Ch19

In addition to triacylglycerols, a variety of components are found in dietary fats which are important in maintaining health. These non-glyceride constituents of fats may explain some of the inconsistencies in epidemiologic and experimental studies.

Fat-soluble vitamins

Vitamin A and vitamin D occur prominently in butterfat and fish oils. Red palm oil is a potent source of B-carotene, a provitamin A. In many developing countries, especially in West Africa, crude palm oil is an important source of ß-carotene, providing much of the vitamin A which is required by the populations.

The processing of edible oils, however, often results in the total removal of the carotenoids present in the crude oil. For example, crude palm oil, a rich source of carotenoids (500-700 ppm) may lose all of its carotenoids in the refining process.

However, it is possible to use mild techniques for processing crude palm oil that result in the retention of a major portion of the carotenoids while removing the undesirable free fatty acids and peroxides of the crude oil. The resulting red palm oil, with its high carotenoid content, could become an important dietary component in the battle against vitamin A deficiency in many developing countries and its use should be encouraged.

Many vegetable oils and the products made from them, contain appreciable concentrations of vitamin E (tocopherols) which may be reduced by some processing methods as well.

Ubiquinones

It is not known if ubiquinone Q₉ is biologically active in humans, but ubiquinone Q₁₀ is active as a mitochondrial electron carrier. Ubiquinone₁₀, together with a -tocopherol, appears to protect low density lipoproteins from oxidation (Tribble et al., 1994).

Antioxidants

Substances other than vitamin E act as antioxidants, but tocopherol is the principal fat-soluble antioxidant in the body and is found in lipoproteins, especially LDL. It is found within membranes both inside and outside cells, enhancing the cell's protection against free radical attack. Vitamin E enhances immune function (Meydani et al., 1990) and can play a role in the repair of damaged membranes (Newmark and Mergens, 1981; Bright-See and Newmark, 1983).

It has been hypothesized that dietary antioxidants protect against a variety of age-related conditions, including cardiovascular disease and cancer. The role of tocopherol and other antioxidants in protecting unsaturated fatty acids from oxidation gained attention with the recognition that oxidized LDL could be involved in the atherosclerotic process (Jurgens et al., 1987). Oxidatively modified LDL occurs in the atherosclerotic lesion and appears to be the link between plasma LDL and the development of early lesions (Yla-Herttuala et al., 1989). Further evidence that lipid oxidation could be a factor in atherosclerosis came from the finding that the susceptibility of LDL to oxidation was correlated with the severity of atherosclerosis in young male survivors of myocardial infarction (Regnstrom et al., 1992).

Antioxidant supplementation of men with low antioxidant status and a high intake of fat lessened the capacity of platelets to aggregate and to produce thromboxane A₂ (Salonen et al., 1991). In line with this finding, stored serum samples of 16 European groups provided evidence of an inverse relationship between plasma a -tocopherol levels and mortality rates from coronary heart disease (Gey et al., 1991). When a cohort of male volunteers received supplementation with cx-tocopherol, compared to a placebo, during a 3 month period, this led to enhanced plasma and LDL a -tocopherol levels and decreased susceptibility of LDL to oxidation (Jialal and Grundy, 1992). In two large prospective studies, one among women (Stampfer et al., 1993) and another among men (Rimm et al., 1993), intake of vitamin E, primarily in the form of supplements, was associated with a substantially reduced risk of myocardial infarction. Among men, intake of dietary carotenoids was associated with lower risk as well, especially among individuals who had smoked cigarettes prior to or during the study. Current evidence suggests that antioxidants play an important role in the prevention of coronary heart disease, but further scientific support is required.

Fat-soluble antioxidants, particularly carotenoids and tocopherol, have been hypothesized to reduce the risk of various cancers. The best evidence indicates a relationship between carotenoid intake and risk of epithelial malignancies, particularly lung cancer (Steinmetz and Potter, 1991). Relatively few data exist relating vitamin E intake to the risk of cancer, in part because of the difficulty of assessing intakes of this nutrient. Nevertheless, blood levels of vitamin E (Knekt et al., 1988) and vitamin E supplements (Gridley et al., 1992) have been positively related to lower risk of several cancers. Also, vitamin A, both preformed and from a carotenoid precursor, has been found to be inversely related to the risk of breast cancer (Hunter et al., 1993).

There is evidence from animal and epidemiological studies that [l-carotene and possibly a -carotene, may have anti-cancer properties. Epidemiological studies have consistently shown associations between high intakes of [l-carotene-enriched foods with reduced risk of certain cancers (Connett et al., 1989; Le Marchand et al., 1989; Peto et al., 1981).

Tocotrienols

Apart from their vitamin E activity, tocotrienols exhibit certain physiological properties not observed with the tocopherols. When administered in the diet of animals and humans, tocotrienol concentrates have been reported to manifest a hypocholesterolemic effect (Burger et al., 1984; Qureshi et al., 1991 a, b; Tan et al., 1991). It has been suggested that the cholesterol-lowering potential of the tocotrienols may be mediated by their ability to decrease levels of hepatic HMG-CoA reductase activity (Qureshi et al., 1986). In addition, tocotrienols have been shown to influence certain hemostatic parameters (Qureshi et al., 1991 a) and to reduce the occurrence of chemical-induced tumours in rats (Tan and Chu, 1991; Gould et al., 1991).

Phytosterols

Plant sterols are not well-absorbed by humans and may inhibit cholesterol and bile acid absorption. They can have appreciable effects on LDL cholesterol levels, even at relatively low intakes (Grundy and Mok, 1977; Lees et al., 1977; Heinemann, Leiss and von Bergmann, 1986). While the principal mechanism of action of phytosterols has not been established, they can influence micellar cholesterol solubilization (Child and Kuksis, 1986) as well as the rate of cholesterol synthesis and degradation (Bober, Akerlund and Bjorkhem, 1989, Ikeda and Sugano, 1983; Heinemann, Leiss and von Bergmann, 1986; Heinemann et al., 1991).

A group of ferulic acid esters of triterpene alcohols and plant sterols has shown hypocholesterolemic effects as well, perhaps by inhibiting cholesterol absorption and enhancing sterol and bile acid excretion (Nicolosi, Ausman and Hegsted, 1990, 1991 a, b). Oryzanol, a ferulic acid ester, constitutes as much as 20 percent of the non-saponifiable fraction of crude rice bran oil (Rogers et al., 1993). In addition, ferulic acid is a potent antioxidant which stabilizes vegetable oils.

Conclusions

Current data on antioxidants, specifically tocopherol, suggest a protective effect against coronary heart disease in humans. However, these data do not yet warrant specific recommendations for their intake. The hypothesis that oxidized LDL is an important atherogenic factor is generally regarded as attractive. Although oxidized LDL has been detected in atherosclerotic plaque, solid evidence that lipoprotein oxidation in humans is causally related to atherosclerosis has not been reported yet.

A number of studies seem to support the hypothesis that antioxidants can prevent oxidative modification of LDL. However, before these studies can be extrapolated to atherosclerosis, several points should be considered: first, demonstrating a protective effect in humans is difficult because the available techniques are limited in their ability to measure ongoing lipid peroxidations; second, the implications of the measurement of LDL oxidation in vitro for the in vivo situation are not clear; and, third, the efficacy of the individual antioxidants in the prevention of atherosclerosis has yet to be established in large intervention studies.

Foods high in polyunsaturates should contain at least 0.6 mg tocopherol equivalents per gram of polyunsaturated fatty acids. Higher levels may be necessary for fats rich in fatty acids containing more than two double bonds.

The introduction of low-fat spreads and dressings reduces vitamin E intake. In light of the emerging evidence on the importance of vitamin E, it may be prudent to have the same amount of vitamin E per gram of product in such low-fat products as occur in corresponding high-fat products.

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