Conclusions

Basic laboratory research findings strongly suggest that oxidative stress may play an important role in the development of atherosclerosis. Basic and animal studies suggest that antioxidant vitamins may delay or prevent various steps in the pathophysi-ologic process. Several observational studies have demonstrated an association between antioxidant intake either from foods or supplements and subsequent risk of CVD. However, neither basic research nor observational research can provide conclusive evidence. Because of these results and an increasing use of antioxidant supplements despite lack of documented benefit, many large-scale trials of antioxidant supplements have been completed and others are ongoing to test further the efficacy both of single supplements and combinations in varied populations.

Clinical trials of vitamin E alone for primary prevention of CVD have not generally supported the observational results, but the largest trials may have used sub-therapeutic doses (ATBC and the Chinese Cancer Prevention Trial) or had inadequate follow-up time (PPP). Secondary prevention trials of vitamin E supplementation have shown minimal or no benefit. One of the first trials (CHAOS) found benefits for vitamin E, but subsequent large trials have not confirmed those results. Patients with high oxidative stress (e.g., hemodialysis patients in the SPACE trial) may benefit more from vitamin E supplementation. Both the CHAOS and SPACE trials demonstrated a risk reduction after less than 2 years of vitamin E supplementation while longer and larger trials (GISSI, HOPE, and HPS) found no benefit. Subsequent larger studies with longer follow-up have not demonstrated an overall CVD benefit to vitamin E supplementation (WHS, WACS), and the PHS II is scheduled to be completed in 2007. There have not been any large randomized trials evaluating antioxidants consumed in natural food sources.

Primary prevention trials of beta-carotene in well-nourished populations have demonstrated no reduction in CVD or cancer (ATBC, Skin Cancer Prevention Study, CARET, PHS, WHS), and some studies have raised the possibility of harm (ATBC, CARET). The few secondary prevention trials have also failed to show any benefit of beta-carotene supplementation. A meta-analysis of major beta-carotene trials found a slight increase in both total and CVD mortality. At this time beta-carotene supplementation cannot be routinely recommended for either the primary or secondary prevention of CVD.

The only completed large trial of vitamin C in primary prevention (Chinese Cancer Prevention Trial) found no effect on cerebrovascular mortality but did not have adequate power to analyze CVD outcomes. In a secondary prevention trial (WACS), vitamin C did not have any beneficial effect on CVD. A large randomized trial of vitamin C in a well-nourished population is scheduled to end in 2007 (PHS II).

Antioxidants may be most effective when taken in particular combinations. A few trials of combinations have shown a CVD benefit (Chinese Cancer Prevention Trial, ASAP), while others show no benefit (SU.VI.MAX) or raise the question of increased risk (CARET, HATS, WAVE). Two large-scale secondary prevention trials have demonstrated no beneficial effect on total CVD from antioxidant combinations (HPS,

WACS), but one these trials found a reduction in stroke with the combination of vitamin E and vitamin C (WACS). One large ongoing primary prevention trial was specifically designed to test the effect of antioxidant supplements both alone and in various combinations in order to identify potential therapeutic interactions (PHS II).

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