Overnight food is not harmful if it has not spoiled within a certain period of time, but if it has spoiled, bacteria multiply in it and can produce a lot of nitrite. Usually, overnight food is not harmful if it has not been spoiled within a certain period of time, but if it has been spoiled, because the food has been cooked, it is not only easier to be digested by the human body, but also easier to be digested by the bacteria, so overnight food often contains a large number of bacteria. After the decomposition of bacteria, overnight vegetables can produce a large number of nitrites, which, after being ingested by the human body, can oxidize the low iron hemoglobin in the blood into high iron hemoglobin, losing the oxygen-carrying capacity, which in turn causes tissue hypoxia. Therefore, the deterioration of overnight food should be avoided, eat fresh meals, daily cooking according to the amount of food to do, try to avoid leftovers.