Beta-carotene has consistently demonstrated antioxidant activity in vitro, although the mechanism of action is poorly understood. At low partial pressures of oxygen, such as those found in most tissues under physiological conditions, it exhibits good radical-trapping antioxidant behaviour, whereas this capacity is lost at high oxygen pressures in vitro with autocatalytic, pro-oxidant effects observed (Burton & Ingold 1984).

Beta-carotene has been shown to quench singlet oxygen, scavenge peroxyl radicals and inhibit lipid peroxidation in vitro; however, there is ongoing debate as to beta-carotene's ability to act as an antioxidant in vivo, with some evidence suggesting this varies from system to system for reasons that are poorly understood (Pryor et al 2000).

Beta-carotene acts synergistically with other antioxidants, such as vitamins E and C, or other dietary components as part of the antioxidant network (see Vitamin E monograph). A combination of beta-carotene and alpha-tocopherol has been shown to inhibit lipid peroxidation significantly more than the sum of the individual inhibitions in a membrane model (Palozza & Krinsky 1992) and a synergistic effect has also been demonstrated in vitro and in vivo with vitamins E and C (Bohm et al 1997, 1998).

In vivo studies A number of studies have demonstrated that beta-carotene has antioxidant activity in vivo. Supplementation with 180 mg of beta-carotene for 2 weeks was found to increase the beta-carotene content of LDL and significantly reduce plasma lipid peroxidation and LDL susceptibility to oxidation, as analysed by © 2007 Elsevier Australia

malondialdehyde generation (Levy et al 1996). Similarly, lipid peroxidation as measured by breath pentane output was found to be significantly reduced in healthy subjects by 4 weeks of daily supplementation with 120 mg of beta-carotene (Gottlieb et al 1993). In a case-controlled trial involving 20 patients with long-standing type 1 diabetes mellitus, as well as age- and sex-matched controls, supplementation with 60 mg/day of natural algae-derived beta-carotene for 3 weeks was found to significantly reduce malondialdehyde and lipid peroxide production and the increased susceptibility towards LDL oxidation seen in the diabetic subjects (Levy et al 2000). Beta-carotene supplementation was also found to significantly reduce serum lipid peroxidation in a dose-dependent manner in a number of double-blind, placebo-controlled trials (Greul et al 2002, Lee et al 2000).

These results contrast with those from a number of studies that failed to demonstrate any in vivo antioxidant activity. A study of 79 healthy volunteers found that normal concentrations of carotenoids in plasma and tissues did not correlate with total antioxidant capacity of the plasma or breath pentane measurements (Borel et al 1998). In other studies supplementation was seen to increase LDL beta-carotene without changing LDL susceptibility to oxidation (Princen et al 1992, Reaven et al 1993).

It is possible that beta-carotene is more likely to demonstrate antioxidant activity in conditions of increased oxidative stress. This is suggested by the results of a randomised, double-blind controlled trial involving 42 non-smokers and 28 smokers who received either 20 mg of beta-carotene or placebo and showed that beta-carotene reduced lipid peroxidation as indicated by breath pentane output in smokers but not in non-smokers (Allard et al 1994). It is further supported by a study of whole-body irradiation in rats that found that supplementation with natural algae-derived beta-carotene protected against the reduction in growth rate and the selective decline in 9-c/s beta-carotene and retinol seen in irradiated animals, as well as partially reversing the effect of irradiation when given after the irradiation (Ben-Amotz et al 1996). Algae-derived beta-carotene was also found to protect against CNS oxygen toxicity in rats, as indicated by a significant increase in the latent period preceding oxygen seizures in supplemented animals (Bitterman et al 1994). Isomer differences Individual isomers and isomer mixtures demonstrate different antioxidant properties in vivo. The 9-c/s isomer, which is present in greater amounts in natural beta-carotene, exhibits higher antioxidant potency than the a\\-trans isomer in vitro (Levin & Mokady 1994). Natural beta-carotene, such as that obtained from algal sources, also exhibits greater antioxidant activity than synthetic beta-carotene in vivo (Takenaka etal 1993).

In humans, supplementation with natural algal beta-carotene containing a 50:50 mix of all-fransand 9-c/s isomers has been shown to be a more effective lipophilic antioxidant than all-trans beta-carotene (Ben-Amotz & Levy 1996). Human supplementation with natural algal and synthetic beta-carotene has also been shown to produce similar reductions in LDL oxidation, despite the synthetic beta-carotene producing double the rise in LDL beta-carotene content (Levy et al 1995).

These studies are contrasted by in vitro studies that suggest that 9-c/s beta-carotene and all-trans beta-carotene have equal antioxidant activities (Liu et al 2000) and that synthetic beta-carotene is twice as effective as algal beta-carotene in inhibiting LDL lipid peroxidation following LDL incubation with copper ions (Lavy et al 1993).

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