Cruciferous vegetables are a rich source of
glucosinolates and their hydrolysis products, including
indoles and
isothiocyanates, and high intake of cruciferous vegetables has been associated with lower risk of lung and
colorectal cancer in some epidemiological studies.
Glucosinolate hydrolysis products alter the metabolism or activity of
sex hormones in ways that could inhibit the development of
hormone-sensitive
cancers, but evidence of an inverse association between cruciferous vegetable intake and breast or
prostate cancer in humans is limited and inconsistent. Organizations such as the National Cancer Institute recommend the consumption of five to nine servings of fruits and vegetables daily, but separate recommendations for cruciferous vegetables have not been established.
Isothiocyanates and
indoles derived from the hydrolysis of
glucosinolates, such as
sulforaphane and
indole-3-carbinol (I3C), have been implicated in a variety of anticarcinogenic mechanisms, but deleterious effects also have been reported in some experimental protocols, including
tumor promotion over prolonged periods of exposure. Epidemiological studies indicate that human exposure to
isothiocyanates and
indoles through cruciferous vegetable consumption may decrease
cancer risk, but the protective effects may be influenced by individual genetic variation (polymorphisms) in the metabolism and elimination of
isothiocyanates from the body. Cooking procedures also affect the bioavailability and intake of
glucosinolates and their derivatives. Supplementation with I3C or the related dimer
3,3'-diindolylmethane (DIM) alters urinary
estrogen metabolite profiles in women, but the effects of I3C and DIM on
breast cancer risk are not known. Small preliminary trials in humans suggest that I3C supplementation may be beneficial in treating conditions related to human papilloma virus
infection, such as
cervical intraepithelial neoplasia and
recurrent respiratory papillomatosis, but larger randomized controlled trials are needed.