While reheating food is not necessarily unhealthy, it can be dangerous if not done properly. One of the most important factors in safely reheating food is ensuring that the food is promptly refrigerated or frozen within two hours of cooking, according to Foodsafety.gov. Some foods lose nutrient value during reheating.
Refrigerated foods should be reheated and eaten within three to four days after initial cooking, while frozen foods stay edible for up to four months, suggests The Health Site. Frozen leftovers should be thawed in the refrigerator for several hours or defrosted in a microwave while covered. To be sure that potentially harmful bacteria have been eliminated by reheating, use a food thermometer to check that the temperature has reached a minimum of 165 degrees Fahrenheit.