Many doctors strongly urge MS patients to exercise. What are the results of exercise? Better general health. Now there is a study that concludes that reduction in brain size corresponds with Body Mass Index (amount of body fat).

Study:
http://n.neurology.org/content/91/24/e2256.long

Conclusions:
Higher BMI appears to be associated with greater reductions in nGMV and nBPV, which is relevant because, in particular, nGMV loss portends greater longer-term disability. Because obesity is modifiable, further studies should explore these relationships in detail, and evaluating the effect of reducing BMI on imaging and clinical outcomes in MS may be warranted.