Obesity in adolescence appeared to be key
Being obese as a teenager may be linked with an increased risk of multiple sclerosis as an adult, researchers say.
A 40-year study of 238,000 women found those who were obese at 18 had twice the risk of developing MS compared to women who were slimmer at that age.
Yet body size during childhood or adulthood was not found to be associated with MS risk, the US researchers report in Neurology.
But an MS charity warned more research was needed to confirm the findings.
Researchers from Harvard School of Public Health used data from nurses taking part in a large study on diet, lifestyle factors and health.
Over the course of the study, 593 women were diagnosed with MS, a condition caused by the loss of nerve fibres and their protective myelin sheath in the brain and spinal cord, which causes neurological damage.
The researchers compared the risk of the disease with body mass index (BMI) - a ratio of weight to height - at age 18.
Participants were also asked to describe their body size using a series of diagrams at the age of five, 10 and 20.
The study showed that those with an "obese" BMI of 30 or larger at age 18 had more than twice the risk of developing MS.
There was also a smaller increased risk in those who were classed as overweight .
The results were the same after accounting for smoking status and physical activity level.
When comparing the risk of MS with self-reported body shape, the researchers found no association between childhood obesity and the future chances of developing the disease.
They also found no risk associated with adult obesity.
But women who had a larger body size at 20 years of age also had almost twice the risk of MS compared to women who reported a thinner body size.
Previous research has linked high levels of vitamin D with a reduced risk of MS and the researchers point out that obesity is associated with low vitamin D levels in the body.
The researchers suggest fatty tissue produces substances that affect the immune system, which may also provide a link with the chances of developing MS.
Further research should look at confirming the findings in men and individuals from different ethnic groups as well as comparing with vitamin D levels, they said.
"Our results suggest that weight during adolescence, rather than childhood or adulthood, is critical in determining the risk of MS," said study author Kassandra Munger, ScD, of Harvard School of Public Health in Boston.
"There's a lot of research supporting the idea that adolescence may be an important time for development of disease, so what we have found is consistent with that."
She added: "Teaching and practicing obesity prevention from the start - but especially during teenage years - may be an important step in reducing the risk of MS later in life for women."
Susan Kohlhaas, research communications officer for the MS Society, said: "This study does not account for several other factors that may play a role in causing MS. Based on that, more work is needed.
"As such, it is difficult to determine whether teenage obesity could be a possible factor in causing MS in women."