The leftover food becomes saltier the more it is reheated, mainly because when a dish is freshly cooked, much of the salt is in the sauce. Letting it sit overnight allows more salt to seep into the food, increasing its saltiness. When it is reheated, the water evaporates, concentrating the sauce. This highly concentrated, salty sauce then coats the food, making it taste saltier. Leftovers, especially leafy vegetables, have a high nitrate content. Over time, these nitrates can convert into nitrites. Although nitrites have a slightly salty taste, this saltiness is not enough to account for the increased saltiness of the leftovers. It is important to note that nitrites are harmful to the human body and can cause food poisoning, so leafy vegetables are best cooked and eaten immediately.
It is recommended that to prevent leftovers from becoming saltier upon reheating, the most important step is to use less salt and other salty seasonings during the initial cooking. For most leftovers, you can discard the sauce and add a small amount of water before reheating.