TUESDAY, Nov. 10, 2009 (HealthDay News) -- While there are plenty of good reasons to avoid obesity in your teens, a new study now suggests that extra weight in adolescence may increase your risk of multiple sclerosis (MS) later.
Reporting in the Nov. 10 issue of Neurology, Harvard researchers found that being obese at age 18 more than doubles a woman's risk of developing MS later in life compared to her slimmer peers.
"This is one more study that shows obesity leads to another unhealthy outcome, and obesity during adolescence may be critical in determining MS risk," said study author Kassandra Munger, a research associate at the Harvard School of Public Health in Boston.
Multiple sclerosis is a chronic disorder of the central nervous system, according to the National Multiple Sclerosis Society. Although no one is certain what the exact cause of the disease is, scientists believe it's an autoimmune disease. That means the body's immune system mistakenly targets its own cells. In the case of MS, the immune system destroys myelin, a fatty substance that covers nerve fibers, according to the MS society.
The current study included more than 238,000 women who participated in the Nurses' Health Study or the Nurses' Health II study between 1976 and 2002. The women were between 25 and 55 years old at the start of the study.
Body mass index (BMI) was calculated using information given by the women about what their height and weight were when they were 18 and at the start of the study. A BMI of 18.5 to 24.9 is considered normal, according to the U.S. Centers for Disease Control and Prevention, while 25 to 29.9 is overweight and above 30 is obese.
The women were also asked to look at pictures of nine different body silhouettes that ranged in size from very thin to extremely obese, and to identify which silhouette most closely matched hers at ages 5, 10 and 20.
Women who were obese at age 18 had a 2.25 times greater risk of developing MS, according to the study. Being overweight seemed to increase the risk slightly, but not to a level that was statistically significant, Munger noted.
A large body silhouette at age 20 increased the risk of MS by 96 percent, the researchers found. A large body size at age 5 or 10 was not associated with an increased risk of MS, provided that the woman had slimmed down by age 20.
Although this study wasn't designed to find the reason behind the increase, Munger said low vitamin D levels have been suspected of playing a role in the development of MS, and that people who are obese may have lower circulating levels of vitamin D. Another possibility, suggested Munger, is that fat tissue secretes a lot of substances that can affect the immune system.
"We're beginning now to get clues about things that might predispose a person to MS," said Dr. John Richert, executive vice president of research and clinical programs at the National Multiple Sclerosis Society. "Up until recently, we've looked at MS as a disease for which the onset can't be controlled, and though it's still mostly that way, maybe there are circumstances where people might be able to lower their risk a little bit. If all the incoming data [from this and other studies] is correct, not smoking and maintaining an ideal weight might lower the risk of MS."
But, of the current study, he added, "As with so many interesting observations, this study raises a lot more questions than it answers."
Munger noted that the study was done solely in women, so the results may not hold true for men. And, additionally, she said, the study participants were almost all white, so it's not clear if this association would hold up for other races.
Learn more about potential causes of MS from the National Multiple Sclerosis Society.