There are a number of human studies that suggest that supplementation with beta-carotene can reduce the oxidative stress associated with different pathological conditions or stressors such as intense exercise or radiation. Supplementation with beta-carotene (30 mg/day) and vitamin E (500 mg/day) for 90 days has been found to enhance the antioxidant enzyme activity of superoxide dismutase and catalase in the neutrophils of sportsmen (Tauler et al 2002). Similarly, in a study of 13 professional basketball players, 35 days of antioxidant supplementation with 600 mg alpha-tocopherol, 1000 mg ascorbic acid and 32 mg beta-carotene led to a significant decrease in plasma lipid peroxides, with a significant decrease of lactate dehydrogenase serum activity and a non-significant increase in the anabolic/catabolic balance being observed during a 24-hour recuperation time after exercise (Schroder et al 2001).
The results of an Israeli study of 709 children exposed to radiation from the Chernobyl accident suggest that natural algae-derived beta-carotene may act as an in vivo lipophilic antioxidant or radioprotector. This study found that exposed children had increased susceptibility to lipid oxidation and that supplementation with 40 mg of natural 9-c/sand a\\-trans equal isomer mixture beta-carotene twice daily for a period of 3 months reduced plasma markers of lipid oxidation (Ben-Amotz et al 1998).
A double-blind study has also found that beta-carotene supplementation reduced the severity, but not the incidence, of post-endoscopy pancreatitis which is thought to be mediated by oxidative stress (Lavy et al 2004). A further small, controlled trial involving 1 5 patients with RA found that 3 weeks of supplementation with natural beta-carotene resulted in a significant increase in plasma antioxidants, but did not change indicators of disease (Kacsur et al 2002).
Further evidence for beta-carotene's ability to reduce oxidative stress comes from a study of conditioning therapy preceding bone marrow transplantation. This therapy consists of high-dose chemotherapy and total body irradiation and has acute and delayed toxic effects that are considered to be due to peroxidation processes and exhaustion of antioxidants. Supplementation with 45 mg beta-carotene, 825 mg alpha-tocopherol and 450 mg ascorbic acid daily for 3 weeks, however, was found to © 2007 Elsevier Australia
increase serum antioxidant levels and reduce the post-conditioning rise in plasma lipid hydroperoxides in patients receiving the conditioning therapy prior to transplantation (Clemens et al 1997). Similarly, a combined antioxidant supplement increased plasma antioxidant levels and antioxidative enzyme activities, and lowered LDL lipid peroxides in male hyperlipidaemic smokers, although higher doses of the supplement did not have an additive effect (Chao et al 2002).
These studies are contrasted by two small studies that showed no change in oxidative stress in healthy subjects after beta-carotene supplementation. Normal concentrations of carotenoids in plasma and tissues were not correlated with clinical markers of antioxidant and oxidative stress in a study of 79 healthy volunteers (Borel et al 1998), and a placebo-controlled, single-blind study found that daily supplementation with 1 5 mg of beta-carotene for 3 months did not significantly improve biomarkers of oxidative stress in healthy males (Hininger et al 2001).
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