Finnish researchers tracked the serum vitamin D levels of 3,173 people aged 50-79 over two years, and followed them up 29 years later.

The researchers found patients with low vitamin D levels were three times more likely to suffer from the disease, and risk was dose dependent. Those with higher levels of vitamin D were less likely to develop Parkinson's disease.

Are the findings significant?
Previous work suggested Parkinson's disease may be caused by continuously inadequate vitamin D levels leading to a chronic loss of dopaminergic neurons in the brain.

To test this theory, Dr Paul Knekt and colleagues from the National Institute for Health and Welfare in Helsinki assessed patients' vitamin D levels from 1978 to 1980 and followed them up in 2007.

Individuals with a serum vitamin D concentration of at least 50nmol/L had a 65 per cent lower risk of developing Parkinson's disease than those with levels below 25nmol/L.

But the study had some weaknesses. Low numbers of disease outcomes affected the study's statistical power.

Vitamin D serum levels were measured just once, which failed to account for seasonal variation. Also, the study did not account for long-term change in serum levels.

The authors also acknowledge that the study recorded diagnoses for Parkinson's disease, rather than confirmed cases, which they said may have biased the estimates of strength of association.

Could pills prevent Parkinson's?
Dr Knekt said that, although their results were 'in line with the hypothesis that low vitamin D status predicts the development of Parkinson's disease', it was not clear how vitamin D protects against the disease.

Kieran Breen, director of research at charity Parkinson's UK, said: 'The study provides further clues about the potential environmental factors that may influence or protect against the progression of Parkinson's.

'However, further research would be required to find out whether low levels of vitamin D may increase an individual's risk of developing Parkinson's or whether low vitamin D levels could predict against Parkinson's developing later.'

Dr Breen advised that a balanced healthy diet should provide the recommended levels of vitamin D, but that it remained unclear whether dietary supplements or increased sunlight exposure may affect the development of Parkinson's.

• Arch Neurol 2010; 67 :795-7

Informing Patients
Low vitamin D levels may increase the risk of developing Parkinson's disease, while high levels may protect against it. The relationship appears to be dose-dependent, although further studies are needed to support this suggestion. A balanced healthy diet is the best source of vitamin D, but more research is needed to show the impact of dietary supplements on Parkinson's.