In this study in England (UK), 6,824 people who were 45 years old had a 25 hydroxy vitamin D test as part of a larger study. In women, but not men, vitamin D levels of 75-99 nmol/l was associated with a 7.7% risk of having Chronic Widespread Pain (CWP) vs. a 14.6% risk for those with levels under 75 nmol/l.
Although this does prove that low vitamin D contributes to having Fibromyalgia, it suggests that getting 2,000 units of vitamin D a day, the sunshine vitamin, may be optimal (get sunshine, not sunburn).
For more info on vitamin D and how it can save your life, see Vitamin D Could Save Your Life!
Vitamin D and chronic widespread pain in a white middle-aged British population: evidence from a cross-sectional population survey. Kate Atherton, Diane J Berry, Tessa Parsons, Gary J Macfarlane, Chris Power, and Elina Hypponen; Ann Rheum Dis, August 12, 2008.