In addition to making people feel hollow in the abdomen, there is a desire to eat, and it also causes a series of reactions in the brain, liver, pancreas, kidney and other important organs. In order to let everyone understand these reactions and their impact on health, we set a simplified scenario: a healthy adult, who has been eating normally for several years, and after eating a normal lunch (12 o''clock) on a certain day, no longer Eat and go hungry. During this period, Ta''s physical activity was as usual, with no illness, trauma or other accidents. Let''s take a look at the reactions of important organs in his body (the time limits of "4 hours" and "10 hours" in the following are not absolute and are only approximate reference times; the weights of "15 grams" and "90 grams" are not accurate. Measured, only a rough reference quantity)

　　After 5pm (after lunch 4, 5 hours), the digestion and absorption of lunch foods (especially sugars) is completed, and the blood sugar (glucose in the blood) drops to before the meal At this time, there are a total of 3 or 5 grams of glucose in the blood. Organs such as the brain and muscles continue to consume glucose, and blood sugar tends to continue to decline. If the blood sugar level is too low (hypoglycemia), the brain will be difficult to work properly (blood sugar is almost the only energy source of nerve tissue), so the body will make the following adjustments to maintain blood sugar stability:

　　①Muscle cells stop Ingest blood sugar and switch to glycogen stored in the muscles (about 180 to 300 grams). ②The liver cells decompose the glycogen stored in it (about 70 to 100 grams) into glucose and output it as blood sugar, which is mainly provided to the brain for use. ③Heart, kidney, liver and other energy-consuming organs mainly use oxidized fatty acids as energy sources and rarely consume glucose. In short, the energy consumption during this period is mainly glycogen (reserved during normal diet), muscle glycogen supplies muscles, and liver glycogen supplies the brain (and a few tissues such as red blood cells). Other organs use fatty acids as energy sources.

　　The next morning (after more than ten hours), glycogen (especially liver glycogen) is exhausted (the weight of the liver has decreased, and the weight of the muscle has also decreased), and blood sugar is facing again Too low the risk, in order to avoid hypoglycemia from affecting the brain, the body makes the following adjustments again:

　　①The liver cells start to synthesize glucose by themselves, the raw material is some non-sugar substances, such as glycerol (from fat breakdown) And amino acids (from muscle protein breakdown). Among them, glycerol is used to synthesize 10 to 15 grams of glucose (in 24 hours), and amino acids are used to synthesize 90 to 120 grams of glucose (in 24 hours). ②Kidney can also synthesize glucose, but its output is lower than that of liver. ③ Muscle, heart, kidney, liver, etc. further reduce the consumption of glucose (mainly consumption of fatty acids), to "save" glucose and supply the brain (and a few tissues such as red blood cells). ④The liver cell mitochondria also use fatty acid metabolites (acetyl coenzyme A) to synthesize "ketone bodies" (acetone, acetoacetic acid and β-hydroxybutyric acid). Ketone bodies are transported by the blood to the heart, kidneys, muscles, brain, etc. for energy use.

　　 Continue hunger (do not eat or eat very little). ①The liver continues to synthesize glucose, and the production of glucose by the kidneys also increases, supplying glucose to the brain. ②Muscle protein is forced to break down continuously, and its product is amino acid, which is the main raw material for the synthesis of glucose. The result of losing the car (muscle) and keeping the handsome (brain) is that the muscle weight is reduced and the weight is reduced. In theory, the production of 90-120 grams of glucose requires 180-200 grams of protein (amino acids). Muscle protein breakdown, and possibly visceral protein breakdown, will have a very bad impact on life. To this end, the brain makes adaptive adjustments. ③The brain uses ketone bodies (directly from liver synthesis and indirectly from fat breakdown) as the main energy source to save glucose and thereby save muscle or visceral proteins (amino acids). The daily consumption of protein (amino acid) is about 35 grams.

　　All in all, in the early stages of starvation, the glycogen stored in the body is quickly depleted, and muscle protein is decomposed, because glycogen and muscle have a large amount of water, combining 2 and 4 times their own weight of water, Therefore, the weight will be significantly reduced. Gives the illusion of weight loss. Starvation continues, and the body turns to increase fat (and ketone) utilization to slow down muscle breakdown. Fat provides more energy, has a very low water content, and loses weight more slowly.