Dietary Antioxidants and Human Cancer

Abstract

Reactive oxygen species (ROS) are by-products of metabolism that increase in the body during inflammation, smoking, and exposure to agents such as radiation and certain drugs. ROS cause damage to DNA and other molecules, and they are implicated in the development and progression of cancer. Antioxidants from endogenous and dietary sources can neutralize and destroy ROS and may play a role in cancer prevention. Experimental and animal studies have shown that treatment with single or combined antioxidants prevents neoplastic transformation of normal cells and inhibits tumor growth. Human observational studies suggest that high intake of antioxidant-rich foods, notably fruits, vegetables, and grains, is inversely related to cancer risk. However, prospective human studies that began to accrue data in the 1990s did not always confirm this relationship. Randomized, controlled clinical trials using specific antioxidant supplementation have also produced mixed results, but study limitations and confounding factors often make it difficult to derive definitive conclusions. Antioxidant supplementation during cancer treatment remains controversial. Radiotherapy and certain chemotherapeutic agents rely on ROS to destroy cancer cells. Although antioxidant supplementation may help protect normal cells from ROS damage and may have palliative effects during cancer treatment, studies suggest that cancer cells may also be protected from ROS damage, thereby reducing treatment efficacy and patient survival. This article reviews evidence for the impact of antioxidant supplementation and antioxidant-rich diets on cancer risk and mortality. It also outlines some of the factors that may have contributed to the conflicting outcomes reported.