Those who believe that milk can be drunk like water do so under the assumption that milk is a highly nutritious beverage, and therefore, the more the better. Some people have a misunderstanding of "nutrition," interpreting it as the quantity and quality of various nutrients. Milk is rich in nutrition because of its high calcium content, high-quality protein, and relatively high levels of Vitamin B2 and Vitamin A, which are often insufficient in the Chinese diet.
It is said that milk can be drunk like water because nearly 90% of its composition is water. Water is one of the most important nutrients for sustaining human life. When the body loses more than 2% of its water, its functional performance is affected. Without water replenishment, human life can be in danger within days. Therefore, when the water supply is insufficient, milk can naturally serve as a source of water to replenish the body's precious fluids.
However, milk cannot be drunk like water. This is because we can drink as much water as we need, but not so with milk. The reason is that milk also contains many other components such as fat, protein, sugar, calcium, and phosphorus, and the intake of all these has limits. If our daily water needs are met entirely by milk, the intake of nutrients like fat may become excessive. Therefore, milk cannot be consumed in unlimited quantities as a substitute for water. "Highly nutritious" foods are not something that "the more, the better."
Firstly, so-called nutrition refers to the reasonable combination of various foods in terms of quantity and variety to meet the body's needs. Due to current limitations in nutritional science, this balance is often expressed in terms of the quantity and proportion of various nutrients. However, this does not mean that simply combining proteins, vitamins, calcium, iron, zinc, and other nutrients creates a "highly nutritious" food. A balanced diet should include a certain proportion of grains, legumes, vegetables, fruits, meat, eggs, milk, and other foods, combined with water, to form our daily meals. Using large amounts of milk instead of water actually disrupts this balance.
Secondly, analyzing its nutritional content, milk contains about 3% protein and fat, 5% sugar, and about 0.1% calcium. According to the Chinese Dietary Guidelines, which recommend 1.2 liters of daily water intake, if milk is used as a substitute for water, calcium intake would become excessive, and fat and energy intake would also be too high. The saturated fatty acids in milk are detrimental to cardiovascular health, and excessive energy can easily lead to obesity [Ten Causes of Obesity; Sleep is the Simplest Way to Lose Weight]. Even if milk replaces only half of the daily water intake—600 milliliters—the fat and energy consumed would still be excessive. This is why Westerners choose skim milk for health reasons. As for the protein in milk, it is not something we need to overemphasize. Due to the current abundance and variety of food supplies, and the relatively high proportion of animal-based foods, protein intake is no longer the main nutritional concern for the urban population in China. For rural residents, especially those in less developed areas, milk plays a more significant role in improving protein nutrition.
Finally, a high-protein, high-fat, and high-energy diet has become one of the main culprits behind modern "civilization diseases" such as diabetes, cardiovascular diseases, and cancer. Therefore, some scholars hold a very negative view of the Western habit of consuming large quantities of milk and drinking it like water. Based on reports linking milk to ovarian and prostate cancer, some scholars even advocate not drinking milk at all, which is somewhat extreme. The occurrence of cancer is related to many of the foods and eating habits in our daily lives, such as consuming large amounts of animal products, fried and grilled foods, and smoked and pickled foods, as well as excessive energy intake and low consumption of vegetables and fruits. These factors have a much greater correlation with cancer than milk. Therefore, we cannot "stop eating for fear of choking," nor can we simply interpret these studies as a direct causal relationship between a certain food and cancer. We should recognize that any food has both beneficial and harmful aspects; it is beneficial in moderation and harmful in excess. The key is to consume a variety of foods in appropriate amounts.
Therefore, if we can drink some milk daily, it can improve our calcium intake, and it can also be a temporary source of hydration when drinking water is inconvenient. However, using large amounts of milk as a long-term substitute for drinking water by no means constitutes a balanced diet, let alone a "highly nutritious" one; it may even bring health risks.