I've had this IRL discussion too many times, where the non-veg person says that a lack of meat affects their sleep, energy level, digestion, etc. I'm not a dickhead, and I don't argue with people's lived experience, even if it feels very sus.
I've done a good faith search for evidence of how certain people might have negative health effects from not eating meat, and nothing turns up. (Short of just having a bad veg diet) Maybe I'm missing something. It's just frustrating how often people use this as an excuse, and they're often anti-vegan in their wider ideology.
I get really politely pedantic and start asking them how they know this, what the nature of their experimental protocol and controls are, if they made any effort to do blinded testing. If they're smart they realize how silly their refusal to openmindedly consider their conviction is based on false assumptions is and if not they get big mad and make sillier claims that tend to get progressively easier to deboonk
Here's a 76 hour YouTube series that answers all these questions. No I don't have timestamps, you just have to listen to the whole thing.
If you're smart, you'll understand that um, aktuly beef is a universal panacea and salad cause cancer.