What an opportunity and an adventure! I've never been to Kenya but I would presume that hospitality is a cornerstone of the cultures there; soooo, I'd try to at least try everything they offer. But if you truly cannot, tell them it is for a health concern- and then never ever let them see you try the dish later. If you're especially picky when they offer food, it may come across at first-world snootiness. At least this has been my experience in other countries, long before I ever went PB.
Bottom line: limit unhealthy foods as you can, keep good snacks on you, but enjoy the trip and accept the locals' hospitality. =)