TUESDAY, Jan. 25, 2005 (HealthDayNews) -- While you may not have been pleased about their arrival at the time, it turns out that if you had a baby brother or sister before you were 6 years old, your risk of developing multiple sclerosis may be greatly reduced.
That's what a new Australian study found when researchers compared people with multiple sclerosis (MS) to a control group of people who didn't have the disease.
People with more than five years of infant contact before they were 6 years old had almost a 90 percent reduced risk of MS, according to the study published in the Jan. 26 issue of the Journal of the American Medical Association.
"Higher infant sibling exposure in the first six years of life was associated with a reduced risk of MS, possibly by altering childhood infection patterns and related immune responses," the authors wrote.
About 400,000 people in the United States have multiple sclerosis, and most are diagnosed between the ages of 20 and 50, according to the National Multiple Sclerosis Society. Many more women than men have the disorder. Symptoms include fatigue, loss of balance and muscle coordination, cognitive problems, dizziness, pain and vision problems.
The cause of MS is unknown. While there appears to be a genetic component to the disease, it is not directly inherited. Instead, people probably inherit a genetic susceptibility or predisposition to the disease, explained Dr. Patricia O'Looney, director of Biomedical Research Programs for the National Multiple Sclerosis Society.
"Genetics plays a role, but it's not the complete story," O'Looney said. "Something triggers the immune system in genetically susceptible individuals."
The authors of the new study suggest that sibling exposure may protect against such an immune system trigger by exposing children early and often to infections in childhood. Such early exposure may reduce the number of allergic and autoimmune disorders because they boost the immune system and help develop cells that respond effectively to viral threats, explained the authors.
The researchers compared 136 people who were diagnosed with MS to members of two control groups who were age- and gender-matched for each person with MS. The average age of the study participants was 43 and almost all were born in Tasmania. There were about twice as many females as males in the study.
Each participant completed a standardized verbal questionnaire and provided information on the number of siblings and their dates of birth, smoking exposure, sun exposure, breastfeeding history and illness history.
Birth order was not significantly associated with MS risk, but the number of younger siblings was.
Infant contact for more than five years during the first six years of life conferred almost a 90 percent reduced risk of MS. Three to five years of contact with an infant sibling reduced the risk by 60 percent, while one to three years of infant exposure before age 6 lowered the risk by 43 percent, the researchers said.
People with at least one year of infant exposure in their early years also had a reduced risk of getting infectious mononucleosis, according to the study.
"This is an interesting and intriguing study, but it needs to be replicated," said Dr. Robyn Wolintz, co-director of the MS Center at Maimonides Medical Center in New York City.
Looney agreed that further research needs to be done on this issue. She pointed out that the researchers mentioned that sunlight exposure seemed to exacerbate the sibling exposure effect, but they weren't able to separate out what effect the sun exposure might have had.
Sunlight exposure, probably because it triggers the production of vitamin D in humans and vitamin D helps to regulate the immune system, has also been implicated as a potential risk factor for MS, Looney said.
Looney emphasized that the new research doesn't mean that, if you're an only child, you will develop MS.
"You're not at a higher risk for MS just because you don't have one sibling," she said.
More information
To learn more about multiple sclerosis, visit the National Multiple Sclerosis Society.
