NEW YORK (Reuters Health) ¿ The relationship of body mass index (BMI) with the body composition in patients with rheumatoid arthritis differs from that in patients with osteoarthritis and healthy controls, according to UK researchers.

"Patients with rheumatoid arthritis may have more fat in their bodies ¿ and experience the adverse health effects of fatness ¿ with weight and BMI that are not traditionally associated with obesity in the general population," senior investigator Dr. George D. Kitas told Reuters Health. "This is important for the management of individual patients with rheumatoid arthritis."

Kitas of the Dudley Group of Hospitals NHS Trust and colleagues studied 174 rheumatoid arthritis patients, 43 osteoarthritis patients and 82 healthy controls. A further 324 rheumatoid arthritis patients served as a validation group. The findings appear in the Annals of the Rheumatic Diseases.

Osteoarthritis, also referred to a degenerative joint disease, is the most common type of arthritis, causing join pain, swelling and reduced range of motion. While it may occur in any joint, it usually affects the hands, knees, hips and back.

Rheumatoid arthritis, on the other hand, is an autoimmune disease, a condition in which the immune system attacks itself. It causes chronic inflammation of the joints, the tissue around the joints, as well as other organs in the body. It is a chronic systemic disease that can be progressive.

BMI is the ratio of height to weight and is often used to classify people as underweight (less than 18.5), normal weight (18.5 ¿ 24.9) or overweight (25.0 ¿ 29.9). People with BMIs over 30 are considered obese.

The team found that significant BMI differences among groups were mainly due to age. However, for a given body fat content, rheumatoid arthritis and osteoarthritis patients showed lower BMI levels than did controls.