"The Duke team concluded that depression, therefore, is more likely to contribute to inflammation in the body as opposed to arising as a consequence of inflammation."
But saying that because they didn't find a pathway that inflammation is unlikely to cause depression doesn't really make any sense, given that major triggers for depression are surgery, infection, autoimmune diseases, etc.
I don't think anyone is disputing that depression also causes inflammation, but the idea that the causality only goes one way just seems very counter to everything else we know.
The point is that there's no clear causation in either direction, and certainly not in even a significant minority of cases (let alone all or even a majority). We just don't know nearly enough about how the brain works to be able to chalk up something as complex as major depressive disorder to "your brain's inflamed; take some Aleve".
In fact, the DSM definition of major depressive disorder/episode (or at least this summary thereof (I unfortunately don't have a copy of any DSM version on hand): http://www.mental-health-today.com/dep/dsm.htm ) explicitly excludes cases where the symptoms are caused by some other physical case:
"Note: Do note include symptoms that are clearly due to a general medical condition, or mood-incongruent delusions or hallucinations."
I would think brain inflammation would count as a "general medical condition" in this context, though whether or not "clearly" is applicable is admittedly very unlikely. Regardless, the DSM seems to maintain a distinction between "depression symptoms caused by some other disorder, whether psychological or physical" v. "only exhibiting symptoms of one or more major depressive episode(s)". In the former case, yeah, totally work on fixing that inflammation with lifestyle changes or whatever. In the latter case, it ain't really that simple.
I think the article I linked to in my other comment has it correct, where depression should be considered as something that exists within all people on a spectrum rather than as a mental illness. Now obviously if depression gets to the point where it's causing health problems and interfering with your ability to work and enjoy life then that may be a diagnosable issue, but it's a mistake to think of depression as being something that you either have or don't have in the more general case.
"The Duke team concluded that depression, therefore, is more likely to contribute to inflammation in the body as opposed to arising as a consequence of inflammation."