Older people with low blood levels of the sunshine vitamin (vitamin D) and high blood levels of parathyroid hormone are more likely to be depressed, Dutch researchers report.

But it remains unclear whether these abnormalities are a cause or a consequence of depression, Dr. Witte J. G. Hoogendijk and colleagues from Vrije Universiteit in Amsterdam say.

Past studies have linked altered levels of vitamin D and parathyroid hormone with depression, but the relationship "has never been studied systematically," Hoogendijk and colleagues note. To investigate, they looked at 1,282 men and women aged 65 to 95 years participating in a long-term study of aging.

Nearly 40 percent of the men and 57 percent of women had low levels of vitamin D in their blood.

Among the 169 people found by self-report and diagnostic interviews to be suffering from minor depression, as well as the 26 with major depressive disorder, vitamin D levels were 14 percent lower than for people who were free from depression, the researchers found.

And blood levels of parathyroid hormone, which increase with vitamin D deficiency, were 5 percent higher in people with minor depression and 33 percent higher in those with major depression, relative to levels seen in seniors without depression.

There are a number of plausible ways that low vitamin D levels could influence mood, the researchers note, given that the vitamin plays a role in several neurological and hormonal processes.

If vitamin D deficiency is a cause rather than a result of depression, they add, this suggests supplementation with the vitamin and increased exposure to sunlight could help treat the mood disorder.

Long-term studies with repeated assessments are needed to explore the question of whether decreased vitamin D levels and increased parathyroid hormone levels precede depression or follow it, the researchers conclude.

SOURCE: Archives of General Psychiatry, May 2008.

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