A lack of Vitamin D, due to reduced sunlight, has been linked to depression and the symptoms of Seasonal Affective Disorder (SAD), but research by the University of Warwick shows there is no clear link between the levels of vitamin D in the blood and depression.

Exposure to sunlight stimulates vitamin D in the skin and a shortage of sunlight in the winter has been put forward as one possible cause of SAD. However Warwick Medical School researchers, led by Dr Oscar Franco, have discovered low levels of vitamin D in the blood may not be connected to depression.

In a study published in the Journal of Affective Disorders, the team recruited more than 3,000 people and tested levels of vitamin D (25-hydroxyvitamin D) in the blood. They then carried out a questionnaire with the participants to assess the prevalence of depressive symptoms.

Vitamin D deficiency exists when the concentration of 25-hydroxy-vitamin D (25-OH-D) in the blood serum occurs at 12ng/ml (nanograms/millilitre) or less. The normal concentration of 25-hydroxy-vitamin D in the blood serum is 25-50ng/ml.

The researchers found there was no clear association between depressive symptoms and the concentration of vitamin D in the blood.

Dr Oscar Franco, Assistant Clinical Professor in Public Health, said: "Few studies have explored the association between blood 25-hydroxyvitamin D concentrations and depression in the general population. A deficiency of vitamin D has also been attributed to several chronic diseases, including osteoporosis, common cancers, autoimmune and cardiovascular diseases."

This study was carried out in collaboration with colleagues from the Institute for Nutritional Sciences, Chinese Academy of Sciences in China.

The team recruited 3,262 community residents aged 50-70 from Beijing and Shanghai in China as part of the Nutrition and Health of Aging Population in China (NHAPC) project.

Dr Franco said his study did not evaluate whether the depressive symptoms were seasonal and suggested more studies needed to be done.

Dr Franco said: "Previous studies into the effects of vitamin D supplementation have produced mixed results. More studies are still needed to evaluate whether vitamin D is associated with seasonal affective disorders, but our study does raise questions about the effects of taking more vitamin D to combat depressive symptoms."