http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4293152/
"Total vitamin D intake ≥400 versus <400 IU/d was associated with a lower risk of total lung cancer among participants who received the CARET active intervention (HR=0.56, 95% CI=0.32–0.99) and among those who had total vitamin A intake ≥1,500 μg/d Retinol Activity Equivalent (RAE; HR=0.46, 95% CI=0.23–0.91). The beneficial associations were attenuated among those who did not receive the CARET active intervention or who had total vitamin A intake <1,500 μg/d RAE (P-interaction=0.02 for current smokers). Our observation suggests that vitamin A may assist vitamin D in preventing lung cancer among smokers."
Cheng T-YD, Goodman GE, Thornquist MD, et al. Estimated intake of vitamin D and its interaction with vitamin A on lung cancer risk among smokers.International journal of cancer Journal international du cancer. 2014;135(9):2135-2145. doi:10.1002/ijc.28846.
