- Vitamin A supplementation and plasma retinol levels: a randomized trial among women. Willett, W.C., Stampfer, M.J., Underwood, B.A., Sampson, L.A., Hennekens, C.H., Wallingford, J.C., Cooper, L., Hsieh, C.C., Speizer, F.E. J. Natl. Cancer Inst. (1984)