The Role of Vitamin D in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is an autoimmune disease that affects the synovial joints between bones. Vitamin D intake may modulate RA, because calcitriol modulates the adaptive immune system. Like all autoimmune diseases, rheumatoid arthritis is caused by the adaptive immune system.

RA can affect the main synovial joints of the body:

  • Elbow joints

  • Finger and toe joints

  • Hip joints

  • Knee joints

  • Shoulder joints

  • Wrist joints

The synovial joints are surrounded by a joint capsule. The inner lining of the joint capsule is made of tissue called synovium, which secretes synovial fluid into the synovial cavity to provide smooth movement of the joint. The following figure shows a typical synovial joint.

A healthy synovial joint.
A healthy synovial joint.

In RA, the immune system attacks the synovial tissue, which stops producing the synovial fluid. Without synovial fluid the joints don’t move smoothly. It’s like removing the oil from the engine of a car — you get a lot of friction without the oil.

In RA, the joints heat up, swell, and become tender. Over time they also become stiff and don’t move well. As the disease worsens, the joints lose their motion and become deformed. The next figure shows a synovial joint affected by rheumatoid arthritis.

A synovial joint with inflammation in the synovial membrane.
A synovial joint with inflammation in the synovial membrane.

Signs and symptoms of rheumatoid arthritis

RA affects women at a rate three to five times higher than men. It mainly affects the joints but also has signs and symptoms that affect the entire body.

The major signs and symptoms include the following:

  • Inflammation of the joints caused by cytokines produced by T cells with swelling, heat, and tenderness that’s usually (but not necessarily) symmetric

  • Inflammation of the lungs, the tissue surrounding the heart (called the pericardium), the tissue surrounding the lungs (called the pleura), and the white part of the eye (called the sclera)

  • Destruction of the cartilage, the smooth surfaces that permit unhindered movement of the joint, leading to joint deformity and loss of joint mobility

  • Nodular lesions under the skin, called rheumatoid nodules

  • Increased susceptibility to heart attacks and strokes

  • Loss of bone near the inflamed joint (cytokines stimulate bone loss by activating osteoclasts)

Constitutional symptoms include the following:

  • Fatigue

  • Loss of appetite

  • Loss of weight

  • Low-grade fever

  • Morning stiffness

Diagnosis involves a blood test for a protein called the rheumatoid factor. This test isn’t flawless — 15 percent of patients who have the disease might not have rheumatoid factor in their blood whereas 10 percent of the normal population might have it there. Newer tests are being developed. Doctors use a multitude of anti-inflammatory and other drugs for RA, with variable results.

Vitamin D and rheumatoid arthritis

A number of findings have linked vitamin D to rheumatoid arthritis. Among the most important findings are the following:

  • RA patients with serum 25-hydroxyvitamin D levels less than 20 ng/ml [50 nmol/L] have more tender joints and more evidence of inflammation compared to people with higher serum levels.

  • Among 29,368 women who were followed for 11 years, those who took more than 400 IU of vitamin D as a supplement each day had a lower risk of developing RA.

  • Vitamin D insufficiency (serum 25OH D of less than 15 ng/ml [37.5 nmol/L]) is common in African Americans with recent-onset RA.

  • * Treating with a drug similar to calcitriol reduced joint inflammation and reduced RA in rats.

None of the studies so far are randomized controlled trials, but those types of studies are getting underway. As a result, no consensus exists on the vitamin D dosage that may protect a person from RA or that may lessen the effects of the disease in an already affected patient. However, based on what is known so far, getting the blood level up to 20 ng/ml [50 nmol/L] simply makes sense in protecting against this disease.

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