Not that kind of temptation... what kind of blog do you think this is? I mean the temptation of sweet treats and baked goodies that co-workers bring in. Donuts, cake, candy... there free for the taking. It might be tough but you have to just walk away. It can be a slippery slope because just taking one little slice can can quickly turn into another, and another. And suddenly your plan to eat nothing but real food is derailed.
Sometimes in a situation like someone's birthday, you feel awkward saying no. So just say what you need to say. Often, "No thanks, I'll pass" is enough. Or just say you're trying to lose weight. Most people understand that instantly.
Keep a couple of healthy treats in your desk for situations like this so you don't feel completely deprived. Kind Bars are good alternatives. Or maybe a few dark chocolate covered almonds.