Carotenoids

The carotenoids are a family of bright yellow-, orange- and red-coloured compounds found in fruit, vegetables and some animal products such as salmon, lobster and egg yolk. Carotenoids can be divided into the provitamin A group, such as beta-carotene, and xanthophylls such as lutein, zeaxanthin and lycopene, which are important fat-soluble antioxidants. Of the 600 or so carotenoids known to exist in nature, approximately 20 are found in humans. In plants, carotenoids play a vital role in photosynthesis and participate in the energy-transfer process, as well as protecting plants from oxidative damage. The red, orange and yellow colour of these compounds is because they preferentially absorb blue light, which is the most energetic and hence the most biologically damaging part of the visible spectrum.

In animals, carotenoids have many functions. In addition to providing direct photoprotection via absorption of blue light, carotenoids act as powerful fat-soluble antioxidants linked to oxidation prevention, as well as playing a role in cellular communication, including stimulation of gap-junction communication, which is important for cancer prevention by regulating cell growth, differentiation, apoptosis and angiogenesis. Carotenoids may also be involved in detoxification of carcinogens, DNA repair and immunosurveillance. These properties are believed to contribute to their antioxidant, immune-enhancing, anticarcinogenic and photoprotective activity.

Beta-carotene was the first of the carotenoids to be discovered, being initially isolated from carrots. The bioavailability of beta-carotene is dependent on its source, with the amount being absorbed from raw foods such as carrots, where it forms part of a protein-polysaccharide matrix, being only about 20% of that absorbed from supplemental forms. Although beta-carotene is lipid soluble its absorption requires only a limited amount of fat (Roodenburg et al 2000); however, there is a wide individual variation in serum response to beta-carotene administration (Bowen etal 1993, Pryoretal 2000).

Although it has been suggested that different carotenoids compete for absorption, this was not confirmed by a postprandial study (Tyssandier et al 2002). Beta-carotene is absorbed in the intestine and released into the lymphatic circulation within chylomicrons. It is then taken up by hepatocytes and released into the blood Beta-carotene 78

and transported predominantly within LDLs. It is distributed to adipose tissue and the

skin and excreted in the faeces (Micromedex 2003). The time to reach peak concentration is up to 4-6 weeks with oral dosing (Mathews-Roth 1990a).

Animal feeding studies suggest that a natural algae-derived beta-carotene isomer mixture is more readily absorbed than synthetic a\\-trans beta-carotene and that this higher bioavailability can be enhanced by increasing dietary lipid levels (Mokady & Ben-Amotz 1991). Natural algal beta-carotene has also been shown to have higher accumulation in rat liver than synthetic a\\-trans beta-carotene (Ben-Amotz et al 1989, Takenaka et al 1993) with at least a 10-fold higher accumulation having been observed in chick and rat liver (Ben-Amotz et al 1989).

Animal studies suggest that there is some bioconversion within the body between different stereoisomers of beta-carotene (Ben-Amotz et al 2005) and further studies in humans suggest that, regardless of the isomer mix, there is preferential absorption or transport of the all-trans isomer in comparison with the 9-c/s isomer, with plasma levels of the all-trans isomer being around 10-fold that of the 9-c/s form (Gaziano et al 1995a, Jensen 1987, Morinobu et al 1994, Stahl & Sies 1993, Tamai et al 1993).

It is suggested that Helicobacter pylori infection may impair the protective role of alpha-tocopherol and beta-carotene in the stomach, because infected people have been found to have reduced beta-carotene concentrations in gastric juice and the presence of gastric atrophy and intestinal metaplasia is associated with reduced mucosal alpha-tocopherol and beta-carotene concentrations (Zhang et al 2000).

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