The new study, which uses Mendelian randomization, finds a causal role of obesity in MS risk.
The study, led by Dr. Brent Richards from the Jewish General Hospital in Quebec, Canada, is published in PLOS Medicine.
According to the National Institutes of Health (NIH), many experts believe multiple sclerosis (MS) is an autoimmune disease, whereby the body's immune system attacks its own tissues. With MS, the body attacks its own nerve-insulating myelin.
Most people encounter their first MS symptoms between 20-40 years of age, and initial symptoms include blurred or double vision, red-green distortion, or blindness in one eye.
The researchers from this latest study say that an elevated body mass index (BMI) has been shown to promote a "proinflammatory state," affecting the immune system.
They add that "it has been proposed that adipose-derived hormones, such as leptin and adiponectin, might mediate this, providing a possible mechanistic link between obesity and risk of MS."
Although previous observational studies have suggested a link between obesity in early adulthood and MS risk, the researchers say bias due to confounding and reverse causation could have influenced the findings.
'Findings provide evidence for causal role for obesity in MS etiology'
To decrease the likelihood that exposures linked to obesity - such as smoking - can explain such findings, Dr. Richards and colleagues conducted a Mendelian randomization study in large population datasets, in order to examine whether genetically determined obesity was linked with increased MS risk.
"Mendelian randomization offers a way to investigate potentially causal relationships by using genetic associations to explore the effect of modifiable exposures on outcomes," explain the researchers.
Results showed that a change in BMI from overweight to obese - which is equivalent to an average adult woman increasing in weight from 150 to 180 pounds - was linked with an increase of 40 percent in MS risk.
The researchers say their findings have important implications for public health, given the high prevalence of obesity in many countries, including the United States.
They add that since the median age of MS onset is 28-31 years, their findings should provide motivation "to combat increasing youth obesity rates by implementing community and school-based interventions that promote physical activity and nutrition."
Currently in the U.S., about 17 percent of children aged 2-19 years are obese. The researchers therefore suggest their study provides further rationale to investigate whether interventions that promote a healthy lifestyle could help curb MS risk.
The team concludes:
"Genetically elevated BMI is associated with risk of MS, providing evidence for a causal role for obesity in MS etiology. While obesity has been associated with many late-life outcomes, these findings suggest an important consequence of childhood and/or early adulthood obesity."