If you often eat leftovers, will it cause cancer

 If you often eat leftovers, will it cause cancer

A few years ago, you may have seen such a forwarding in your relatives, friends and colleagues wechat group - the next nights meal is fatal! Quickly forward it to your friends, overnight water is like poison! Stop drinking! u300b, if nothing happens to you, you cross the wechat rumor just as another ridiculous one.

Open the refrigerator. There are always some leftovers in it except for the ingredients

Should I pour the overnight dishes or eat them hot?

Is nitrite lethal

Some people think that, of course, overnight food can be eaten. Just change the way of thinking. Eating overnight food is equivalent to eating the dishes made in the morning in the afternoon, which can instantly eliminate the psychological burden of eating overnight food.

Yearning for life

And the most controversial part of overnight food is whether the content of nitrite exceeds the standard. I believe you often see this term.

Nitrite is an inorganic nitrogen-containing salt, which is not carcinogenic in itself. And we usually eat all kinds of food, such as vegetables, meat, eggs, milk contain trace amounts of natural nitrite.

The vegetables you eat usually also contain nitrite, but it is not harmful / unsplash

Under normal diet, the nitrite in human body is mainly the nitrate in food and drinking water, which is transformed under the action of bacteria in mouth and stomach. The real carcinogenic risk is the nitrosamine produced by nitrite in gastric acid and intestinal environment. [1

However, it is believed that everyone has heard the saying, it is rogue to talk about toxicity without dose..

In fact, as long as the content of nitrite is within the range of national standards, nitrite in food will not cause health risks. Trace nitrite can even be converted into nitric oxide, which is beneficial to health [2].

Although there is no standard for nitrite content of overnight food, there are clear regulations on the use limit and residue of nitrite in food in China and other countries around the world.

According to the joint food additive Expert Committee of FAO and who, the daily allowable intake of nitrite is 0-0.2mg/kg body weight.

The Chinese compulsory standard also stipulates that the content of nitrite in dairy products shall not be higher than 0.2mg/kg. In the production of meat products, the maximum limit of sodium nitrite used alone is 0.15g/kg; the residue of finished products shall not exceed 30mg / kg. [3

However, if a large amount of nitrite is ingested orally in a short period of time (by mistake or excessive intake), it is easy to cause acute poisoning, which is called methemoglobinemia. When the intake reaches 0.2-0.5g, it can lead to poisoning, and when the intake exceeds 3G, it can cause death [2].

So, whats the nitrite content in the overnight dish?

In a study of Suzhou University, taking three common dishes as examples, the nitrite content measured at different storage conditions and different time points is shown in table [2].

There is nitrite in the overnight dish, but the content is not so exaggerated / the selection and analysis of the determination method of nitrite content in the overnight dish

It can be seen from the table that although the content of nitrite increases gradually with the increase of storage time, the measured content of nitrite is far less than the national standard.

The nitrite content of the preserved dishes was lower than that of the normal temperature.

It can be seen that even if there are terrible nitrites, the risk of carcinogenesis and even death of leftovers is not one meal. Only long-term and large amount of overnight food can we worry about the intake of nitrites.

It is worth noting that the content of nitrite in carrot is lower than that in cabbage. In addition to the high content of nitrate in stem and leaf vegetables, they are easy to be reduced to nitrite, because the rich vitamin in carrot can remove certain nitrite [4

The real security risk of overnight food

Compared with nitrite, polycyclic aromatic hydrocarbons (PAHs) is a strange name. It is a highly toxic substance with carcinogenicity, mainly produced in incomplete combustion of hydrocarbons.

When you heat the overnight dish again, again and again, its produced. In all kinds of heating methods, smoking, barbecue and microwave heating after attached packaging will greatly increase the generation of PAHs [5].

Microwave heating is too common, but not healthy

The analysis of epidemic characteristics of food poisoning in the third quarter of 2018 in China shows that bacterial food poisoning accounts for the highest proportion in both the number of incidents and the number of people poisoned, and 81.25% of food poisoning is caused by food pollution, deterioration and cross contamination of raw and cooked food.

However, only 2.5% of the cases of food poisoning caused by nitrite were caused by the misuse of condiments.

There are many kinds of bacteria in leftovers. Air with mold, yeast, common Salmonella, E. coli and so on, inadvertently into the meal.

When eating, peoples saliva is the hotbed of bacteria breeding when they pick up vegetables; the continuous agitation of chopsticks will also make dishes more fully exposed to the air, increasing the possibility of bacteria breeding.

In addition, bacteria can also be produced in the cooking and storage of food. For example, there is a high risk of Salmonella breeding in the undercooked overnight loose eggs; there may be Escherichia coli, Bacillus cereus and other bacteria in the refrigerator [7] [8].

On January 9, 2017, a peddler thawed prawns in a seafood market in Qingdao. The seafood also had bacteria / SHiPA Photo Agency

These bacteria cause vomiting, diarrhea, coma and even liver failure.

For example, the incubation period of Salmonella varies from several hours to three days, which may cause high fever, diarrhea and other symptoms; Vibrio parahaemolyticus, which is often found in immature aquatic products of animals, may cause vomiting and abdominal pain; Staphylococcus aureus may cause meningeitis and other diseases after invading the body [9] [10].

According to a survey of 600 families, 88.7% of the respondents think that freezing can kill all or part of the bacteria in the food. But in fact, the vast majority of bacteria will not be killed at low temperature, but enter the dormancy period [11].

Once the food is thawed, the bacteria wake up, so the food that has been thawed for many times also contains a lot of bacteria - over and over again, putting the food that cant be eaten in the refrigerator cant toss the bacteria, you can only toss yourself.

On January 9, 2017, a peddler thawed prawns in a seafood market in Qingdao. The seafood also had bacteria / SHiPA Photo Agency

In addition to the risk of bacterial contamination, nutrient loss is also the reason why leftovers are not recommended.

The content of vitamin C in green vegetables will be prolonged and decreased gradually, and the loss will be faster after heating. Some vitamins, such as water-soluble vitamin C and vitamin B, are unstable. When they are heated when they contain more soup, the nutrition is easily lost [12] [13].

Ways to reduce the risk of overnight food

If you have to eat overnight food, you can also try to slow down the deterioration of food through some correct storage methods. At the same time, you should also pay attention to several foods with high risk after the overnight.

Seafood is delicious, but not storable / unsplash

There are also techniques and methods for storing and reprocessing food.

First of all, leftovers should not be placed for more than two hours at room temperature, or more than one hour in hot weather. After that time, the bacteria will multiply rapidly.

Even if its stored in the refrigerator, it doesnt mean everything is going to be OK. The temperature of the refrigerator where food is stored should be below 4 u00b0 C to inhibit the growth of bacteria.

However, different refrigerators have different refrigeration systems. Some researchers found that even if consumers know what the temperature should be, 21.5% of refrigerators do not maintain the correct storage temperature [15].

Therefore, the vegetables in the refrigerator should not be more than 24 hours, and the meat should not be more than 1 to 2 days.

A string of joss sticks is used to separate the ingredients into small containers, which helps keep the temperature low

And when the food is placed, keep the raw and cooked separately. You can put the cooked ones on the top and the raw ones on the bottom. This can not only prevent food from catching other odors, but also prevent the bacteria in the refrigerator from cross breeding.

We should also pay attention to the material of utensils containing leftovers. For example, overnight soup is better not to put salt or less salt, and kept in glassware, because the material of stainless steel or aluminum may react with salt, vinegar and other spices, resulting in the precipitation of harmful substances [17].

Glassware for vegetables is healthier than Metalware

In addition, attention should be paid to reheating food. Overnight dishes should be heated to at least 74 degrees for half an hour, or 85 degrees for 15 minutes to kill most of the bacteria. For overnight soup and other foods with more water, they should be covered and heated until boiling [18].

For nitrite in food, although whether the dishes are cooked or not, nitrite in vegetables and meat may increase during storage.

Fruits with high vitamin have many benefits, and reducing the harm of nitrite is just one of them / pexels

However, when eating the overnight vegetables with some fruits with high vitamin C content, it can also reduce the harm of nitrite in the overnight vegetables to a large extent.

In any case, eating leftovers is neither delicious nor healthy. Its the right way to solve the problem of eating by yourself.

This paper has been reviewed by Zhang Xiuning, master of public health, Shandong University


[1] Zhou Jiahua, Yang Huirong, & Li bina et al. (2011). Food additives (pp.1163). Beijing: Chemical Industry Press

[2] Lu Yuankai (2017). Determination method selection and content analysis of nitrite in overnight vegetables. Masters thesis, Suzhou University, Suzhou

[3] National standard for food safety and standard for use of food additives (2014). National standard of the peoples Republic of China. Gb2760-2074

[4] Zhan xiuhuan, Wang Ziyun, & Su Yajuan. (2016). Effect of vitamin C on nitrite content in vegetables under simulated gastric juice. Jiangsu agricultural science, 44 (02), 312-314

[5] Liu Yiqi, & Hu Changying. (2019). Research progress in the safety of polycyclic aromatic hydrocarbons in food. Food science, 40 (19), 353-362

[7] (2019). Which foods cant be eaten overnight?. food and fermentation technology, 55 (03), 115

[8] Wang Shuying (2012). Three kinds of refrigerated food are most likely to deteriorate. Decision making exploration (the first half of the month), 10, 88

[9] Wang Bin, Yan Bin, Lv Bin, Wang Guozhen, & Zhang Huifeng. (2019). Research progress of food borne pathogens. Journal of Jilin Medical College, 40 (05), 376-378

[10] Zhang Jingbo, Ren Yanfang, Wang Jinghui, Wei Zeming, & Wang Yongquan. (2019). Analysis of surveillance results of foodborne pathogens in Xicheng District, Beijing, 2015-2017. Chinese Journal of health inspection, 29 (22), 2795-2797 + 2801


[12] Yang Jian, Wang Zhifei, Cui Lin, & Wang Xuedong. (2005). Effect of heating on ultrastructure and nutrients of carrot. Acta electronica Sinica, 04420

[13] Mo Baoqing, Zhou Jingping, & Zhou Ming. (2015). Effect of repeated heating and storage on the nutritional and hygienic quality of stir fried green vegetables. Chinese Journal of health inspection, 25 (06), 810-812 + 815




[17] Shenyang daily. (2016). Corrosion prevention: use stainless steel pot to put salt later