Free radicals are chemical species characterized by an odd number of orbital electrons or by pairs of electrons of similar directional spin isolated singly in separate orbitals. Consequently most of these agents are highly reactive and usually exhibit an extremely short half‐life, although due to steric and resonance effects some exceptions occur. Some radicals and their precursors, such as the diradical O2 which exists in the triplet state, represent a critical and essential element of normal metabolism of aerobic organisms where, under normal circumstances, controlled reduction of reactive oxygen species occurs via the cytochrome oxidase or cytochrome P‐450 mixed function monooxygenase systems. In addition to reactive oxygen species, organisms may be subjected to a wide‐range of other free radicals or their precursors, including those of both exogenous and endogenous origin. Elaborate defense mechanisms have evolved to avoid cellular damage from these highly reactive species. Enzymes, such as the superoxide dismutase, the glutathione peroxidase/reductase system, and catalase; interactions with conjugated diene systems such as those found in melanins, carotenoids, and tocopherols; and direct reduction by sulphydryl compounds, phenols, and purines represent but a few of these natural defense systems. Despite a strong rationale for considering free radicals as pathologic agents, progress in implicating these agents, or their reactions, in pathologic processes has been arduous. The fore‐most hurdle to providing definitive evidence for free radical involvement rests with the highly transient nature of these species, hardly reaching measurable levels in vivo and thereby making rigorous testing of the hypothesis extremely difficult. Indeed, free radical damage has been studied, for the most part, by indirect means–usually by measurement of known free radical reaction intermediates and products from which free radical involvement is implied. Nevertheless, free radical formation has been shown to occur in UV‐irradiated skin and a considerable body of circumstantial evidence has been amassed that strongly infers that these agents, or reactions initiated by them, are responsible for at least some of the deleterious effects of UV upon skin.

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In patients with a history of nonmelanoma skin cancer, a low-fat diet reduces the incidence of actinic keratosis.

Carotenoid pigments, particularly β-carotene and lycopene, are consumed in human foodstuffs and play a vital role in maintaining health. β-carotene is known to quench singlet oxygen and can have strong antioxidant activity. As such, it was proposed that β-carotene might reduce the risk of cancer. Epidemiological studies found inverse relationships between cancer risk and β-carotene intake or blood levels. However, clinical trials failed to support those findings and β-carotene supplementation actually increased lung cancer incidence in male smokers. Early experimental animal studies found dietary β-carotene inhibited UV-induced skin cancers. Later studies found that β-carotene supplementation exacerbated UV-carcinogenic expression. The discrepancies of these results were related to the type of diet the animals consumed. Lycopene has been associated with reduced risk of lethal stage prostate cancer. Other carotenoids, e.g., lutein and zeaxanthin, play a vital role in visual health. Numerous studies of molecular mechanisms to explain the carotenoids’ mode of action have centered on singlet oxygen, as well as radical reactions. In cellular systems, singlet oxygen quenching by carotenoids has been reported but is more complex than in organic solvents. In dietary β-carotene supplement studies, damaging pro-oxidant reactivity can also arise. Reasons for this switch are likely due to the properties of the carotenoid radicals themselves. Understanding singlet oxygen reactions and the anti-/pro-oxidant roles of carotenoids are of importance to photosynthesis, vision and cancer.

The effect of a low-fat diet on occurrence of non-melanoma skin cancer was examined in a 2-year dietary intervention trial. A total of 101 skin-cancer patients were randomized either to a control group that consumed, on average, 38% of caloric intake as fat, and in which no changes in dietary habits were introduced, or to a low-fat dietary-intervention group, in which patients were instructed to limit their calories from fat to 20% of total caloric intake. Patients were examined at 4-month intervals by dermatologists blinded to their dietary assignments. Nutrient analyses, conducted at each of the 4-month follow-up visits, indicated that the % calories of fat consumed in the intervention group had been reduced to 21% at 4 months and remained below this level throughout the 2-year period. There were no significant differences in total calories consumed, or in mean body weights, between the control and the intervention groups. Nor were there significant group differences in P/S ratios until month 24. Numbers of new skin cancers treated at each examination were analyzed in 8-month periods of the 2-year study. Comparisons of skin-cancer occurrences revealed no significant changes in the control group from baseline values. However, cancer occurrence in the low-fat intervention group declined after the first 8-month period and reached statistical significance by the last 8-month period. Patients in this group had significantly fewer cancers in the last 8-month period than did patients in the control group. In addition, there was a significant reduction in the number of patients developing skin cancer in the last 8-month period, as compared with the first 8-month period, within the low-fat intervention group. There were no significant changes in the control group. These data indicate that a low-fat diet can significantly reduce occurrence of a highly prevalent form of cancer.