Water is the most important part of our body, accounting for about 50%~70% of our body weight. Water is not only the structural basis of blood, skin, muscle, fat, bone and other tissues, but also the basic condition of physiological processes and biochemical reactions such as transporting nutrients, discharging metabolic wastes, supplying energy and keeping body temperature normal. Ensuring a good hydration state is a necessary condition for maintaining health.
Adults lose about 2.5 liters of water every day through urination, defecation, body surface evaporation, breathing and sweating, so we need to replenish water accordingly. In addition to the water produced by food intake and metabolism, we also need to drink water actively. Dietary Guidelines for China Residents (2022) suggests that in mild climate, adult males with light physical activity level should drink 1.7 liter of water every day, and adult females should drink 1.5 liter of water every day. If a glass of water is about 200ml, the recommended amount is about 8 glasses of water a day.
How did this suggested value come from? Zhang Na, an associate researcher in the Department of Nutrition and Food Hygiene of Peking University College of Public Health, gave an explanation. 20 10 from July to August, researchers from Peking University and China CDC investigated the drinking water of adults in four cities in China. The data show that drinking water accounts for 56% of the total intake of adults in China; The median amount of drinking water was 1488 ml, including 1679 ml for males and 1370 ml for females. According to this survey, the research group proposed that the suitable daily intake of drinking water in China is 65,438 0.7 liters for men and 65,438 0.5 liters for women. Later, this suggestion was written into the Reference Intake of Dietary Nutrients for China Residents (20 13), which became the authoritative data in the field of nutrition in China.
In other countries, there is also a saying of "drinking 8 glasses of water every day", but its more complete version is called "8 8" method, that is, drinking at least 8 glasses of 8 ounces (about 237 ml) of water every day, that is, about 1.9 liters of water every day.
However, the origin of this "8 8" statement is not very clear. Heinz Valtin, a physiologist at Dartmouth Medical College in the United States, once verified the origin of "8 8" and published a commentary on it in 2002. One possible source of this statement can be traced back to 1945. At that time, the Food and Nutrition Committee of the American Academy of Sciences suggested in a guide that in most cases, adults should consume 2.5 liters of water a day; For different individuals, the water demand can also be calculated as per 1 kcal 1 ml of water. It may be because the order of magnitude of 2.5 liters is similar to that of "8 cups", so the saying of "8 8" is gradually derived.
Walding also proposed another possible origin of "8 8". From 65438 to 0974, the famous nutritionists Fredrick J. Stare and Margaret McWilliams co-authored the book Nutrition for Health. At the end of the book, there is a sentence that says, "For the average adult, it is appropriate to drink 6 to 8 glasses of water every 24 hours." However, this sentence does not quote any references, and it is not the focus of this book, so it may be just some ideas added by the author.
After careful literature search and communication with colleagues, experts and relevant media, Valtin found no scientific evidence to support the "8 8" method, but he also said that he did not find any evidence that "drinking water is not absolutely harmful". Drinking water anxiety
Walding did not find the authoritative source of "8 8". After the article was published, it attracted some controversy, but this does not mean that the statement of "drinking 8 glasses of water a day" is wrong. Zhang Na said that "8 glasses of water" was used because it was easier to remember than the specific water consumption data. After all, it is impossible and unnecessary for ordinary people to drink water in cups. Of course, we also need to pay attention to the size of the "cup". According to the Dietary Guidelines for China Residents (2022), the standard portion of each glass of water is about 200ml.
First of all, it should be pointed out that the human body has an accurate and sensitive mechanism for regulating body fluid balance. Under normal circumstances, people are not easy to dehydrate, resulting in serious consequences. When human body loses water, the osmotic pressure of plasma and extracellular fluid will increase, thus activating osmotic pressure sensory neurons in hypothalamus and transmitting signals to promote the release of vasopressin from hypothalamus and posterior pituitary. After vasopressin acts on renal tubules, it can increase the reabsorption of water by the kidneys, thus reducing the water output; The greater the increase of plasma osmotic pressure, the more vasopressin is released. At the same time, the cerebral cortex will also receive the signal of increasing osmotic pressure, which will produce thirst and make people drink water actively.
Usually, when the osmotic pressure of plasma rises less than 2%, people will feel thirsty. A classic study shows that people will start to feel thirsty before body fluids are lost to plasma osmotic pressure, plasma sodium and potassium concentration and other indicators change significantly. However, some studies have found that the threshold of plasma osmotic pressure to promote vasopressin release is lower than that to make people feel thirsty, which means that the osmotic pressure regulation mechanism in the body begins to work before thirst.
In short, under the action of these mechanisms, the osmotic pressure of human plasma can usually be maintained very stably at 280 ~ 290 mmol/kg, and the fluctuation range rarely exceeds 2%. There is no sudden illness such as severe vomiting and diarrhea, sweating like marathon, bicycle, triathlon or heavy manual workers, and it is difficult for us to dehydrate in extreme environments such as hot desert, with serious consequences. On the other hand, it also means that when you feel thirsty, your body has lost some water.
Zhang Na introduced that when water loss reaches about 1% of body weight, people will not only feel thirsty, but also their physical fitness will be affected. When water loss reaches 2% of body weight, urine will be less. Many experts will use 3% dehydration (equivalent to at least 5% increase in plasma osmotic pressure) as the standard to define dehydration or not. Although these conditions are not serious enough to cause obvious diseases and serious consequences, it does not mean that there is no impact on the body.
On the other hand, if you drink water without a lid, it may really cause serious consequences. The kidney can excrete at most about 0.7~ 1 liter of water per hour (this is not equal to the upper limit of water consumption). If drinking too much water in a short time exceeds the upper limit of renal excretion capacity, it will lead to the dilution of sodium in the blood, causing acute water poisoning and hyponatremia. Symptoms include confusion, anorexia, seizures, muscle spasms and so on. -If endurance athletes just drink water and don't have enough supplements, then drinking water should pay attention to "extremes meet".
3. Not just drinking "water"
In the guidelines issued by the Food and Nutrition Committee of the American Academy of Sciences 1945, after the two sentences behind the water consumption, one sentence is easily overlooked: most of the water intake every day comes from food intake. Perhaps because I didn't notice this sentence, some people think that drinking 2.5 liters of water every day means drinking 2.5 liters of water. Similarly, "the sources of water can include coffee, tea, milk, drinks, beer and so on." Drink 6~8 glasses of water every 24 hours, and fruits and vegetables are also ideal water sources. "
It may be exaggerated to say that most of the water intake comes from food, but the water in food is indeed a source that cannot be ignored. Zhang Na reminded that due to the different eating habits of different countries, cultures and individuals, the intake ratio of food and water is significantly different. China residents' cooking methods are mainly steaming, frying and stewing, which not only retains most of the original water in food, but also adds extra water during processing. Therefore, the water intake of China residents from food may reach more than 40% of the total intake. In contrast, western-style fast food is often cooked by frying and frying, which will reduce the water intake from food.
Therefore, "water intake" and "water consumption" are not the same, but quite different. If you don't make a distinction, you may misunderstand (or let others misunderstand) some numbers. For example, the Dietary Guide for China Residents (2022) states that the total daily intake of China residents is 2.7~3 liters, but it also shows that this includes drinking water and water in diet. The above figures of 1.7L and 1.5L are only suggestions for drinking water.
Zhang Na specifically explained that "water" here not only refers to pure water, but also includes various types of boiled water, packaged water or drinks. However, it is recommended not to use sugary drinks instead of water, but to drink less or no sugary drinks. The suitable first choice of "water" is all kinds of "white water", such as filtered and purified direct drinking water, boiled water, bottled water and various packaged drinking water. Drinking pure water for a long time is not recommended.
At the same time, water demand is influenced by metabolism, age, physical activity intensity, environmental temperature, diet, disease and other factors, so not only individuals vary greatly, but also the same individual varies under different environmental or physiological conditions. Therefore, in the Dietary Guide, the advice on the amount of drinking water reflects the "general" recommended amount suitable for the "general" people with living environment and physical activity level. When the amount of exercise increases, the environment is hot, some diseases (such as fever, diarrhea and vomiting) appear, and the body is in pregnancy or lactation, the demand for water will increase.
Drinking water is a necessity and instinct for survival. The expression "8 glasses of water" can help us to simply remember the amount of water we need every day, but it can't match everyone's needs. We don't have to worry too much about this, and we don't have to worry too much about "not drinking enough water". We just need to drink as much water as possible.