Studies show that 25%-84% of cancer patients use nutritional supplements containing antioxidants often at doses higher than the recommended dietary allowances during or after cancer treatment to enhance the benefits of treatment, to alleviate side effects and to maintain general well-being.
The role of antioxidants in cancer therapy has become a point of contention among oncologists in recent years. The controversy behind this topic rests on the observation that both chemotherapy and radiation involve the generation of free radicals that contribute to their cytotoxic effects. Some practitioners have therefore assumed that use of antioxidants, which quenches the activity of free radicals would counteract the effects of these therapies.
Although many chemotherapy drugs do induce the formation of free radicals, their anticancer effects do not, in general, seem to depend on the formation of these free radicals. As a result, antioxidant supplementation may in some circumstances help prevent free-radical-induced side effects without inhibiting the positive effects of the chemotherapy.
A study by Keith Block MD, showing a reduction in the toxic side effects of ROS-generating chemotherapies with concurrent antioxidant supplementation showed improved survival rates and tumour response by helping patients complete their prescribed treatment cycles.
In 18 out of the 30 studies evaluated, patients who received oral or intravenous antioxidant supplements experienced significantly lower toxicity than the control groups. Glutathione, melatonin and vitamin E showed the most consistent and promising effects. The only study that reported significantly greater toxicity in the antioxidant group than the control group utilized vitamin A.
This is of no surprise due to the well documented toxicities of high-dose vitamin A. In addition, of the 19 studies in the review that reported tumour response and/or survival rates, all but one of the antioxidant supplemented groups experienced the same or better response than the control group.
Oxidative Stress and chemotherapeutic effectiveness
Free radicals and other reactive oxygen species (ROS) are essential for life. ROS are produced in all respiring organisms in the process of ATP generation. Non-essential production of ROS i.e. oxidative stress, can also be induced by drugs and environmental toxins.
Excessive oxidative stress i.e. generation of greater than normal amounts of ROS, interferes with the cytotoxic effects of antineoplastic agents on cancer cells through its influence on the length of the cell cycle. Excessive ROS results in lipid peroxidation. The rate of DNA synthesis and the rate of proliferation of cancer cells and normal cells are inversely related to the degree of lipid peroxidation; oxidative stress prolongs the G1 phase or may result in cells entering the G0 phase.
Tumour cells in the non-proliferative G0 state are little affected by anticancer drugs and are able to re-enter the division cycle after chemotherapy is completed, resulting in recurrence of disease. Anticancer drugs are cytotoxic only when tumour cells are proliferating rapidly, this provides a mechanism whereby oxidative stress, which slows or arrests cell growth, interferes with chemotherapeutic effectiveness.
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