Rheumatoid arthritis is a relatively common autoimmune disease for which the cause is still unknown.  Researchers are investigating the contributing factors for developing disease, including the impact of vitamin D.  Vitamin D deficiency has been implicated in the development of autoimmune disease such as type I diabetes and multiple sclerosis.

Vitamin D is a fat-soluble hormone which is synthesized in the body as sunlight (ultraviolet light UVB) reaches exposed skin.  Vitamin D is found naturally in very few foods, such as fatty fish, and is available as a dietary supplement in over-the-counter and prescription form.  Many people do not produce enough vitamin D from UVB exposure nor get enough from food or supplementation.  As RA patients might avoid sun exposure due to photosensitivity, they may be especially susceptible to vitamin D deficiency.  Vitamin D is essential for calcium absorption in the gut and for bone growth and mineralization (preventing osteoporosis).

Reduced vitamin D intake has been linked to increased risk of developing RA and vitamin D deficiency has been found to be associated with disease activity and musculoskeletal pain in patients with RA.  In a recent study, researchers evaluated vitamin D status in 44 patients with RA and looked for any relationship between vitamin D serum levels and disease activity.  A control group of 44 persons was evaluated as well. 

Vitamin D circulates in the body in two forms.  The liver converts vitamin D to 25-hydroxyvitamin D3 [25(OH)D3], also known as calcidiol.  The kidneys convert calcidiol to activated vitamin D, also known as 1,25-dihydroxyvitamin D [1,25(OH)2D] or calcitriol.  When measuring vitamin D levels in the blood, the recommended test measures serum concentration of 25(OH)D3, reported as nanomoles per liter (nmol/L) and/or nanograms per milliter (ng/mL).  

Persons who have serum concentrations of 25(OH)D3 less than 12 ng/mL are considered deficient in vitamin D.  Levels between 12 and 20 ng/mL are considered inadequate in healthy persons.  Greater than 20 ng/mL is considered adequate in healthy persons.  According to NIH, levels greater than 50 ng/mL may cause undesirable adverse effects.  However, some rheumatologists (including my own) recommend serum concentrations between 50-80 ng/mL in patients diagnosed with autoimmune disease.  

[My 25(OH)D3 level was 76 ng/mL the last time it was tested after years of monitored vitamin D supplementation, whereas it had been 7.6 ng/mL shortly after my RA diagnosis when I wasn’t taking supplements and was experiencing more pain.]

In the study, levels of C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), and parathyroid hormone were also measured.  Disease activity was assessed using the 28-joint Disease Activity Score (DAS28).  What was observed is that 25(OH)D3 levels were low in the RA patients compared with healthy controls, 15.26 ± 1.07 ng/ml and 25.8 ± 1.6 ng/ml, respectively.  Results from the additional blood tests were elevated in the RA group with parathyroid hormone levels at 71.08 ± 7.02 pg/ml (normal values 10.0–65.0 pg/ml), CRP at 7.6 ± 1.57 mg/litre (normal values < 3 mg/litre) and ESR at 38.0 ± 4.6 mm/h.  The DAS28 index was 4.26 ± 0.26 in the RA group.