What are cruciferous vegetables?
These are also called the Brassicaceae family and include broccoli, cabbage, cauliflower, Chinese cabbage, common radish, horseradish, kohlrabi, turnip and many others. These vegetables contain several substances that have been chemically identified (e.g. isothiocyanates and indole-3-carbinol) and extensively studied. These so called phytochemicals have been repeatedly proven protective against different phases of cancer development in various laboratory tests.
Are these “natural chemicals” really helpful in humans?
Several large clinical studies have been performed, consistently showing beneficial effect of consuming these vegetables on both cancer prevention and cancer treatment.
The most recent one was the Shanghai Breast Cancer Survival Study: it was performed in a group of women diagnosed with breast cancer in different stages of the disease and the results of this study show that the more cruciferous vegetables you eat the better chance of survival and the lower risk of recurrence of the disease you have. It is important to say that these women consumed the vegetables in addition to a standard cancer treatment (i.e. this study does not postulate that if you just eat crucifers you will cure your cancer!).
An older study (from 2004) has shown that women of reproductive age have lower risk of getting breast cancer if they eat cruciferous vegetables (the more they eat, the lower the risk).
The good news is that the beneficial effects of cruciferous vegetables are not only limited to breast cancer: other clinical trials have shown a favorable effect of these vegetables in preventing bladder cancer, oral cavity/pharyngeal cancer, breast and kidney cancer.
What is the mechanism of action?
We now only partially understand why these vegetables have some favorable effects in preventing and treating cancer: the chemicals that they contain stimulate enzymes (special kind of protein) in the body which are capable of stopping harmful substances (toxins, carcinogens) from damaging the cells. Another mechanism of action of these substances could be the reduction of oxidative stress – i.e. reducing the amount of harmful oxygen-based molecules (which are called oxygen free radicals) that are produced in the body. A different mechanism is inducing the so called programmed cell death (apoptosis) in cancer cells. However, we are still far from knowing the exact mechanism of action of these phytochemicals and further research is needed to clarify their cancer-protective effects.
How much of these vegetables should we eat?
Unfortunately, there is no clear answer to this question yet. From most of the clinical trials we can see that the more of these vegetables we eat, the more protection against cancer we get.
But is there any possible harm these vegetables can cause? Not much. These vegetables are known to contain the so called goitrogens – substances that make your thyroid gland produce lower amounts of thyroid hormones and make your thyroid gland grow into a goiter. This risk is particularly increased in those who suffer from iodine deficiency or have a history of insufficient thyroid function, but if you have enough iodine in your diet and your thyroid gland is fine then there is no danger in consuming these vegetables.
In my opinion, a reasonable approach is to eat broccoli, cauliflower, radish, horseradish, cabbage, kohlrabi or turnip at least once a week and to make sure you eat your five pieces of fruit or vegetables a day. In addition to the cancer-protective effect, regular consumption of cruciferous vegetables may also lower your risk of getting type 2 diabetes, so how about cooking a delightful broccoli soup for dinner or trying some crunchy radishes for snack?