Well, if you think about it, there are loads of different causes of (for example) heart disease - if you can narrow down your chances of getting heart disease, or senile dementia, or cancer any other disease by changing one thing, surely the research is valid and useful?
My grandmother suffered from senile dementia and she was obese. It affects all kinds of people, and probably for all kinds of different reasons. If we can find out what some of these reasons are then we can all take steps to reduce our chances of being affected, whether it's by losing weight, stopping smoking, taking up exercise etc. Funnily enough these 3 seem to crop up as ways to reduce the dangers of pretty much everything. This does of course make them obvious subjects for research.
