EXPOSURE to vitamin A might increase the risk of one of the deadliest forms of cancer, scientists said yesterday.
Laboratory tests conducted by the Medical Research Council (MRC) have shown a link between the nutrient and a particular type of gullet, or oesophageal, cancer.
It is the latest health warning to confuse people who thought they were taking a healthy diet - limited consumption of vitamin A, also known as retinol, is recommended by experts.
The researchers hope the discovery will lead to new treatments. There are around 7500 new cases each year in the UK, with a mortality rate of almost 100%.
Vitamin A is found in cheese, eggs, oily fish such as mackerel, milk, fortified margarine and yoghurt. Liver and liver pate are also good sources - probably too good, especially for pregnant mothers, since too much vitamin A can harm unborn babies.
Excess consumption of vitamin A is known to be harmful for other reasons, but this is the first link to be made with gullet cancer, in particular the type which develops from pre-existing abnormalities in the lining of the gullet known as Barrett's oesophagus.
This condition occurs in 10% of people with symptoms of heartburn, but precisely what triggers the abnormalities has been unclear.
Scientists at the MRC's Cancer Cell Unit who made the discovery hope it will lead to new ways of preventing the cancer, rates of which have risen eight-fold in the UK in the last 30 years.
Laboratory tests showed that treatment with vitamin A, in the form of retinoic acid, turned the normally scaly lining of the oesophagus into a glandular lining, like that found in the stomach or intestine. This is typical of the abnormalities seen in Barrett's.
When the altered tissue was treated with vitamin A inhibitors, the cells reverted to their normal state.
Dr Rebecca Fitzgerald, who led the scientists, said: "We are very excited about these findings. Vitamin A inhibitors could allow us to reverse Barrett's oesophagus, which would prevent the lesions it provokes from causing oesophageal adenocarcinoma.
"Until now it had been thought the changes to the cells in the oesophageal lining was limited to the top layer of those cells, the epithelium. This research shows that the change is actually more fundamental. This ties in with recent work in many cancer areas which suggests that we have neglected the cell environment for too long in our thinking about cancer."
The findings, published on-line in the journal Gut, were being discussed at the Barrett's Oesophagus National Symposium in London yesterday.
Grant Fullarton, a consultant surgeon specialising in gullet cancer at Gartnavel General Hospital in Glasgow, said: "Clearly this has to be investigated further. This is a complicated area and there are probably a number of different factors. It will be some time before the findings, if they are confirmed, could be translated into treatment, as vitamin A inhibitors could have a knock-on effect elsewhere."
Vitamin A has a number of important functions, contributing to the health of skin and mucus linings, immunity from infections and vision in dim light.
Because it is fat soluble, any which is not needed at the time is stored for future use, so it needn't be taken every day.
The Food Standards Agency recommends an average daily intake of 0.7 mg a day for men and 0.6 mg a day for women, but more than 1.5 mg a day is thought to be a risk factor for brittle bones.
Many multivitamins contain vitamin A, as do supplements such as fish liver oil, so those taking them should check levels carefully.
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