The unexpected finding suggests that plans to introduce mandatory folate fortification of bread and cereals should be reconsidered as a high folate consumption might actually increase cancer risk.

Previous observational studies suggested that high dietary folate levels could be protective against colorectal cancer.

But this was not supported by the results of a Swedish study which compared the circulating folate levels from blood samples taken from patients who went on to develop colorectal cancer with those of healthy controls.

The study included 226 patients with colorectal cancer diagnosed between 1985 and 2002. The levels of circulating folate were measured in serum samples taken from patients on average 4.2 years prior to colorectal cancer diagnosis. These were compared with samples taken at the same time from 437 age and sex-matched controls.

The researchers found that patients who developed colorectal cancer had higher average levels of circulating folate than controls. Those with folate levels of 5-8nmol/l were 72 per cent more likely to develop colorectal cancer than those with a circulating folate level of under 5nmol/l. Those with folate levels of 8-12nmol/l were twice as likely to develop colorectal cancer.

Lead researcher Ms Bethany van Guelpen from the department of medical biosciences at Umea University said that the finding was unexpected: 'Dietary studies have quite consistently shown that a higher intake of folate was associated with a lower risk of colon cancer, but that was not what we found.

'We were not able to draw any conclusions about a very high folate intake, as intake was generally low across the population we looked at, but we saw that those with moderate folate levels had a doubling of risk compared to those with the lowest levels.

'Advice to the population is not going to change based on this one small study, especially considering all the health benefits of a diet high in fruits and vegetables. But it will have to be taken into consideration when it comes to decisions about folate fortification.'

- Gut online