Vitamin A and lung cancer: a perspective.

Sixteen studies of the relationship between lung cancer and vitamin A in humans are critically reviewed: eight dietary studies and eight serum studies. Of the eight dietary studies, only five had case samples large enough to warrant meaningful statistical analysis. Three dietary studies reported a negative association between vitamin A intake and lung cancer after proper adjustment for cigarette smoking and socioeconomic effects; the results suggested that daily consumption of green-yellow vegetables reduces the risk of lung cancer in all categories of smokers. The reduction in risk was found to be greatest in persons in the higher socioeconomic strata. Four dietary studies suggested that high intake of dietary vitamin A has a protective effect against the development of squamous and small cell carcinoma of the lung in smokers. Three studies revealed a lower risk of lung cancer among persons who consume carrots and milk daily. None of the eight serum studies reviewed had large enough samples of lung cancer cases to allow for meaningful statistical analysis. However, four of the serum studies reported significantly lower serum vitamin A levels in lung cancer cases than in controls. Analysis of the above studies demonstrated shortcomings in each. Future studies should include all of the following factors: more complete vitamin A indices, examination of the histological type of lung cancer, a large enough pool of both male and female patients for statistical analysis, and proper adjustment for the effects of age, sex, smoking, and socioeconomic status.