Hmmm, being on the medical side of this, I have a different view.
It's a fact that whatever medical condition you have, they will be more difficult to treat if the person has depression. The ratio of people likely to have depression in a certain setting does affect our way of searching for a solution. Whether we can deal directly with the medical condition itself, or are we going to have to consider the mental state of the patient to a large extent so that it will change the choice of immediate therapy?
So of course, someone is going to dig into the Qs that come up this way.
Using statistcal studies to prove something is also one of the main streams of medical research now, called evidence based medicine. Statistical data are taken (they have to meet a number of requirements) then the data analysed, and conclusions drawn as to which method of diagnosis or therapy is the most effective, in time, in cost and in the resolution of the particular illness.
How to apply that data is another thing. We always have to keep in mind that, if say, 25% of patients suffering from a certain disease is likely to be sufferin from depression as well, one in four patients that we see could have depression as well, and we need to treat that as well. If the data is 10%, then we watch for that one out of ten. For the patient him/herself, though, if you have depression as well as, then that's 100% for him/her, while if he/she doesn't, then it's 0%.
While I disagree on the finding that obese people have a lesser rate of substance abuse (in the case of morbid obesity, there is a contention that food itself is the substance being abused so, if that's taken into account, the data is likely to change) the general statistical data is interesting, and it helps shape the drs minds in what to look out for.
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Study shows strong link between obesity and depression - Newstarget.com 
So long as you are putting your best foot forward, then, 
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