Best approach for Picky Eaters

So im a picky eater, and I barely eat to be completely honest. But as I’m getting older I just feel it’s rude to not try someone’s food even if I know I shouldn’t cause it’ll make me sick or whatever. Like I’m diabetic I’m supposed to be staying away from sweets but co workers always are the worst. But what’s the best approach to say you’re not interested? Ive always try to dodge the situation like keep moving around the room. Sometimes I say I just ate, or I brought my own food today. What are some suggestions?