I assume you are talking about invitations to Qatari houses and the etiquette there. They will offer drinks and food to guests. Usually tea/coffee, sweets and pastries. As suggested above, just let them know gently what you do take as part of your diet (make it simple like water, vege and fruit). This is so that they, as hosts, have something to offer you which you will eat and drink. Qataris are exceptional hosts and unlikely to get offended if such things are explained (and given alternatives) in a polite way to them. If possible, forewarn them (at the point of invitation) if you are going there for a meal.