Their findings offer new insight into how carrot consumption may protect against cancer, as previously demonstrated in epidemiological studies.
Falcarinol protects carrots and other vegetables in the same family from fungal diseases. Previous research on the compound, which is toxic to humans in large doses, has concentrated on its actions on plant disease defence.
"It was simply not considered interesting for humans because it is toxic in high amounts," explained study author Dr Kirsten Brandt, a senior lecturer with Newcastle University's School of Agriculture, Food and Rural Development.
However falcarinol is also present in ginseng, a long-established medicinal plant, and initial findings showing that it could protect against cancer led a team from Newcastle University in the UK, the University of Southern Denmark and the Danish Institute of Agricultural Sciences to look more closely at the compound.
Their results, published today in the Journal of Agricultural and Food Chemistry (10.1021/jf04519s), show that after 18 weeks, rats with pre-cancerous tumours who ate a popular variety of carrots along with their ordinary feed, and another group that consumed falcarinol in a quantity equal to that in the carrots, were one third less likely to develop full-scale tumours than the rats in the control group.
"We already know that carrots are good for us and can reduce the risk of cancer but until now we have not known which element of the vegetable has these special properties," said Dr Brandt.
The researcher told NutraIngredients.com that the findings lead to a potential explanation for the confusion surrounding the widely researched carotenoid beta-carotene, another important component in carrots.
"Beta-carotene has been widely investigated in extensive intervention studies. One of the big conundrums was that beta-carotene alone was found to raise risk of cancer yet people who ate a lot of carrots did not experience this elevated risk," she said.
"This led to a simple explanation that it must be something else in the carrot that has a protective effect as it can't be the beta-carotene."
The mechanism for the anti-cancer action is unclear at this stage.
"We have some vague ideas, such as the theory that it could stimulate the immune system. Greenhouse workers exposed to falcarinol on their skin can become allergic to the compound as the immune system becomes overstimulated," said Dr Brandt.
"This shows that there is some kind of chemical reaction, and it might also stimulate the immune system in a positive way," she said.
The researchers also need to find out how much falcarinol is needed to prevent the development of cancer and if certain types of carrot are better than others.
But Dr Brandt noted that extracting the compound for use in supplements would present significant safety issues, likely to prevent its launch on the market.
"You could in principle isolate it and let people take it in a pill. But I don't see this as an option. It can kill you in high doses, and while people would never be able to eat the 200 carrots required to reach these fatal doses, they may overdose on pills."
Further research will instead seek to pinpoint the optimal dose needed to fight off cancer, while Dr Brandt will focus on implications for industry, such as whether processing conditions like boiling carrots or making juice changes the levels of this compound. The current study used raw carrots.
"We could also expand our research to include other vegetables. For consumers, it may soon no longer be a case of advising them to eat five portions of fruit and vegetables per day but to eat particular types of these in certain quantities," added Dr Brandt.
The research may lead to more tailored advice for growers. The Newcastle team will shortly study whether organic carrots have higher levels of the compound.