Some vegetables may contain carcinogenic substances, but the risk of cancer is relatively low. Fiddlehead ferns, for example, contain a substance called "proanthocyanidin," which has been shown to be carcinogenic in animal studies. However, the amount of fiddleheads consumed by human beings is usually less than the dose in animal studies, and the carcinogenic substances in fiddleheads need to reach a certain dose before they can cause cancer in human beings.

In addition, the World Health Organization's International Agency for Research on Cancer classifies fiddlehead fern as a group 2B carcinogen, meaning that it may be carcinogenic to humans, but the evidence is not yet strong. Thus, while eating certain vegetables may increase the risk of cancer, this risk is relatively low and a high intake is required to reach a carcinogenic dose.

Overall, no single food should be over-consumed, but rather a balanced diet with a varied intake of a wide range of vegetables and fruits should be maintained for overall nutrition and health. In addition, foods that contain carcinogens do not cause significant harm to the human body as long as they are consumed in appropriate amounts.