VITAMIN D2

Vitamin D(2) is a group of fat-soluble secosteroids. Vitamin D is known as the sunshine because sunlight helps to synthesize it. Research has found that vitamin D may play a significant role in joint health, and that low levels may increase the risk of rheumatologic conditions such as arthritis. Several studies have found low blood levels of vitamin D in patients with osteoarthritis of the hip and knee. In another study of more than 2,000 people, researchers found that vitamin D deficiency was strongly associated with disabling symptoms among those with rheumatoid arthritis.

Source: http://www.everydayhealth.com/arthritis/arthritis-and-vitamin-d-whats-the-connection.aspx
Vitamin D deficiency in rheumatoid arthritis.                                          

Source: http://arthritis-research.com/content/12/6/R216