What if OP isn't fat? It is possible that the co-workers are just douchebags.

You are a victim of workplace harrassment. Report it to your HR dept right away. That type of behaviour is inexcuseable.

That's horrible! It's not up to them to decide what your weight is, don't listen do what you want!

Somewhere in your office one of your co-workers is walking aorund thinking to themselves, "where the hell did I leave my SlimQuick?"

Mean and cowardly. FYL for having to work with that type of people.

Well your coworkers are asshole. Regardless of whether they are wrong or not. This is very rude.

People need to keep there opinions to themselves. They may have been trying to help you without actually saying anything,they have gone to the trouble to buy them for you and don't wan to talk about it because they or you may be embarrassed. That said they really they have no right to make judgement on your weight. Do what you need to do and when you feel you need to do it. Don't do anything on their agenda because only you know when you have they motivation to do it. If there is no motivation nothing will change. Good luck :)

Plot twist: somewhere one of OP's diet-crazed co-workers is scratching her head and saying, "Now where did I put my $100 supply of slim quick...?" She then goes to post an FML about it but sees this first and realizes what has occurred.

Next day i would bring the biggest cake i could find and eat it in front of everyone.

Your weight is none of their business. If they don't like your size, they should suck it up.