That's what I thought too but it has been refuted by studies.
The normalisation of blood glucose has been noted to occur as soon as 48-72 hours post surgery and at most within a week. This goes against the previous belief that it was a weight loss effect.
I don't currently have Athens access to see the full article but this paper from the July 2013 issue of Science suggests a possible mechanism:
http://science.sciencemag.org/content/341/6144/406
One further mechanism that I have heard suggested is the alteration of gut microbiota:
That's interesting. Thanks for the follow up. I hadn't looked at this since 2008 or so. So you have any info on the refutation studies? I work at a hospital and am in a position to let people who are making these recommendations know about the info.
Perhaps refuted is a bit strong of a word for me to use! I think there is growing evidence that it is related to other factors than overall weight loss but that probably contributes too.
It's hard because I don't have academic access to journals right now as you know most of them are behind expensive paywalls! I think the best people to ask would be your endocrinologist colleagues - they can do a Medline search or if the hospital has a good library with a librarian they can do it for them to look for the latest reviews.
The other good source of information is bariatric surgeons they tend to be up to date with the latest research - that is how I found out about this information myself initially and I've read various supporting articles on Medscape etc.
Further in discussions I have had with my own endocrinologist who works directly as part of the bariatric team he is definitely of the opinion that weight loss is not the main cause of resolution. I might ask him for more information at my next appointment.
Actually I just found this article which suggests there may still be some controversy over it although it is from 2013:
http://care.diabetesjournals.org/content/36/Supplement_2/S287
Here is one from Nature Reviews but it is behind a paywall and the abstract doesn't really say much:
http://www.nature.com/nrendo/journal/v9/n10/full/nrendo.2013.159.html