Absolutely you should socialize. Isolation is bad.
Do your companions really care what you're eating and drinking? If I'm invited to a chain restaurant I just forget any chance at decent food and will nurse an iced tea, seltzer, or wine while chatting. Eating is never so urgent to justify toxins--I can always cook before or after.
If your friends truly need to watch you chew and swallow it's only because of precedent. If they become accustomed to you not doing so then they'll stop expecting it. Don't discuss your nutrition strategy, just say "not hungry" or "already ate".