A woman drank 8 glasses of water a day and developed blindness? 6 truths about drinking water

Recently, a piece of news about drinking water has become a hot topic. The gist is that a woman in Changsha set an alarm to remind herself to drink 8 glasses of water a day for health reasons. As a result, she drank 1000ml of water at a time, which led to a sudden onset of acute angle-closure glaucoma. This disease can cause blindness in severe cases. So many friends are anxious. Can drinking water cause blindness? How should we drink water?

The picture comes from the Internet

Let me first say the conclusion. This situation is relatively rare. The main reason this time is that the woman already had high-risk factors for glaucoma. Ordinary people don’t have to worry about drinking water affecting their eyes.

So, what is the relationship between drinking water and glaucoma? It turns out that drinking a lot of water at one time will dilute the blood and reduce the plasma osmotic pressure, which will increase the speed of aqueous humor production in the eyeball, and indirectly lead to increased intraocular pressure. When this happens, ordinary people can adjust the amount of aqueous humor by accelerating metabolism to keep the intraocular pressure within the normal range. However, people who have high-risk factors for glaucoma have poor ability to adjust intraocular pressure and cannot eliminate excess aqueous humor in time. Drinking too much water at one time will increase intraocular pressure and may cause the disease.

Therefore, ordinary people's eyes will not be affected simply by drinking more water, but for people at high risk of glaucoma or patients, it is indeed necessary to pay attention to the principle of drinking small amounts of water multiple times.

It should also be noted that the statement that "drinking 8 glasses of water a day is good for health" lacks scientific evidence. As long as you drink small amounts of water multiple times and don't make yourself thirsty, you don't need to worry too much about whether you drink eight glasses.

Besides, with so many types of cups, who should be counted? | Tuchong Creative

However, for those friends who often neglect sleep and eat because of work or study, they do need to remind themselves to drink two to three hundred milliliters of water every once in a while to avoid dehydration.

In fact, in addition to "eight glasses of water a day", there are still many misunderstandings about drinking water. Below, we will answer 6 questions about drinking water, uncover the truth, and let everyone drink water clearly!

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01

What temperature of water is safest to drink?

It is best not to exceed 40℃

That's right, the safety of drinking water, in addition to water quality, temperature is also a big issue that cannot be underestimated. For example, drinking too hot water can easily cause problems. In life, many people like to drink hot water, scalding water, hot drinks, and hot soups. But what many people don't know is that hot drinks over 65°C have been listed as Class 2A carcinogens by the International Agency for Research on Cancer (IARC) under the World Health Organization (WHO), which means they may cause cancer to humans.

Our esophageal mucosa is very fragile, and too high a temperature will burn it. The most dangerous thing is that the esophagus is not sensitive to temperature, so you don't feel much when you get burned. Therefore, eating hot food frequently is a chronic irritation to the esophageal mucosa. Before the damage is repaired, it will suffer a new "severe injury", which will increase the risk of cancer in the long run.

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For the delicate esophagus, 10-40℃ is the most suitable, 50-60℃ is barely tolerable, and above 65℃, it will cause burns. Therefore, whether you are drinking water, soup or eating food, you must be vigilant: do not put hot food into your mouth, you can cool it down before eating, and it is best not to exceed 40℃! Before eating, you can test it with the back of your hand or your lips.

In addition, if you are too impatient to drink a cup of hot water, it may cause severe damage to your tongue and wallet. For example, according to a netizen, he used to drink a cup of warm water every morning, but on the morning of the incident, he was in a hurry and had no time to dry the water, so he drank a cup of hot water directly, resulting in a scalded tongue. The surgery cost more than 5,000 yuan. This water is really a double irritation to the wallet and tongue. I feel sorry for this netizen again...

▲Picture: From the Internet

02

Can’t you mix hot and cold water?

Just drink it with confidence!

Water is too hot to drink, but sometimes it takes time to let the boiled water cool down naturally! And the more thirsty and anxious you are, the slower the water cools down! You ask why not mix it with cold water to cool it down? Isn't there a saying that mixed hot and cold water is "yin and yang water", which will give you diarrhea if you drink it!

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In fact, it is completely possible to mix hot and cold water and drink it together without causing diarrhea, but the premise is to ensure that both the cold water and the hot water are boiled water or bottled water that meets the hygiene standards. If you do this, it is fine to mix water of different temperatures and drink it together.

But if it is raw water, it is hard to say, because raw water has not been treated with high temperature and may contain pathogenic bacteria, so it is not recommended for drinking.

03

Can’t drink boiled water or overnight water?

That's not true!

I believe many people have heard this statement: the nitrite content in repeatedly boiled water will increase. Nitrite is a carcinogen. If you drink such water frequently, it may cause nitrite poisoning. Nitrite will also turn into the carcinogenic substance nitrosamine after being acted upon by stomach acid!

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The explanation of the principle seems logical and clear, but in fact, whether it is boiled water or overnight water, it is safe to drink.

To verify this fact, someone conducted an experiment to test the nitrite content in water boiled by an automatic water boiler and a household electric kettle. The results showed that the nitrite content in municipal tap water was undetectable; after boiling and leaving it for 24 hours, the nitrite content in the water was 2.37 μg/L; after heating it for another minute, the content was 2.76 μg/L… and after boiling the tap water repeatedly eight times, the nitrite content was 3.38 μg/L. [1]

▲Figure: Sanitary Standards for Drinking Water

According to the "GB5749-2006 Drinking Water Hygiene Standard", the limit of nitrite in drinking water is ≤1 mg/L (1000 μg/L). Therefore, as long as the tap water itself is hygienic and the nitrite content does not exceed the standard, whether it is overnight water or repeatedly boiled water, the final nitrite content will still be within the safe range, and there will be no cancer risk if consumed as rumored.

04

Can drinking hot water improve metabolism?

Quite the opposite, actually.

There is a saying that "drinking hot water can increase metabolism and help lose weight" because of two reasons:

① Drinking hot water will make you sweat, and sweating will cause you to lose weight.

② They believe that drinking hot water will increase the body temperature, and when the body temperature rises, the metabolism will increase accordingly.

In fact, drinking hot water does not increase metabolism. If the body temperature rises, it will indeed increase metabolism, such as when you are sick and have a fever. However, drinking hot water has almost no effect on the body temperature of healthy people. Even if you drink more hot water, the body temperature is still constant, so the effect on basal metabolism is minimal.

However, surprisingly, the results of a human experiment showed that room temperature water has no effect on metabolism, while ice water can increase metabolism. This experimental result was also published in the Journal of Endocrinology and Metabolism. The experiment divided a group of healthy adults into three groups. Before the experiment, their metabolism was measured. Some people were given room temperature distilled water, some people were given room temperature saline, and the other group was given 3 ℃ ice water.

The results showed that drinking either room temperature distilled water or saline did not increase metabolism; however, drinking 3°C ice water increased the metabolism of these people by 4.5%, and this effect lasted for 60 minutes. [2]

Unfortunately, if you want to achieve your goal of losing weight by drinking cold water to increase metabolism, your idea will fail. Although cold water can increase metabolism, it is only 4.5%. If the basal metabolism of an adult woman is 1200 kcal, it can only increase 54 kcal. This calorie difference can be reduced by half a tablespoon of oil when cooking. There is really no need to drink more cold water for this purpose, especially for some people who feel uncomfortable drinking cold water, so it is not recommended to try.

05

Can soda water regulate the acid-base constitution?

This is exaggerated propaganda.

Some soda water merchants will claim that "alkaline soda water can adjust the acid-base constitution" and "drinking ×× soda water can improve the acid-base balance of the body."

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In fact, under normal circumstances, the pH value in a healthy human body will remain in a constant range. Eating and drinking cannot adjust the pH value of body fluids. For example, the pH of blood will remain constant at 7.35-7.45, and it will not change just because of drinking soda water. Unless you have a serious disease, such as cancer, but in this case, soda water alone will not change anything.

Therefore, the claim that drinking soda water can change your acid-base constitution is nonsense. Instead of worrying about what alkaline foods to eat, it is better to pay attention to food diversification and make your nutritional intake more balanced and comprehensive.

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06

Purified water has no nutritional value, so there is no need to drink it?

Drinking water also requires a positive attitude

Purified water is often complained about: It has no nutrition, what's the point of drinking it? Ah... this... do you expect to get enough nutrition by drinking water? The main purpose of drinking water is to replenish water. As for nutrients such as vitamins and minerals, they can be supplemented by food. Why bother with water?

Moreover, even if mineral water contains a little trace element, the content is very limited. According to the national standard "GB 8537-2018 Drinking Natural Mineral Water", the mineral components in mineral water must meet the following conditions:

▲Figure: "Standards for Drinking Natural Mineral Water"

According to the recommendation of the "Dietary Guidelines for Chinese Residents", the recommended amount of water to drink is 1500-1700 ml per day. Let's take 1500 ml as an example. If you drink 1500 ml of mineral water every day, you will take in 0.3 mg of zinc and 0.015 mg of selenium through water, which is 4% and 25% of the recommended zinc and selenium intake for adult women respectively. Therefore, drinking water to supplement zinc is basically negligible.

What about the mineral selenium? According to WHO survey data, about 1 billion people are currently facing the problem of selenium malnutrition. The global distribution of selenium intake is extremely uneven, and most countries are in a state of "hidden hunger" with insufficient selenium intake. The daily selenium intake of adults in my country is also significantly lower than the recommended selenium intake (RNI) of 60 μg/d for Chinese residents. [3] Therefore, if you want to supplement selenium, you need to increase your intake of aquatic products and egg and dairy products in your diet. As for water, it is recommended to choose selenium-rich mineral water as a supplementary source of dietary selenium.

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Summarize:

In the hot summer, we often sweat a lot, so it is very important to replenish water in time every day. So, don't be confused by the many rumors about drinking water. If you understand the knowledge of drinking water, you can drink water with confidence.

In addition, don’t wait until you are thirsty to drink water. Drink water anytime, anywhere and in small sips. Women should drink at least 1500 mL and men should drink at least 1700 mL a day!

Author | Xue Qingxin, member of Chinese Nutrition Society, registered dietitian, registered nutrition technician, health manager, public nutritionist

Review | Song Shuang, Associate Researcher, Institute of Nutrition and Health, Chinese Center for Disease Control and Prevention

Review | Jia Changkai, deputy chief physician, Xiamen Eye Center, Xiamen University

Editor | Ding Zong

References:

[1] Wu Fei, Wang Hui, Shu Jingwei, Wang Anshuo, Chen Wenjun. Study on the correlation between residual chlorine and nitrite content in tap water and heating conditions[J]. Journal of Food Safety and Quality, 2017, 8(11): 4458-4462.

[2]Brown CM, Dulloo AG, Montani JP. Water-induced thermogenesis reconsidered: the effects of osmolality and water temperature on energy expenditure after drinking. J Clin Endocrinol Metab. 2006 Sep;91(9):3598-602. doi: 10.1210/jc.2006-0407. Epub 2006 Jul 5. PMID: 16822824.

[3] Yuan Lijun, Yuan Linxi, Yin Xuebin, Qin Liqiang. Research progress on the physiological functions, intake status and countermeasures of selenium[J]. Progress in Biotechnology, 2016, 6(06): 396-405.

Scientific rumor refutation