Vitamin D seems to be important in regulating immune functions. In one study 2007 study, the effects of serum vitamin D level were explored further in clinical patients with early inflammatory poly arthritis.

Patients in the Norfolk, UK region who had early inflammatory arthritis, known as having synovitis of 2 or more peripheral joints for a minimum of 4 weeks, were taken into the Norfolk Arhtritis Register (NOAR). They measured Vitamin D metabolites, and 1,25-dihydroxy Vitamin D and were compared in patients based on the clinical features of an outcome of 1 year.

The 206 patients that suffered from early inflammatory arthritis, where 2/3 were women at about 59 years old. 35 percent of the patients met the ACR classification requirements for RA, while 45 percent met the 1 year criteria. The subjects who fulfilled the ACR criteria for RA had significantly lower means of 1,25-dihydroxy vitamin D levels compared to those who did not meet this RA criteria. For subjects who had not fulfilled the ACR criteria had a trend to lower their levels of Vitamin D metabolites. Every 10mg/ml increased in 1,25-dihydroxy vitamin D was linked to a 42.7 percent lower tender joint count, 25 percent lower CRP. 0.30 lower DAS28 score and decreased odds of being in the higher HAQ group by 39 percent. Every 10pg/ml as a result increased in the 12,5-dihydroxy vitamin D was linked to a 21 percent lower risk of being in the higher HAQ group.

In about one year, the 125-dihydroxy vitamin D and Vitamin D metabolites were the two dramatically lower in subjects fulfilling the ACR criteria for RA. Each of them saw a 10ng/ml increase in baseline vitamin D metabolites was linked to a 46 percent decrease in tender joint count at one year and had lower odds of being in the HAQ group by 41 percent less. Every 10 pg/ml was higher in the baseline of 1,25-dihudroxy vitamin D thus being associated with a 26 percent lower odds of being in the higher HAQ group.

In the end, the lower levels of vitamin D metabolites were linked to the clinical outcomes in patients with the early inflammatory poly arthritis.