How can low-carb diets be harmful to your body?

Yes: when on low carbs, your body might not have enough glucose to run the metabolism of your brain, your heart, your kidneys, other organs. It might rely on the breakdown of fats for energy- called ketoacidosis. It is not the best way to get energy and it can lead to problems in organs mentioned, esp the kidney if you dont have enough fluids to flush out the ketones.