Introduction: Patients with chronic fatigue syndrome (CFS) may be at risk of osteoporosis due to their relative lack of physical activity and excessive time spent indoors, leading to reduced vitamin D synthesis. We hypothesized that serum 25-OH vitamin D levels are lower in CFS patients than in the general British population.

Subjects and methods: We performed a retrospective survey of serum 25-OH vitamin D levels in 221 CFS patients. We compared this to a group of patients attending the hospital for other chronic conditions and to a large British longitudinal survey of 45-year old women, using a variety of appropriate statistical approaches.

Results: 25-OH vitamin D levels are moderately to severely suboptimal in CFS patients, with a mean of 44.4 nmol/L (optimal levels >75 nmol/L). These levels are lower and the difference is statistically significant (p<0.0004) than those of the general British population from a recent national survey, but similar to those in patients with other chronic conditions.

Conclusions: This data supports the recommendation made in recent NICE guidelines that all patients with moderate to severe CFS should be encouraged to obtain adequate sun exposure and eat foods high in vitamin D. Oral or intramuscular vitamin D supplementation should be considered for those whose levels remain suboptimal.