As a former college student who carried a backpack stuffed with textbooks and a laptop sometimes for miles around campus, I didn't notice any difference between when my thermos (carried in backpack side pocket) was empty vs when it was full, and that's 20 floz of liquid which is roughly equivalent in weight to the glass containers I use for food prep.

When people complain about how soft modern society it, it's this crap that they're talking about. "Oh I already carry around 30 lbs of stuff, 31 will break my back!". If you're carrying around so much that 1 lb will literally make the difference between injured and not injured, you need to seriously re-think your system. Possibly save up for a better backpack, possibly learn how to carry weight and do some exercises to strengthen your back, possibly re-arrange your schedule so you aren't carrying as much weight at one time.

And I'm serious about the back strengthening, people (including myself) ruck-march with 20-40 lb packs for exercise, and our backs aren't falling apart. Lifting and carrying weight with good form is great for your back, and strengthens it to the point where you can even use bad form for limited time and avoid injury.