Steinberg also noted that the serotonin found in the brain is not linked to what's happening in the digestive tract serotonin. He said it's strictly reflective of "periphery serotonin," and that the serotonin found in the brain, some five per cent of total serotonin in the body, does not interact. The two pools of the same hormone are separated by what's called a blood brain barrier, and do not mix.

It leaves the question of the link between depression, serotonin levels and obesity up in the air, something Steinberg says he can't answer just yet.

So my understanding is, "probably not, but they can't be entirely sure yet."

Indeed, there may be some connection there. But frustratingly, we don't really know what the connection is, or if it has multiple factors--and if so, how those factors relate to one another. To date (to the best of my knowledge), reports of weight gain from people taking SSRIs have been anecdotal. There is a large enough sample size to draw a clear connection; the weight gain isn't imaginary. (It occurs in about 25% of SSRI users, and the risk for weight gain increases over the duration of SSRI usage.) But there haven't been controlled, clinical studies of of the effect. Do SSRIs affect metabolism in some way? Possibly. Do they affect appetite instead, or in addition to, metabolism? Possibly. Do they affect gut microflora? Possibly. Could be any of these things; could be all of these things; could be something we haven't even hypothesized yet. There is a relationship; we don't yet know the directionality of cause and effect, or the factors involved, or their individual significance in the causal web.

These new results are fascinating, and they give further impetus for further study.