Bioactive Dietary Compounds in Edible Oilseeds: An Appraisal of Tocopherols and Tocotrienols

Abstract

Phytochemicals are naturally occurring and biologically active chemical compounds found in plants. Most of these phytochemicals are known to exhibit antioxidant properties and thereby provide numerous health benefits for humans more than those attributed to macronutrients and micronutrients. In fact, dietary antioxidants are understood to reduce the risk of several life-threatening diseases, including cardiovascular diseases and cancer types. Synthesized only by plants and photosynthetic microorganisms, tocochromanols are a group of natural compounds (lipid-soluble antioxidants). In particular, tocopherols (α, β, δ, and γ) and tocotrienols are tocol-related compounds, belong to the vitamin E family, and are recommended for their health benefits owing to their unique antioxidants qualities. These are provided to the human body in varying amounts mainly from dietary sources such as vegetable oils, some oilseeds, and nuts. Seeds (edible oilseeds) often, dominate other plant parts in terms of the abundance of total tocopherol (T-tocopherol). This chapter aims to appraise relevant literature available on the chemistry of tocopherols and tocotrienols (or vitamin E), major sources of tocopherols and tocotrienols in plants, and the major roles of tocopherols and tocotrienols in human health. The outcomes of discussion may help devise future research on edible oilseeds and their human health benefits.

1. Introduction

Because of their antioxidative properties, tocopherols (α, β, γ et δ) and tocotrienols compounds related to tocochromanols are particularly significant bioactive components in vegetable oils. Notably, lipid oxidation is an autocatalytic chain that takes over the active and nutritional functions of unsaturated fatty acids. The presence of antioxidants can greatly limit the proportion and extent of lipid oxidation fluctuations. In fact, vitamin E is the collective name for tocopherols and/or tocotrienols, which are found together with lipids in various natural ingredients. They are considered the best-known lipid-based natural antioxidants in food and biological environments [1, 2]. Since they are strong antioxidants, vitamin E derivatives make sense in biological processes, particularly in preventing lipid oxidation caused by nonenzymatic processes [3]. Particularly tocotrienols, which are forms of tocopherols, have demonstrated therapeutic potential primarily in the prevention of cardiovascular diseases and lipidaemia atherosclerosis, hyper-, neurodegenerative diseases, osteoporosis, and recently different cancers [3]. However, the exact role of tocopherols particularly tocotrienols in biological careful processes still requires investigation. This chapter enlightens the chemistry of tocopherols and tocotrienols (or vitamin E), overviews the major sources of tocopherols and tocotrienols in plants, and appraises the literature available on major roles of tocopherols and tocotrienols in human health. The discussion outcomes may help devise future research on edible oilseeds and their human health benefits.

2. Chemistry of tocopherols and tocotrienols (or vitamin E)

Vitamin E was discovered in 1922 to be an essential compound to maintain pregnancy in a study in rats [4]. In fact, vitamin E refers to a group of structurally related plant-derived natural products comprised of tocopherols and tocotrienols or Tocochromanols. Tocochromanols, considered as vitamins become the dominant chromanols as a part of the family of lipid-soluble antioxidants coexisting in the plastids of plants [5]. In addition, tocopherols are a class of components produced only by plants and photosynthetic microorganisms. The eight most frequently mentioned forms of vitamin E are made up of four tocopherols and four tocotrienols, which vary in the quantity and orientation of their methyl groups on the primary 6-hydroxychroman (Figure 1) [5]. In fact, the general configuration of both vitamin E types consists of the 6-hydroxychroman form and the phytol subchain of the isoprenoid moiety (see Figure 1), with tocopherols and tocotrienols having similar chemical configuration, but the phytol sub-chain changes less. Tocopherols have a saturated phytol moiety chain, but tocotrienols have three unsaturated sites on side chain carbon atoms 3′, 7′, and 11′ [5].

Figure 1.

Schematic representation of the naturally occurring four different chemical structures of tocopherols and tocotrienols (α-, β-, γ-, and δ-forms) [5].

Biological tocopherols have a 2R,4′R,8′R (RRR) stereochemistry, whereas tocotrienols have an R stereochemical core only at C2 (Figure 1). Synthetic α-tocopherol contains a racemic mixture of eight stereoisomers (four pairs of enantiomers: RRR-SSS, RSR-SRS, RRS-SSR, and RSS-SRR) from three chiral centers from positions C2, C4′, and C8″ [5]. Due to its structure, the components of the vitamin E family are lipid-soluble and amphipathic, allowing them to bind to membrane lipids [4]. Although the various vitamin E components are structurally similar, their natural functions are not the same [4]. Vitamin E is a scavenger of free radicals generated in cell membranes and plasma lipoproteins [6]. Indeed, tocols are integrated into the amphipathic phospholipid bilayer of cell membranes because of their structural makeup. As a result, they can shield plant organs, membrane lipids, and photosynthesis-related organs from oxidative stress [7, 8].

3. Major sources of tocopherols and tocotrienols in plants

Numerous bioactive components, such as dietary fiber, carotenoids, tocotrienols, tocopherols, phytoestrogens, and vitamins, are present in whole grains [9]. Tocopherols and tocotrienols are usually found together with other lipids, especially in vegetable oils that are high in unsaturated fatty acids (approximately 100–1000 ppm mixed tocopherols), and in smaller amounts in fish oils (250–350 ppm) and animal fats (up to 100 ppm). Fish and animal fats are primarily composed of a-tocopherol, while oil seeds are primarily composed of g- and a-tocopherols. There are some special cases, for example, palm oil and nopal resin are rich in tocotrienols, and linseed oil contains high amounts of plastochromanol-8 [2]. There are currently fewer sources of tocotrienols known, but plant foods high in unsaturated fatty acids are good sources of tocopherols. Animal food products’ tocol concentration is influenced by what they eat [10]. The main sources of tocols are oilseeds, nuts and other common edible oils (such as palm, rice bran, olive, corn, and wheat) (Table 1) [11]. Although due to their low little lipid content, they are negligible in most fruits and vegetables [12]. Furthermore, seeds and other plant-processing by-products are emerging as novel sources of these bioactive [13]. In terms of food chemistry, the presence and/or concentration of tocols may be influenced by different feedstocks [14], cultivars, crop year, climate and stress conditions, life cycle, soil quality, storage, and processing conditions [15]. A regular diet mainly provides large amounts of tocopherols: α-tocopherol and γ-tocopherol. This last one is the most common dietary source in the United States due to the high consumption of soybean, sesame, and corn oils. However, α-tocopherol is most commonly found in the European nutrition [16]. Since vegetable oils and animal fats often contain tocopherols/tocotrienols, they retain their natural properties when stored and used in large quantities at room or lower temperatures. When stored at high temperatures, such as when fried. However, tocopherols break down quickly, resulting in limited antioxidant effects in oils and fried foods. This is particularly important when fried or semi-fried foods with compromised tocopherol/tocotrienol content are frozen and thawed before use [16]. Table 1 illustrates the high concentrations of α-tocopherol present in wheat germ oil. This compound is also present in common edible oils, including barley, cottonseed, olive oil, palm, safflower, sunflower, and its derivatives in special seed oils (such as grape, lemons, oranges, and papayas) (Table 1) [11]. Cashew oil is the only ingredient that contains b-tocopherol in its major form [17]. Similarly, only one feedstock (passion fruit seed oil) among the examples provided displayed δ-tocopherol as the primary one. Only traces of β-tocopherol were found in linseed and rapeseed [18], whereas the same has been recorded for the presence of d-tocopherol in cottonseed [18]. Nuts are well recognized sources of tocopherols [19]. Several tree nut oils have been examined [20], including walnuts, pecans, pine nuts, pistachios, almonds, Brazil nuts, and hazelnuts. The study mentioned above indicates that the main homologs found in these samples were α- and γ-tocopherols. For peanuts, the same pattern has been observed [21]. Macadamia, on the other hand, appears to be an exception among the nuts that have been reported, with tocopherols being found to have either negligible or no effect [22]. It has been recorded that α-tocopherol is commonly present in the chloroplasts of plant cells, whereas β-, γ-, and δ-tocopherols are usually present outside these organelles. Among the natural sources of tocopherols, wheat germ oil is the most abundant followed by sunflower, safflower, almond, cottonseed, and rapeseed.

Feedstock	α-Tocopherols	β-Tocopherols	γ-Tocopherols	δ-Tocopherols
Almonds	nd-34.9	nd	1.26–1.77	nd
Barley	14.2–20.1	0.60–1.90	3.50–15.1	0.90–4.60
Brazil nuts	nd-2.2	nd	13.8–16.8	nd
Cashew	nd-7.84	134	30.0	0.3–0.63
Cottonseed	30.5–57.3	0.04–0.30	10.5–31.7	tr
Grape	11.8–18.8	nr-0.01	2.22–60.1	nr
Hazelnut	15.7–42.1	nd	9.7–13.6	nd-to 0.3
Lemon	10.2	0.22	0.13	1.9
Linseed	0.54–1.20	nd–tr	52.0–57.3	0.75–0.95
Olive	11.9–17.0	nd–0.27	0.89–1.34	nd–tr
Orange	30	nd	nd	1.86
Palm	6.05–42.0	nd–0.42	tr–0.02	tr–0.02
Papaya	5.18	0.21	0.18	1.89
Passionfruit	Nd	5.40	16.7	27.9
Peanut	8.86–30.4	nd–0.38	3.50–19.2	0.85–3.10
Pecan	nd-1.82	nd	44.0–4.73	nd-0.7
Pine	2.2–16.6	2.26–3.18	23.0–24.7	nd-0.7
Pistachios	nd-32.8	nd	3.06–4.78	nd-2.3
Rapeseed	18.9–24.0	nd–tr	37–51	0.98–1.90
Safflower	36.7–47.7	nd–1.20	tr–2.56	tr–0.65
Sunflower	32.7–59.0	tr–2.40	1.40–4.50	0.27–0.50
Walnut	nd-3.80	nd	34.9–37.6	1.98–5.40
Wheat germ	151–192	31.2–65.0	tr–52.3	nd–0.55

Table 1.

Content of types of tocopherols (mg 100 g⁻¹ of oil) in oils from different sources [11].

nd: not detected.

While tocotrienols are typically absent from green plant parts, they can be found in the bran and germ portions of some seeds and cereals (Table 2) [23]. Tocotrienols are much less prevalent than tocopherols. Small evidence of α-tocotrienol has been found in high concentrations (Table 2) [11, 24]. As per Zhu et al. [23], certain seeds exhibited comparable or elevated levels of γ- and β-tocopherols; therefore, it is suggested that the oilseed should contain α-tocotrienols. Furthermore, macadamia also seems to be an exception among other nuts, as previously mentioned [21, 25]. Saidi and Keum reported that tocotrienols are also predominant in wheat germ oils while in sesame, sunflower, and safflower oils are in minor contents [26].

Feedstock	α-Tocotrienols	β-Tocotrienols	γ-Tocotrienols	δ-Tocotrienols
Barely	46.5–76.1	nd-12.4	8.50–18.6	0.50–2.60
Corn	0.94–1.50	Nd	1.30–2.00	nd-0.26
Macadamia	1.25–4.84	Nd	0.87–3.43	0.30–1.77
Olive	Nd	Nd	nd	nd
Sesame	Nd	Nd	0.34	nd

Table 2.

Content of types of tocotrienols (mg 100 g⁻¹ of oil) in oils from different sources [11].

nd: not detected.

4. Major roles of tocopherols and tocotrienols in human health

The bioavailability and metabolism of vitamin E isoforms vary despite their similar structure and antioxidant activity [27]. While all isoforms exhibit biological activity, only α-tocopherol is found in plasma and tissues at elevated concentrations [28]. It has been reported that the number of hydroxyl groups in vitamin E isomers affects their antioxidant activity [28]. Ninety percent of α-tocopherol is present in plasma and bodily tissues, whereas other forms of vitamin E are broken down and eliminated [29].

Some studies have shown that tocotrienols have lower antioxidant activity than tocopherols [30]. This may be due to their lower position in the cell membrane phospholipid bilayer and other efficient binding to lipid peroxyl radicals [31]. However, their oral bioavailability is lower compared to tocopherols [32]. Furthermore, they react better when they become free radicals and can easily form potentially cytotoxic adducts [33]. In addition, other forms of vitamin E, such as γ-tocopherol, δ-tocopherol, and γ-tocotrienol, also have unique antioxidant properties. α-tocopherol was found to have the ability to inhibit nitrogen-reactive forms. This reaction was not observed with γ-tocopherol [34]. The non-antioxidant properties of tocopherols and tocotrienols have also been demonstrated at the molecular level [33]. This activity is due to interactions with enzymes, structural proteins, structural lipids, and transcription factors [35]. Recent research has indicated that γ-tocopherol’s long-chain metabolites have anti-inflammatory properties [36]. The levels of different forms of vitamin E and its metabolites in blood and tissue are crucial information for comprehending the natural fate and function of these substances. This is particularly true for the tocopherols and tocotrienols in their γ and δ forms, which have been found recently to have beneficial effects on health [36]. In fact, there is evidence that tocopherols and tocotrienols have effective therapeutic effects on cardiovascular diseases, certain types of cancer tumors, as well as metabolic disorders, neurodegeneration, and oxidative stress [36]. Vitamin E’s main role in the human body is to maintain membrane integrity and function as a fat-soluble antioxidant. Compared with tocopherols, tocotrienols have stronger functions in preventing tumors and cardiovascular diseases [36]. Vitamin E also stimulates the immune system capacity and repairs DNA damage [36]. Researchers are now focusing on tocotrienols rather than tocopherols because of their improved bioactive qualities and possible health advantages [37]. Several in vitro and in vivo studies have found that tocotrienols exhibit anticancer [38], cardioprotective, and neuroprotective properties [39]. Moreover, it is thought that the unsaturated side chains of tocotrienols, which aid in their integration into cells, are responsible for their health benefits [36]. It is not possible to produce tocotrienols; instead, they can only be extracted from natural sources such as palm oil. Numerous industries, such as cosmeceuticals, pharmaceuticals, and nutraceuticals, can benefit from the use of palm tocotrienols. Tocotrienols (T3s) are a naturally occurring subtype of vitamin E with potential metabolic health-promoting properties. Preclinical studies have shown that tocotrienols possess stronger antioxidative, anti-inflammatory, anti-obesity, and insulin-sensitizing properties as compared to α tocopherol (TF) [40]. A study [41] conducted in patients with nonalcoholic fatty liver disease (NAFLD) demonstrated that a 400 mg/day tocotrienol-tocopherol (T3-TF) supplementation for a year successfully decreased the patients’ hepatic steatosis (Figure 2) [43]. Lately, Pervez et al. [44] released a clinical study contrasting the effects of placebo and delta-tocotrienol (δ-T3) in NAFLD patients. δ-T3 supplementation (600 mg/day) for a period of six months, produced smaller improvements in liver enzymes, inflammatory markers, oxidative stress, and hepatic steatosis compared with placebo. The effects of δ-T3 demonstrated significant control over liver enzymes and other biomarkers compared to the T3-TF mixture. Since α-TF is the only drug recommended for use in patients with nondiabetic nonalcoholic steatohepatitis (NASH), we demonstrated that comparison of δ-T3 with α-TF should lead to more informed studies [42].

Figure 2.

Schematic representation of various functions of vitamin E in human [42].

5. Conclusions

The structures, origins, and consequences of protocols in food and biological systems were covered in this contribution. Although α-tocopherol has been extensively researched, there are currently fewer sources of tocotrienols identified, particularly for newly emerging dietary sources like specialty oils.

Additionally, several studies have confirmed the superior activity of tocotrienols compared to tocopherols regardless of application. Therefore, the production of high tocotrienol content varieties, the exploration of existing tocotrienol sources, and further applications as functional food ingredients and nutraceuticals are promising areas.