it was found that only one enzyme, namely cytochrome P450, family 4, subfamily f, polypeptide 14 (Cyp4f14), was responsible for this activity. The quantitative, and qualitative (i.e., isomer specificity), contribution of this pathway to VE bioavailability warrants further investigation.

52.3

Uptake, Metabolism, and Secretion of VE by Enterocytes

After its extraction from the food matrix and incorporation into mixed micelles, or vesicles, bioaccessible VE can be taken up by enterocytes. It should be stressed here that this extraction is only partial and as a consequence, VE bioaccessibility can be relatively low and is highly variable between food matrices (see Section 52.4 for more details). VE is then transported across the enterocyte before its incorporation into chylomicrons, and also intestinal high-density lipoprotein (HDL), and secretion in the blood circulation via the lymph and the portal vein. These processes are summarized in Fig. 52.3 .

52.4