Like spinach, beets also contain nitrates, which are very useful for your health, but these nitrates become extremely harmful with reheating.
Because after reheating the chemical composition in beet changes, resulting in foods that are non-edible and dangerous. When the beet is reheated it can become very dangerous for your body, as the whole amount of it can become completely nitrate.
When heating is performed, under the action of microorganisms, it transforms nitrate molecules into nitrosamines. They are carcinogenic to the human body. Beet is among is the most harmful vegetables after reheating so it is recommended to be consumed best when fresh. This doesn’t mean that you should not eat this food like other food, but only you should avoid being reheated.