Increased intake of vitamin B6, from diet and supplements, could cut the risk of Parkinson's disease by half suggests a prospective study from the Netherlands.
The researchers, from the Erasmus Medical Center in Rotterdam, recruited 5,289 people over the age of 55 and found that those with the highest total vitamin B6 intake had significantly lower risk of developing the disease than those with the lowest intake.
Parkinson's disease, named after Dr James Parkinson, the London doctor who initially identified it as a particular condition, occurs when nerve cells are lost in a part of the brain called the substantia nigra.
According to the American Parkinson's Disease Association, over two million Americans currently suffer from the disease. InfoPark, an EU funded information service, has estimated that by the year 2050 around four million people in Europe will have the disease.
The Rotterdam Study looked into reports that increased levels of the amino acid homocysteine might promote Parkinson's disease. Numerous studies have reported that higher intakes of folate, vitamin B12, and vitamin B6 decrease plasma homocysteine levels and therefore might offer protection from Parkinson's.
It has also been proposed that vitamin B6 could have antioxidant effects that could reduce the risk of the disease by a mechanism that is not related to homocysteine.
The researchers, led by Dr Monique Breteler, assessed dietary intakes at the very start of the study (October 1990 to July 2003) using semi-quantitative food frequency questionnaires (SFFQ). Physical examinations of the subjects, including neurological exams, were conducted at baseline and at three yearly intervals.
After an average of ten years of follow-up, 72 new cases of Parkinson's disease had been diagnosed.
The average vitamin B6 intake was 1.63 milligrams per day, average B12 intake was 5.3 micrograms per day, and average folate intake of 218.7 micrograms per day.
The researchers found that people who had daily vitamin B6 intakes of 230.9 micrograms or more had an associated risk of developing Parkinson's disease 54 per cent lower than people who had average daily intakes lower than 185.1 micrograms.
No significant reductions in the risk of Parkinson's disease were found for folate or vitamin B12, although the authors could "not completely rule out" an effect from these nutrients.
When the researchers limited their calculations to smokers, who made up 66 per cent of the study population, they found that the protective effects of high vitamin B6 intake appeared to be limited to this group.
"Stratified analyses showed that this association was restricted to smokers," wrote the authors.
"It is unclear why higher vitamin B6 intake was associated with a decreased risk of Parkinson's disease only in smokers," said the authors.
But Breteler and her team noted however several studies have reported that nicotine may protect brain cells and reduce the risk of Parkinson's disease, and suggest that vitamin B6 plus smoking may "mutually reinforce beneficial effects".
Homocysteine is potentially toxic to brain cells, but since no relationship between folate and vitamin B12 was observed, the authors said that the apparent benefits of B6 "may point towards an alternative mechanism… [because] several lines of evidence suggest neuroprotective properties of vitamin B6 through antioxidant capacities."
"Because oxidative stress may be prominent in Parkinson's disease pathogenesis, higher vitamin B6 intake may thus reduce Parkinson's disease risk though antioxidant effects."
The results, which do not agree with the one previous study looking at B vitamin intake and Parkinson's disease, need to be confirmed in both additional observational studies and clinical trials, said the researchers.
The previous study, based in the US, followed a population sample with significantly higher daily intakes of all three B vitamins, which may account for the discrepancy between that study and the Rotterdam Study, said the researchers.
The results are published in the journal Neurology (Vol. 67, pp. 315-318).