At present, the earliest written record about Pigu is found in Zhuangzi Xiaoyao Tour: "The mountain shot by ancient times is inhabited. If the skin is like ice and snow, it is like a virgin, not eating grains, sucking wind and drinking dew, traveling around the world by flying dragons ... "1973 The silk book unearthed from the No.3 Han Tomb in Mawangdui, Changsha also has relevant records and was compiled into Eating Grains and Eating Gas.
Won't the valley starve to death? Although there is a saying in historical records, some people have been dead for years or even decades. However, these legends are not credible. In reality, there are indeed people who have completely fasted and only drank water and survived for more than half a month, 20 days or even longer without being hurt. Because this has gone beyond the general cognition that "you will die if you don't eat for 7 days". So I think there must be something special in the valley, so it is easier to listen to temptation. In fact, you won't starve to death if you don't eat for half a month or 20 days. There is nothing magical about this, which can be completely explained by modern medicine.
The research on chronic hunger has been fully carried out. Even if people completely fast, as long as the water supply is guaranteed, they will not starve to death for a long time, and even there are no signs of injury on the surface. Besides oxygen and water, energy is the most urgently needed nutrient to sustain life. The energy needed by human body is supplied by sugar, protein and fat, which are the three nutrients we usually say.
Under normal circumstances, the daily intake of energy substances supplements the consumption demand and maintains the energy balance. Once fasting, people will enter a state of hunger, and the energy supply of the human body will change adaptively. After several hours of hunger, the decrease of blood sugar level inhibits the secretion of insulin, and the low insulin level inhibits the intake of glucose by peripheral tissue cells such as liver and muscle, and then decomposes fat as an energy source. After fasting for 24 hours, the energy stored in the form of glycogen is exhausted. Because the brain and other tissues usually rely mainly on the function of glucose, the primary task of energy supply transformation is to decompose fat and transform protein into glucose through gluconeogenesis to maintain glucose homeostasis.
After starvation for about 3 days, the liver can form a large number of ketone bodies (acetoacetic acid, D-3- hydroxybutyric acid and acetone), and the brain begins to consume acetoacetic acid (about one third) as a substitute for glucose, and the heart begins to rely on the consumption of ketone bodies to obtain energy. This rapid transformation of energy also inhibits the gluconeogenesis pathway of protein decomposition. Preserving protein as much as possible is the second priority of metabolic adaptation to hunger, and it is also an important mechanism to maintain life. After starvation 1 week, ketone bodies become the main energy source of the brain. The daily demand for glucose in brain tissue decreased from about 65438 0 20 g on the first day of starvation to 40g per day.
In this way, the main energy source of the human body is supplied by ketone bodies converted from fatty acids decomposed by the liver, the demand for glucose is obviously reduced, the pressure of gluconeogenesis is reduced, and the protein decomposition of muscle is reduced from 70g on the first day of starvation to 20g. In a word, when people are hungry, adaptive metabolism changes, the utilization of glucose is limited, gluconeogenesis ensures the stability of the most basic blood sugar level, and the levels of blood fatty acids and ketone bodies increase. Fat decomposition is the main energy source of human body. However, the fat stored in human body will be exhausted one day, and the only energy source is protein decomposition. The decomposition of protein is accelerated, the structure and function of living organs such as heart and liver are destroyed and lost, and death becomes an inevitable outcome.
In addition to energy substances, other macronutrients that must be obtained from food, such as essential amino acids, essential fatty acids, vitamins, minerals and other micronutrients, are either stored or metabolically adapted to some extent. Even if they are lacking, they will not be fatal for a long time.