When you feel hot, you drink cold water. It seems to be a straightforward common sense or human instinct. Yup, after you walk under the scorching sun with sweat all over your body, you gulp down a bottle of cold drink and feel the cold water flush down the throat, the stomach and the intestine, which may be the most blissing moment.
However, some people, mostly elderly, would warn you that cold drinks are harmful to your health. They even share their own experiences with cold drinks, like stomach and abdominal pain, diarrhea, headache, sore throat, cough and even more serious or long-term problems in respiratory and digestive tracts as well as cardiovascular system, etc.
Modern physiology attributes the harmful effects of cold drinks to the so-called “cold stress” of the body. Cold stress is induced by two generalized mechanisms. First, cold drinks cause the blood vessels to constrict, which reduces the blood supplies to the affected organs or tissues. Overtime, the affected organs and tissues will be damaged and make people sick. Second, cold drinks could reduce the body core temperature. To maintain the body core temperature, the body will spend more valuable energy to neutralize cold drinks, and hence other parts of the body will lack energy, running the risks of malfunctions and attenuated immunity, etc.
While this explanation sounds reasonable and is supported by some research studies, many people still don’t accept it, because drinking cold water in hot summer sounds to be a more reasonable common sense than the explanation of modern physiology, not to mention that scientists have not reached a solid conclusion yet.
Here, I would like to elaborate this topic from a TCM point of view and show you how the change of four seasons affects the energy flow in our body and why drinking cold water in summer is not good for your health.
We all know that solar energy is the major energy source on the earth. The movement of the direct sunlight between the Tropic of Cancer (the northernmost latitude where the sunlight directly overheads the earth) and the Tropic of Capricorn (the southernmost latitude where the sunlight directly overheads the earth) creates the year-round temperature change on the earth. People in ancient times observed the temperature change and defined the four seasons. Spring is warm, summer is hot, fall is cool and winter is cold. Interestingly, ancient Chinese people not only observed the temperature change in the four seasons, but also discovered the directions of the energy (Qi, 气) flow in the four seasons. The Yellow Emperor's Classic of Internal Medicine (Huang Di Nei Jing, 黄帝内经), a TCM classic written more than 2000 years ago, has clearly documented that Qi is rising (sheng, 升) in spring, floating（fu, 浮）in summer, falling （jiang, 降）in fall and sinking （chen, 沉）in winter.
According to this TCM classic, here I will describe the flow of Qi in four seasons in modern language. Spring is the season for growth. It is observed that plants have two growth phases in spring. The first is the sprouting phase, in which plants or seeds sprout by mainly using the chemical energy (e.g., ATP or glucose) that was converted from solar energy last year. The second is the photosynthesis phase that occurs after plants grow new leaves and absorb solar energy through photosynthesis. When spring progresses to summer, plants grow more leaves to harvest more energy through photosynthesis. The growth and energy harvest reach their maximum at the Summer Solstice (Xia Zhi，夏至), when the direct sunlight hits the Tropic of Cancer. After that, the direct sunlight starts to move to the Tropic of Capricorn, and summer progresses to fall. When the direct sunlight decreases, plants absorb less energy through photosynthesis and leaves start to fall. Plants start to conserve the energy in the fruits, seeds and roots. In winter, the energy in plants completely sinks under the ground, specifically in roots and seeds that are covered by soil, snow or ice. The energy stored in winter will be used for sprouting and growth in next spring. If we draw a diagram, we can clearly see the rise and fall of Qi flow in four seasons (see figure below).
Chinese medicine believes that like plants, the human body has the same rise-fall pattern of Qi (blood) flow because the universe is holographic and run by the same rule. In spring, the energy of the human body starts to move outward to the surface/perimeter of the body (skin, head and limbs) and eventually reaches outermost in summer. This means in summer the energy in the inner part of the body, such as the internal organs and digestive tracts, is relatively deficient. In fall, the energy starts to move inward and eventually reaches the innermost in winter. Thus, there is relatively more energy in the inner part of the body than the surface and perimeter (see figure below).
This TCM theory can now be explained by modern physiology: the skin is the largest organ of the human body. The rich vascular network under the skin has a huge regulatory effect on the blood volume of the human body. At room temperature, the blood flow to the skin is about 250-500 ml/min, which is only about 5% of the cardiac output. At high temperature, the blood vessels of the skin expand, and the blood flow can reach an astonishing 6-8 liters/min, which is close to 1.5-2 times of the total blood volume (4-6 liters). At cold temperature, the blood vessels of the skin are maximally constricted, and the blood flow can be reduced to 20-50 ml/min, or even lower.
Based on both TCM and modern physiology, in the hot summer the flow of blood and energy is greatly increased in the skin but reduced in the inner part of the body. When people drink cold water, it goes down to the digestive system, where the blood and energy are already deficient. The temperature in the digestive system will quickly drop, and it will consume much more energy and take a much longer time for the body to restore the normal temperature. Overtime, the cold stress or cold shock can cause both acute and chronic health problems, ranging from headache, cough, diarrhea to immunity disorders and cardiovascular incidents. Thus, TCM highly recommends drinking lukewarm water or at least room temperature water in summer. TCM also recommends occasionally eating warm foods, like hot chicken soup and ginger roots, to keep the inner body warm and healthy in summer.
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