It is not good at all, but it will reduce the exercise effect. Muscle exercise mainly provides energy with sugar. After a night of digestion, the sugar content in the body is the lowest in a day. If you don't eat breakfast, the sugar content will be lower. If you practice push-ups and sit-ups in this state, your body has no extra energy to support you to do more quantity and quality, and the exercise effect will definitely not catch up with the usual best effect. If you exercise too much, it will easily lead to hypoglycemia. So it's not only bad, but also bad for fitness.