The simple answer is don't offer.
The complicated answer is saying: "I'd like to give you some of my food, but I want to hold on to it, so please say no."
I hate it more when I offer food and someone says no I'm on a diet. I always think "Bitch just say no thank you. You don't have to make me feel bad for cheating on my diet."
The complicated answer is saying: "I'd like to give you some of my food, but I want to hold on to it, so please say no."