Mutagen

In addition to mutagen-induced DNA changes, spontaneous mutations occur in the dividing cells of the human body every day. The nuclei of the cells have repair enzymes, which remove mutations and restore mutated DNA to its original form. If these natural DNA repair mechanisms fail to keep up with the rate of mutation or the repair mechanisms themselves are defective, disease can result. This latter case is seen in lung cancer due to cigarette smoking, where the nicotine in the smoke is thought to block an important repair process in the lungs.

Aging appears to be due, in good part, to mutagenic oxidants produced as by-products of normal metabolism. These oxidants, such as hydrogen peroxide, are the same mutagens produced by radiation, and cause damage to DNA, proteins, and lipids. The DNA in each cell of a normal rat receives on average about l00,000 oxidative lesions (altered bases) per day. DNA-repair enzymes constantly remove this damage, but they do not keep up: an old rat has over one million oxidative lesions in the DNA of each cell. A human cell receives about ten times fewer lesions than a rat cell, which is consistent with the higher cancer rate and shorter life span of a rat. Decay of mitochondria with time, due to oxidative damage, appears to play a major role in aging. The degenerative diseases of aging, such as cancer, cardiovascular disease, cataracts, and brain dysfunction, are increasingly found to have, in good part, an oxidative origin.

It has been argued that dietary antioxidants, such as vitamins C and E and carotenoids, play a major role in minimizing this damage; however, most of the world's population is receiving inadequate amounts of them, at a great cost to health. The main source of dietary antioxidants is fruits and vegetables. Insufficient fruit and vegetable consumption increases the rate of most types of cancer about two fold. A better diet is thought to be a major contributor to the ever increasing life expectancy in the United States and the decreasing (age-adjusted) cancer death rates in non-smokers.