In the opinion of SmartBarf, fasting is a natural thing for dogs in the wild, but, no, you do not have to fast your pet. You should certainly not fast young dogs as they need regular meals throughout the day. If you decide to fast your dog once a week, you will need to increase the size of its meals on the days that you do feed it. Alternatively, you might decide to fast it on stressful days eg long car journeys, vet trips, etc.