Intervention trials with supplements of beta-carotene have observed either no effect or a harmful effect on lung cancer risk, notes the international team in this month's Cancer Epidemiology Biomarkers & Prevention (vol 13, 40-48).
But because food composition databases for specific carotenoids have only become available recently, epidemiological evidence relating usual dietary levels of these carotenoids with lung cancer risk is limited.
The researchers analysed the association between lung cancer risk and intakes of specific carotenoids using data from seven cohort studies in North America and Europe. Carotenoid intakes were estimated from dietary questionnaires administered at baseline in each study.
During follow-up of between seven and 16 years across studies, 3,155 incident lung cancer cases were diagnosed among 399,765 participants.
Beta-carotene intake was not associated with lung cancer risk and nor were lutein/zeaxanthin and lycopene. These results did not change after adjustment for intakes of vitamin C, folate, other carotenoids and multivitamin use.
The associations were also similar among non-smokers and current smokers. A vitamin A precursor, beta-carotene has been found to increase both the risk of colorectal cancer and lung cancer in smokers, especially those who also drink alcohol, although supplement manufacturers claim that these trials used synthetic beta-carotene, and that the results are not relevant for natural beta-carotene.
In an additional finding, the researchers report that beta-cryptoxanthin reduced the risk of lung cancer by more than 30 per cent for those in the highest quintile of intake. "Although smoking is the strongest risk factor for lung cancer, greater intake of foods high in beta-cryptoxanthin, such as citrus fruit, may modestly lower the risk," conclude the authors.
The carotenoid has previously been shown to protect against rheumatoid arthritis.