That's because there is food residue calcium carbonate in the mouth and teeth. Fitness can cause dehydration. When the mouth is dry, calcium carbonate will produce a bad taste, dehydration will lead to a decrease in saliva, increase the reproduction of anaerobic bacteria in the mouth, and lead to an increase in breath.

Not only for fitness, but also for people who don't drink water in their lives. In fact, many people who exercise have breath, not necessarily smelly, but at least not fresh. Just look at the answers to similar questions, they don't know and don't take them seriously.