Opinions on what to tell the boss?

Wow.  Thanks for all the opinions!!! So when I went in to talk to my boss about time off today I bit the bullet and told her.  I have to say I was extremy surprised by just how supportive she was.   I really wasn't expecting it! Now ths other lady who works in the office is a friend and she's known for a while of my intentions so I'm good there.  I don't plan on telling my other coworkers though at this time.   Probably after.   

None of their business. I struggled with what to tell people as well. Once I decided to not be ashamed or worry about what others thought,it became very EASY to just up the info and proud of the fact that I was lucky to be given the choice of health and quit the battle with food.

This is a tough call. While you should be proud of the decision to have the surgery, it is a fine line with the surgery being thought of as "elective" and justifying the time off work.  They have absolutely no right to judge or deny the time off, but for some management, the resentment will occur.  (IMO)  The company I work for will be awesome when I finally get my surgery date set.  However, my direct supervisor tried to talk me out of a life-saving hysterectomy (ovarian cancer), so I can't imagine how he would be if he knew I was having VSG.   I'm glad your decision worked out for you!

My manager is extremely supportive but she's also a blabbermouth.  I have not told anyone in the office that I'm doing this.  No one will notice whether I eat or not b/c I often go out at lunch / eat at my desk.

I spoke to HR first to find out if I had to tell my manager anything, and at the end of the day all I told her was that I was taking a medical leave of absence.  No other details.  I told her I was 100% mentally and physically able to do my job up until the day I leave and then again upon my return.  That was it.  She was supportive and didn't ask any questions.

you don't need to tell anyone anything..you simply need a note that states you will be off (x) amount of time for a procedure.  If you don't want to tell her what is is for just say "personal" or "womens related issue". 

even working there for twenty years...you don't need to offer anyone more information that you are comfortable with and if you are stating that she can be judgmental, do you really want that along with the why and why not questions that may come from her.

I recently had my panni removal and a couple of really nosey co-workers asked me "are you okay, why were you off" I simply said "to deal with a bothersome ulcer"..sure it's a lie....but that ended the discussion and it's not like they will see me in a bathing suit and will notice the missing stomach.

Yours may be a bit different as you start to lose weight people may ask "are you ill" we noticed you are losing a lot of weight, at that point you might change your mind and tell them you had RNY, or you might simply state that you have taken on a new lifestyle and healthy eating...only you and you're loved ones and close friends ever need to know UNLESS you decide to share.

Whatever makes you most comfortable and at ease