Cellular reactions within human tissues that involve oxygen may bring about the formation of highly reactive and unstable compounds referred to as free radicals. They characteristically feature a number of unpaired electrons which makes the atom or group of atoms interact with surrounding molecules in extremely short time periods of 10-9 to 10-12 seconds, creating a chain reaction that sets off the formation of new free radicals. Studies show that free radical interactions within the cell nucleus may damage DNA, leading to mutagenesis and cancer, if DNA repair systems are unable to cope. DNA damage may also result from reaction with dialdehydes, formed by the free radical peroxidation of unsaturated fatty-acids within cell membranes. It has also been shown that free radicals may interact with amino acids in proteins, leading to a modification of the protein molecule, which may trigger autoimmune responses that attack normal, unmodified protein. Free radicals may also react with unsaturated fatty-acids or amino-acids in plasma lipoproteins, making them unrecognisable to liver LDL receptors. Consequently, they are taken up by white blood cells that become lipid enriched, eventually settling in the walls of blood vessels and thus contributing to the formation of atherosclerosis.
Antioxidants are compounds that can react with free radicals to stop harmful chain reactions from spreading before they can create significant damage. Table 1 shows the major antioxidants that can be derived from dietary sources.
The more the better?
Some studies investigating the effects of elevated blood serum levels of anti-oxidants have indeed shown a positive correlation between blood serum antioxidant levels and decreased incidences of heart disease and cancer. Vitamin C has been shown to positively contribute towards reducing heart disease and cancer risks (Flagg et al, 1995) and Gey et al,1991 showed that elevated levels of plasma vitamin E also reduced the chances of suffering from heart disease. However, these elevated plasma levels were most likely achieved by eating foods rich in Vitamin C and E rather than through supplementation.
However, several other studies testing the effectiveness of supplementing with vitamin C, E, ß-carotene and selenium in order to reduce the risk of contracting cancer or heart disease have been disappointing, if not downright negative.
Two major studies were actually interrupted prematurely due to the negative results that were being observed. The SELECT trial (Selenium and vitamin E Cancer prevention Trial), a study conducted to investigate the effects of selenium and vitamin E supplementation on prostate cancer occurrence had to be stopped early due to increased levels of prostate cancer observed in the supplementation group compared to the placebo group.
In the α-Tocopherol ß-Carotene Cancer Pevention Study (ATBC trial) it was actually shown that ß-Carotene, a form of vitamin A actually increased the incidence of lung cancer in smokers.
When it comes to vitamin intake, more is not necessarily better. Once adequate levels in the body have been achieved, a surplus of vitamins does not seem to have any additional benefit. Water-soluble antioxidants like vitamins C are excreted once saturation levels in the body are reached (at about 100-150mg/day) and fat-soluble antioxidants like vitamin A and E can become toxic if the intake is too high.
The results of these studies indicate that the best prevention for disease is still a diet high in fruit and vegetables and low in saturated fats. It seems that the combination of micronutrients and phytonutrients, rather than isolated vitamins themselves are responsible for the health promoting effects.
So next time you think about purchasing vitamin supplements, make sure that first of all you are getting your 5-8 servings of fruits and vegetable per day. You may just be prolonging your own life!