Rumors and truth about leftover food: It can cause cancer and anemia

Can we eat leftover food? I believe that if this question is raised, you will definitely receive a lot of teachings in the "Loving Family" group, such as leftover food can cause poisoning, leftover food can cause cancer, and I have even seen news that leftover food can cause anemia.

Leftover food seems to be a taboo for healthy eating. However, is this really the case? This article will delve into the truth about leftover food and unveil the mystery of leftover food for you.

First, let’s talk about the cancer-causing effects of leftover food.

Nitrite, the name sounds scary. Indeed, nitrite may be converted into nitrosamine under certain conditions, which is a known carcinogen. However, this does not mean that all foods containing nitrite are carcinogenic.

The reason why leftover food is carcinogenic is mainly because it contains nitrite. Where does nitrite come from?

Certain vegetables naturally contain nitrite, such as beets, cabbage, carrots, celery, spinach, etc. These vegetables are also our main source of nitrite. Within a safe dose (the World Health Organization recommends that the upper limit of daily nitrite intake is 0.06-0.07 mg/kg), nitrite can serve as a nitrogen source for the human body, replenishing the essential element nitrogen for the human body, and plays an important role in the normal function of the cardiovascular system; and in fact, the nitrite in the human body is mainly produced by our body itself. [1]

The fundamental reason why nitrite is harmful to the body is that it is broken down into nitrosamines after entering the body, and the vitamin C in vegetables can inhibit the reaction of nitrite turning into nitrosamines. Therefore, there is a dynamic relationship between nitrite and vitamin C in vegetables. Vegetables rich in vitamin C can better keep nitrite "harmless to humans and animals."

my country's food hygiene standard GB 2762-2022 "Limits of Contaminants in Food" has removed the limit on nitrite content in fresh vegetables and meat, and the residual standard in pickled vegetables is 20 mg/kg. This shows that we are not worried about the threat of nitrite in fresh vegetables to the human body. In addition, experiments have found that the nitrite in overnight vegetables will not exceed the limit.

The Hong Kong Food Safety Center published an experiment[2] that tested the changes in nitrite in Chinese cabbage, amaranth, cauliflower, and vegetable soup when stored at room temperature and under refrigerated conditions. The results showed that:

When stored at room temperature, some cooked vegetables and soups began to show detectable increases in nitrite after 12 hours of storage;

Under refrigerated conditions, low levels of nitrite were detected in some samples only after 72 hours of storage.

This means that there is no need to worry about nitrite in overnight dishes. At the same time, the Hong Kong Food Safety Center recommends

From a food safety perspective, it is recommended that leftover food should be cooled and refrigerated within 2 hours after cooking and consumed within 3 days;

The storage time of cooked vegetables at room temperature should be limited to 4 hours to avoid the growth of harmful bacteria and the formation of toxins.

However, it should be pointed out that nitrite as a food additive is still subject to limited standards. According to the "National Food Safety Standard for the Use of Food Additives" GB2760-2014, the residual standard in processed meat products is sodium nitrite ≤ 30-70 mg/kg.

Can overnight food cause anemia?

In fact, if you look at the news carefully, you will find that the protagonist of the news has eaten less meat (or animal food) for a long time. Animal food is the only source of vitamin B12. Macrocytic anemia occurs due to vitamin B12 deficiency, which has nothing to do with whether the food is left overnight.

The real things to note about overnight dishes

We should try to eat fresh food, but there is no need to create unwarranted panic. We need to know what we should really pay attention to when preserving food:

(1) Do not wait until the food is cold before putting it in the refrigerator to prevent microbial growth

Leaving food at room temperature can easily lead to food being contaminated by microorganisms in the air, or certain microorganisms multiplying in large numbers beyond the human body's tolerance limit. Although stir-frying again can kill some microorganisms, the toxic substances they produce may not be eliminated by high temperature.

Therefore, do not wait until the food is cold before putting it in the refrigerator, and it is best to heat it up before putting it in.

(2) Food in the refrigerator should be separated with plastic wrap or lunch boxes with lids to avoid cross contamination.

(3) Clean the refrigerator regularly

Don’t think that the refrigerator is absolutely safe. Some cold-resistant bacteria (such as Listeria) and molds are not afraid of low temperatures and can still multiply in the refrigerator, contaminating food and causing food poisoning.

(4) It is best not to keep food for more than 2 days and discard it immediately if you notice any odor.

(5) Certain special foods cannot be stored for a long time, such as soaked black fungus. The best soaking time is within 1-2 hours, not overnight. Otherwise, it may cause aflatoxin poisoning, which has a mortality rate of 40%-100% and there is no specific antidote.

(6) Food should include a combination of meat and vegetables to ensure a balanced diet.

Leftover food is not a taboo in healthy eating. The key lies in how we handle and preserve it correctly. Through scientific understanding and reasonable operation, we can enjoy delicious food while ensuring the safety and health of our diet. Let us treat leftover food with a more rational and scientific attitude and enjoy a healthy life.

References:

[1]Ma L, Hu L, Feng X, et al. Nitrate and Nitrite in Health and Disease. Aging Dis. 2018 Oct 1;9(5):938-945.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6147587/pdf/ad-9-5-938.pdf

[2]Centre for Food Safety. Changes in Nitrate and Nitrite Levels of Cooked Vegetables during Storage. 2022.

https://www.cfs.gov.hk/english/programme/programme_rafs/files/programme_rafs_fc_01_46_1.pdf

Author: Jiang Yongyuan, Master of Internal Medicine, Third Military Medical University

Reviewer: Ruan Guangfeng, Deputy Director of Kexin Food and Health Information Exchange Center

The article is produced by Science Popularization China-Creation Cultivation Program. Please indicate the source when reprinting.