Bad aftertaste

I had a slightly odd encounter with a patron at MPOW, and I feel I should have coped with it better. The guy was obviously a hippie type (woolly clothing, patchouli facial hair etc) and, after a straightforward directional enquiry, he leant over the counter and pointed at my bottle of Diet Coke.

"You don't want to drink that you know, full of aspartame."

I was slightly taken aback and muttered something about having drunk so much of the stuff it would have killed me by now, which he followed up thusly.

"I'm a nurse and all my MS patients drink this stuff. Seriously."

I said something else non-commital and he went on his way, but it's been bothering me since for a number of reasons. They are a bit of a tangle, so I'll revert to tried and tested list format.

1. I know the aspartame scare stories are bullshit, but I didn't want to offend the bloke by telling him so. This was partly natural reticence but mainly, I think, because I was trying to be professionally nice at the info desk. But hadn't he shifted the conversation onto a personal level in the first place? Should I have been more bullish in expressing my trust in evidence-based medicine and all the benefits it has brought the world?
2. There is an odious undercurrent in his attitudes towards his patients. "The poor woman has MS but, you know, if she hadn't drunk all of those diet drinks...". It sounds like a blame-the-victim framework to me. I also find it worrying that a professional nurse (if indeed he was) could weigh scare story internet memes above the medical literature.
3. Who looks at somebody as obviously overweight as me and thinks "You know what that bloke's main health problem is don't you? Nutrasweet." But he wouldn't have called me fat would he? That would be rude.