Late - but I thought I'd jump in, anyway.
Until I reached 'middle age', I'd enjoyed a high metabolism rate and was relatively thin, although I always had a good appetite. I began adding pounds in my early 50's - until I was a good 30 pounds overweight by the time of my diagnosis (at age 63). Prior to my AN diagnosis and subsequent surgery, I'd lost the extra weight I'd put on over the past 20 years or so. This fairly rapid weight loss was a result of the loss of my sense of taste (and the resulting loss of appetite) that I'd experienced in the months leading up to my diagnosis . Food tasted like cardboard and so, I just wasn't hungry. Eventually, my stomach actually shrunk (I lost my 'pot'). I later learned that my loss of the sense of taste was due to the AN impacting my 7th cranial nerve (that can affect taste). Post-op, my sense of taste returned and although my appetite returned with it, I managed to avoid a big weight gain (without conscientiously dieting). When I asked my PCP about my (post-op) weight level, he opined that the traumatic brain surgery I'd just been through had possibly 'triggered' my metabolism back to it's original (high) level. He also mentioned that I'd become used to eating less, which helped keep excess weight off, once my sense of taste returned, along with my appetite. This may or may not be 'scientific' but it made sense to me.
Why some AN patients gain weight post-op and others don't is a bit of a mystery to me, except that it may be connected with being housebound for a time (and bored) as well as simple genetics and physiology. I maintain that skipping desserts and snacks, eating smaller portions and getting some exercise, even just walking, can help keep one's weight down - but I'm no expert. Just ask my wife (who looks good but has been trying to lose some weight for years). You can guess how she feels about my being able to maintain a stable, healthy weight without a strict diet. Suffice it to say that we don't discuss weight in my house, anymore.
Jim