You break this when first time meeting someone

What is it that has no weight, can be seen by the naked eye, and if you put it in a barrel, it would make the barrel lighter?

When alone, I can be honest. When I'm with friends, I could've bought you fresh croissants back in the day. And no matter how many we are, always at our best with a dab of mustard.

I help to power your brain and every stride, but too much of me, and pants grow wide. You burn me fast on treadmills and in gyms, then count me up with meals to look slim.

Look, I’m an artist. I need to make art to eat. That’s how I spin it!