Stanford folks studied a bunch of bariatric surgery patients in their alcohol consumption. They discovered that their tolerance went down. I remember when I lost my 50lbs(a few years ago) I couldn't handle caffeine and gave up coffee drinks right away. I also noticed that I couldn't drink as much before getting tipsy.

Bariatric surgery patients have less alcohol tolerance
One drink is enough to put many at the legal limit, study says
Erin Allday, Chronicle Staff Writer June 19, 2007