This is one of those questions that take the cake, even if you don't. If you were truly antisocial, you would not be going to these cake-fests in the first place. Rather than antisocial, you are merely anti-carb, and you have a perfect right to take this position.
However, it will be helpful to take a second look at your behavior in these get-togethers. If you are subtle about your desire to be cake-free, then the problem truly rests with your co-workers. But, if you get on a soapbox and start ranting about the perils of carb consumption, that is another story.
Assuming that you have not opted for the soapbox, you should continue to go to these parties for your co-workers, and continue to be upbeat, positive, and friendly. People who know you should have no problem understanding that your desire to avoid carbs does not mean that you desire to avoid them.
Many companies are actually trying to encourage their employees to eat healthier foods in these types of gatherings. If you ever throw one of these bashes, maybe you could alter the menu. This does not automatically mean serving kelp or tofu, but maybe serving something from your diet could be fruitful, even if you are not actually serving any fruit.
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