In our daily life, many people have such a habit of eating the leftovers of the previous night the next day. In fact, not all the leftovers of the previous night are suitable to be eaten the next day. Some leftovers may lead to poisoning. 

White fungus soup is a kind of high-end nutritional supplements. However, the nutrients will be reduced and harmful substances will be produced overnight. White fungus, whether planted inside the house or cultivated in the open air, contains rich nitrate. The nitrate can be transformed into nitrite through the decomposition of bacteria if the boiled white fungus soup is put aside too long. If people drink such a kind of white fungus soup, nitrite will come into the blood circulation to oxidize the normal hemoglobin into methemoglobin, which can result in the loss of hemopoiesis of the human body.

Dishes including seafood, green vegetables, and cold vegetable dish cannot be eaten overnight. The seafood of the previous night can produce the degradation products of protein the next day which can hurt liver and kidney. Green vegetables of the previous night can also not be eaten the next day. Vegetables contain much nitrate. If green vegetables are cooked for a long time, the vegetables not only become yellow and produce abnormal taste, but also make nitrate change into nitrite which can lead to the cancers. For cold vegetable dishes, they have been polluted during the process of processing. It is greatly possible for them to go bad during the whole night.

Even the boiled water of the previous night can not be drunk the next day. The boiled water should be boiled for another 3 to 5 minutes so as to reduce the harmful substances like nitrite and chloride to the lowest extent. According to some experts, the content of nitrite contained in boiled water is much higher than that contained in raw water. And if the boiled water is boiled too long or is kept for more than 24 hours, the content of nitrite will be increased largely. It is found that the content of nitrite in already boiled water is 1.3 times higher than that in freshly boiled water. Nitrite can form nitrosamine which can cause the cancers inside the human body. Many people like to drink boiled water all over the world. Therefore, in order to keep healthy, people should drink the freshly boiled water. The water which has been boiled for a quite long time should not be drunk.