I agree with what all the other posters have said, but I want to add one thing:
For me, what made me stop feeling bad was working up the courage to respond--politely but strongly--to those comments. If you can figure out a sentence that works for you, so you don't have to think in the shock and hurt of the moment, you can stop people in their tracks. After a few times, they stop doing it any more.
Here are a few possibilities:
'Thanks, but I get my medical advice from my physician.'
'Thanks, but I'm quite aware of my size, and of the risks. Fortunately, they are less than the risks of the eating disorders I suffered from earlier.'
'I'll let you know if I'm looking for nutritional advice.'
And the least polite, which my father once told me I had permission to say to the school nurse: 'Better to be fat in the ass than fat in the head.'