FRIDAY, May 16, 2003 (HealthDayNews) -- A Swedish study finds a relationship between high birth weight and development of rheumatoid arthritis later in life.
It's a puzzlement, as are the other relationships found in the study, says Dr. Lennart Jacobsson, who reports the finding in the May 17 issue of the British Medical Journal. But then, a lot about rheumatoid arthritis is puzzling.
This is not the wear-and-tear arthritis, formally called osteoarthritis, that many people experience as they grow older. Instead, it is an autoimmune disorder, in which the body's immune system attacks joints and surrounding tissue for unknown reasons.
"We know that genetics can explain about 50 percent of cases," says Jacobsson, an associate professor in the Malmö University Hospital department of rheumatology. "We have not yet identified a major environmental factor that is involved."
He and his colleagues tried to identify such a factor by digging up the birth records of 77 people with rheumatoid arthritis who were born in the Malmö area between 1940 and 1960 and comparing them with the records of 308 area residents who don't have arthritis.
They looked at just about everything in the perinatal period, the time around birth, that could be looked at: mother's age, father's occupation, whether the baby was breast-fed, the baby's weight at birth, whether the mother had a previous miscarriage, and so on. And a few associations emerged.
One of them was high birth weight. Babies weighing more than 4,000 grams (about 9 pounds) were more likely to develop rheumatoid arthritis than those of average weight. Another was breast-feeding; breast-fed babies were less likely to develop the disease. Another was the father's occupation. Babies of office workers were more likely to develop rheumatoid arthritis than those of manual laborers.
The birth weight association has been seen in other studies, Jacobsson says, but he admits frankly, "I can't say why it is so."
The journal paper proposes several reasons for the association: the way the immune system develops in the womb, the way the immune system develops after birth, or simply "unmeasured confounding factors." Your guess is as good as his about which might be correct, Jacobsson says.
He does plan more studies to get a clearer picture of the genetic and environmental factors that can lead to rheumatoid arthritis.
Jacobsson's attitude of bewilderment is shared by many in the arthritis medical community. Asked, "What is the cause of arthritis?" on a Johns Hopkins University School of Medicine Web site, Dr. Alan K. Matsumoto, an assistant professor of medicine in the Hopkins division of molecular and clinical rheumatology, posted this answer:
"There are many different types of arthritis and each has different causes. Likely even the same type of arthritis has multiple causes involving a complex interplay of genetic and environmental factors. Ask me again in 50 years."
You can get an overview of rheumatoid arthritis from the Arthritis Foundation or the National Institute of Arthritis and Musculoskeletal and Skin Diseases.