It would of course be highly inappropriate to fast a working dog and / or a high-energy sporting dog. They need the fuel.
I fast once each year on Yom Kippur. No harm done.
Careful. People with working gun dogs know better, fasting depends on the diet.
Do I have to fast my dog the day before a hunt? No. I free feed and she eats when hungry, but it is not that, it is the composition of the feed. It depends on the ingredients in the feed and the amount of expansion taking place from fiber.
Do raw feeders that hunt their dogs have to fast the dog before a hunt? This would include commercially store purchased meat part and organs whole prey model. Answer is no. There is no expansion type fiber. There is fiber from bone and tissue, but that is all good. What is not good are potentially dangerous expansion type fibers.
Do kibble feeders have to fast their working gun dogs the day before a hunt or at least a preset number of hours to fast their dogs before and after the hunt? The answer is yes, and this is considered fairly common knowledge I would hope.
The reason is bloat and it can kill your dog. Without getting into the promotional data science about what causes bloat and how to avoid it, just trust at least this much; expansion of the food does take in the dogs stomach and you have to be very weary of that when it comes to exercising any medium size dog whether it be sporting breed or not. Probably the biggest offender and ingredient to be most concerned about in kibble diets is beet pulp, followed up by tomato pomace. Now if you listen to some folks, the backers of omnivore nutrition, at some of the popular websites, the very diets recommended people put a lot of trust in contain the very ingredients to be concerned about when it comes to bloat and expansion type fibers. Kibble/Water/Exercise can be a dangerous combination for certain breeds, and that goes for the sporting breeds as well.