Antioxidants Protect Against Disease

Many defense mechanisms within the organism have evolved to limit the levels of reactive oxidants and the damage they inflict. Among the defenses are enzymes such as superoxide dismutase, catalase, and glutathione peroxidase. The glutathione S-transferases inactivate reactive electrophilic mutagens, including the aldehyde products of lipid peroxidation.

There are also many structural defenses such as sequestering H202 generating enzymes in peroxisomes and chelating any free iron or copper salts in transferrin and ferritin or ceruloplasmin to avoid Fenton chemistry. Superoxide, however, can release iron from ferritin.

Oxidized DNA is repaired by a series of glycosylases that are specific for particular oxidized bases and possibly by non-specific excision repair enzymes. In the absence of cell division these oxidative lesions are removed from DNA quite effectively and the mutation rate is kept to a minimum. Oxidized proteins are degraded by proteases. Lipid hydroperoxides are destroyed by glutathione peroxidase.

Almost all of these defenses appear to be inducible, as are most other types of defenses, i.e., the amounts increase in response to damage. There is a large literature showing that cells respond to low levels of radiation, an oxidative mutagen, by inducing antioxidant defenses that help to protect them against mutation by high levels of radiation.

There is a tradeoff however, since the induction of these defenses makes the cell more sensitive to alkylating mutagens.

In addition to the protective effects of endogenous enzymatic antioxidant defenses, consumption of dietary antioxidants appears to be of great importance. Fruits and vegetables, the main source of antioxidants in the diet, are associated with a lowered risk of degenerative diseases. Block and her colleagues have recently reviewed 172 studies in the epidemiological literature that relate, with great consistency, the lack of adequate consumption of fruits and vegetables to cancer incidence.

The quarter of the population with low dietary intake of fruits and vegetables compared to the quarter with high intake has double the cancer rate for most types of cancer (lung, larynx, oral cavity, esophagus, stomach, colon and rectum, bladder, pancreas, cervix, and ovary). Data on the types of cancer known to be associated with hormone levels are not as consistent and show less protection by fruits and vegetables: for breast cancer the protective effect was about 30%. There is also literature on the protective effect of fruit and vegetable consumption on heart disease and stroke. Only 9% of Americans eat five servings of fruits and vegetables per day, the intake recommended by the National Cancer Insitute and the National Research Council. European countries with low fruit and vegetable intake (e.g., Scotland) are generally in poorer health and have higher rates of heart disease and cancer than countries with high intake (e.g., Greece).

The cost of fruits and vegetables is an important factor in discouraging consumption. Poorer people spend a higher percentage of their income on food, eat less fruits and vegetables, and have shorter life expectancy than wealthier people. A major contributor to health in this century was synthetic pesticides which markedly decreased the cost of food production and ensured that most of the crops planted would be eaten by humans rather than insects. Synthetic pesticide residues do not appear to be a significant cause of cancer.

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