Is "8 glasses of water a day" true knowledge or pseudoscience? Let's clarify the matter of drinking water today!

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I believe everyone has heard of the phrase "8 glasses of water a day", which was once regarded as a golden rule for drinking water. However, in recent years, some people believe that "8 glasses of water a day" is a pseudoscience that is harmful to people .

So, is there any scientific basis behind "8 glasses of water a day"? How much water should you drink every day to be healthy? Today, I will analyze this rumor that everyone has heard for decades from its origin.

analyze

Where did this saying first come from?

First of all, this saying also has a "Western" origin. The 8 glasses of water here refers to a standard glass of 8 ounces of water, which is approximately equal to 240ml of water. In fact, the idea of ​​8 glasses of water originated from the National Research Council of the US Food and Nutrition Board in 1945, which issued a recommended standard for water intake, that is, 1ml of water is needed for every kilocalorie of energy consumed. For example, according to the standard person's 2000 kilocalorie energy intake, the corresponding water intake is 2000ml, 2000/240≈8!

This is how the Western saying of 8 glasses of water came about!

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In our country, there is also the saying of drinking 8 glasses of water, which actually has some basis.

The average adult loses about 2.5 liters of water through urination, defecation, evaporation from the body surface, breathing and sweating, so we need to replenish water accordingly. In addition to the water generated from food intake and metabolism, we also need to drink water actively. From July to August 2010, researchers from Peking University, China Center for Disease Control and Prevention and other institutions conducted a survey on the drinking water status of adults in four cities in my country. The data showed that the amount of water consumed by adults in my country accounted for 56% of the total water intake; the median amount of water consumed was 1488 ml, of which men were 1679 ml and women were 1370 ml. Based on the results of this survey, the research team recommends that under mild climate conditions, adult men with light physical activity levels drink 1.7 liters of water per day and adult women drink 1.5 liters of water per day. If 1 cup of water is about 200-250 ml, then the above recommended amount is roughly 8 cups of water per day.

Later, this recommendation was also written into the "Dietary Reference Intakes of Chinese Residents (2013)" and the "Dietary Guidelines for Chinese Residents (2022)", becoming authoritative data in the field of nutrition in my country.

The problem is that these 8 glasses of water do not mean that everyone must drink 8 glasses of water, more or less will not work.

The reason why we use the phrase "8 glasses of water a day" is that it is easier to remember than the specific volume of water you drink. So, when you say "8 glasses of water a day", you should pay attention to the volume of the cup.

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In addition, the amount of water that everyone drinks should not be the same. In fact, there are individual differences. Many factors will lead to differences in the amount of water needed by each person:

1. 2000 kcal of energy corresponds to 2000 ml of water. The energy requirements of each person vary.

In fact, the total amount of water we consume every day includes the natural water in food, soup, milk, etc., as well as the water and beverages we drink. If the food and water intake is high, it does not mean that you must drink 8 glasses of water.

In addition, each person's energy requirements vary, so calculating based on energy metabolism water is not accurately suitable for everyone.

The proportion of water intake from food varies significantly due to differences in eating habits among different countries, cultures and individuals. The cooking methods of Chinese residents are mainly steaming, frying and stewing, which not only retains most of the original water in the food, but also adds extra water during processing. Therefore, the water intake from food by Chinese residents may reach more than 40% of the total water intake. In contrast, Western fast food is often cooked by deep-frying and pan-frying, which will reduce the water intake from food.

Therefore, "water intake" and "water intake" are not the same, but there is a big difference. If we do not distinguish, we may misunderstand (or let others misunderstand) some numbers. For example, the "Dietary Guidelines for Chinese Residents (2022)" recommends that the total water intake of Chinese residents is 2.7 to 3 liters a day, but it also states that this includes drinking water and water in the diet. The 1.7 liters and 1.5 liters mentioned above are recommendations for drinking water alone.

2. Water intake and total energy intake may not necessarily correspond one to one.

In fact, the three energy-supplying nutrients (carbohydrates, proteins, and fats) in the food we eat have very different water requirements for metabolism. Protein requires 3.3g of water to be combined with 1g of protein, and carbohydrates stored in the liver and muscles in the form of glycogen also require 2.5-3g of water to be combined with 1g of carbohydrates. In contrast, the storage of hydrophobic fat, the only "oil and water do not mix", does not require much water.

So the question is, although both may consume 2000 kcal per day, if one person follows a high-carb diet with 70% carbohydrates and another follows a ketogenic diet (carbohydrate energy supply <5%), the latter will lose bound water due to the massive decomposition of glycogen and active gluconeogenesis - with the same 2000 kcal, the person on the ketogenic diet obviously needs to replenish more water.

3. The amount of water a person needs must cover the extra water excreted, and there are certain individual differences.

As mentioned earlier, this is a recommendation for individuals with a standard energy intake of 2000 kcal. In fact, differences in each person's age, height, weight, amount of exercise, amount of sweating, and kidney metabolic capacity lead to great "heterogeneity" in energy requirements and water intake.

For example, if both consume 2000 kcal of energy, but one person sweats more and the other sweats less, it is likely that the former will need to drink 10 cups of water, while the latter will only need 7 cups.

Therefore, the recommendations on water intake in the dietary guidelines reflect the "general" recommended amount for people with "average" living environments and physical activity levels. The body's demand for water will increase when the amount of exercise increases, the environment is hot, certain symptoms occur (such as fever, diarrhea, vomiting), and when you are pregnant or breastfeeding.

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The saying "8 glasses of water" can help us remember the amount of water we need to drink every day, but it is impossible to match everyone's needs. We don't have to worry too much about it, and we don't need to worry too much about "not drinking enough water", but we need to drink enough water as much as possible.

So what amount should we refer to for scientific hydration?

This is a technical job. The most accurate gold standard is to use the "double-labeled water method" to test how much energy a person metabolizes every day, and then calculate how much water is used and discharged. Because there is a basic knowledge here:

The amount of water a person needs every day = water consumed by metabolism + water excreted (breathing, sweating, urine, feces)

Therefore, referring to the updated and more comprehensive research in 2022, it was found that the amount of water a person actually needs is related to the following factors:

Physical activity level, gender, economic level, age, and lean body mass.

The first three items are easy to understand. They are "external factors" that directly affect the amount of water excreted every day, including sweat, feces, and urine.

Only age and lean body mass are "rigid factors" related to energy metabolism, which need to be measured according to one's own situation. The paper also gives an example, for example, a 20-year-old man weighing 70 kg, doing moderate physical activity, living in an environment with suitable temperature and humidity, loses about 3.2 liters of water per day. If we calculate based on the fact that drinking water accounts for 50% of the total water intake in my country, then he really needs to drink 8 glasses of water a day.

On the contrary, a 60 kg woman of the same age living in the same environment loses about 2.7 liters of water per day, which means she actually needs to drink 6-7 cups of water.

However, 20 years old is the time when water metabolism is most active in a person's life. So according to this calculation, if a 65-year-old woman weighs only 50 kilograms and has a sudden decrease in exercise, her daily water needs may be reduced. In addition, the elderly may have some urinary system diseases. Forcing yourself to drink 8 cups will not "replenish water" for the body, but will only increase the burden on the kidneys.

How much water should we drink?

First of all, "8 glasses of water" is a baseline. Other non-physical factors may cause you to drink more water, so you can add more water according to your own baseline:

▶ High amount of exercise: The amount of exercise corresponds to a high metabolism and high energy demand, and the amount of water you drink also needs to be increased according to the situation.

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▶ Climate and humidity cause excessive sweating: This is the most critical factor in the hot summer. In this case, since you cannot count the amount of sweating, thirst is the most obvious signal. However, when you drink water when you are thirsty, the urine osmotic pressure is already high. It is recommended that you should take the initiative to drink water regularly, and don't drink only when you feel thirsty. Here we also want to remind everyone that the dehydration caused by excessive sweating actually requires the supplementation of electrolytes at the same time, otherwise it is difficult for the body to maintain the corresponding osmotic pressure, and the efficiency of water replenishment is not high.

▶ Urination and diarrhea: For example, drinking alcohol has an indirect diuretic effect, caffeine can also be a mild diuretic, and diarrhea caused by illness will increase water excretion. In these cases, you need to increase your water intake as appropriate.

▶ Low quality of diet: This factor is often overlooked. According to scientific statistics, for Chinese residents, about 50% of the water comes from the food we eat. In fact, natural foods such as vegetables, fruits, cooked whole grains, lightly processed meat and eggs, and soy products have a high water content, ranging from about 40% to 90%. If your eating pattern is close to the dietary guidelines, it's easy to say. Otherwise, if you over-consume low-water ultra-processed foods every day, such as baked bread, biscuits, hand-grabbed pancakes, fried dough sticks, jerky, protein bars, etc., the available water from these foods will be greatly reduced, and in turn the water you need to drink will increase accordingly-but fortunately, the body is very smart, and friends who eat a lot of snacks must also feel more thirsty.

in conclusion

1. The idea of ​​drinking eight glasses of water a day does have some basis, but there are also individual differences.

2. The human body's water demand not only needs to consider energy metabolism, but also the amount of water excreted due to individual factors to measure the overall amount.

3. Any condition that increases water loss through sweating, urination, diarrhea, etc. requires increasing the amount of water intake on the original basis.

4. People who eat too many low-moisture, ultra-processed snacks also need to drink extra water.

Author: Luo Xiao, registered dietitian in China/Australia, author of "Quit Sugar"

Reviewer: Zhang Na, Associate Researcher, Doctoral Supervisor, Peking University School of Public Health

References:

1. Stookey, JD, & Kavouras, SA (2020). Water Researchers Do Not Have a Strategic Plan for Gathering Evidence to Inform Water Intake Recommendations to Prevent Chronic Disease. Nutrients, 12(11), 3359. https://doi.org/10.3390/nu12113359

2.Yamada, Y., et al. (2022). Variation in human water turnover associated with environmental and lifestyle factors. Science, 378(6668), 909-915. https://doi.org/10.1126/science.abm8668

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