In Parkinson's disease, nerve cells that make a chemical called dopamine have died or don't work as they should in the brain. With less dopamine, you can't control your movements in the usual way, and you may shake, move slowly, have stiff muscles or lose your balance. Studies show that the part of the brain affected most by Parkinson's (called the substantia nigra) has a high level of receptors for vitamin D. This suggests the vitamin is important to how this part of the brain works. So a lack of it, in theory, might contribute to Parkinson's disease.

In the new study, researchers looked at levels of vitamin D in the blood of three groups of people: 100 adults with Parkinson's disease, 97 adults with Alzheimer's disease and 99 healthy adults matched with the other groups for age, sex, race and the area where they lived. The researchers included people with Alzheimer's disease because they were curious whether other diseases that cause a loss of nerve cells (neurodegenerative diseases) might also be associated with low vitamin D.

The study found that 55 percent of people with Parkinson's had below-normal levels of vitamin D, compared with only 36 percent of healthy adults and 41 percent of people with Alzheimer's disease. People with Parkinson's were also more likely to have severely low levels (a vitamin D deficiency), with 23 percent being deficient, compared with 16 percent of people with Alzheimer's and 10 percent of healthy adults.

The fact that people with Parkinson's had less vitamin D than those with Alzheimer's was especially interesting to researchers. This suggests, they say, that low vitamin D may be uniquely linked to Parkinson's rather than to neurodegenerative diseases in general. They point out, however, that low vitamin D might be a result of Parkinson's rather than a cause, as movement problems may limit how often people with Parkinson's go outdoors. This could mean less time in the sun and less vitamin D.

What you need to know. The study suggests a possible link between a lack of vitamin D and Parkinson's disease, but what this link means is unclear. If future research shows that low vitamin D is connected to the development of Parkinson's, then this might point to new ways to prevent and treat the disease. But the research is at an early stage.

What the study does show, however, is that low vitamin D may be more common among people with Parkinson's. This is an important finding in itself. If you have Parkinson's, you may need to be especially careful that you get enough of this nutrient, as a lack of it can cause serious health problems, such as weakened bones.

If you are at all concerned you may not be getting enough vitamin D, be sure to talk to your doctor. He or she may recommend having a blood test to check your level. If it's low, you may need to take regular supplements to increase and maintain your blood levels.

—Sophie Ramsey, patient editor, BMJ Group

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