When I was a child lying down to eat will be taught by adults, from a medical point of view, lying down to eat is not right, but mainly afraid of food into the trachea, causing choking, not afraid of eating overflow. People so think that in space or lying down to eat will spill, because usually when eating are sitting, from the mouth to the anus of the digestive tract in the vertical position, so naturally feel that the reason why food can go down is by the role of gravity. However, humans do not eat from the beginning sitting or lying down. Most mammals eat when the long axis of the digestive tract is also in a horizontal position. As for drinking, animals almost always have to put their heads down, the mouth is instead in the lowest position. In addition, after the food enters the digestive tract, it is not always down, but like a roller coaster through a number of turns. We know that the total length of the human digestive tract is several meters, and from the mouth to the anus in a straight line of less than a meter, it should be understood that food into the digestive tract after the digestive tract can not be all the way down. The movement of food in the digestive tract is not only not always downward, but also not unidirectional. Under normal circumstances, food enters the stomach and stays there for a long time – shorter for fluid and longer for solid food – during which time the food is squeezed and squeezed backwards and forwards, with the lubrication of gastric juice and the action of gastric acid protease. It becomes a paste before it enters the small intestine a little bit. If there is no stomach, the food will go directly to the small intestine, and the sudden increase of carbohydrates in the small intestine will induce a large amount of insulin secretion, causing hypoglycemia and coma, which is the dumping syndrome. After entering the small intestine, the food will occasionally move back and forth in addition to moving toward the final exit, which has the advantage of increasing the time of contact between the food and the mucosa of the small intestine, which is conducive to the full absorption of nutrients. Therefore, even if not in the weightlessness of space, there is some reflux of food, but this reflux is non-gravitational physiological, small in magnitude, and within a particular segment of the digestive tract. If the reflux gets out of control and crosses two GI segments, it can produce discomfort and even lesions. Because the structure and physiological condition of each segment of the GI tract is like an assembly line that can only accommodate the semi-finished product delivered from the previous segment, if the line is suddenly reversed, the worker or robot will be overwhelmed and even cause damage to the machine. For example, the compound squamous epithelium of the esophagus can adapt to the semi-solid food delivered after chewing by teeth, and has strong resistance to mechanical stimuli, but limited ability to remove chemical stimuli; if acidic or alkaline substances from the stomach or duodenum enter the esophagus, the esophagus lacks sufficient neutralization ability, and acid reflux and heartburn will occur. For example, the small intestine is a sterile or less bacterial environment, but the large intestine next door is a bazaar of bacteria, bacteria check and balance each other to maintain the surface peace; if a large number of bacteria into the small intestine, even if the usual more honest bacteria, encounter no competition in the environment will expand, resulting in small intestine bacteria overproliferation, the symptoms of abdominal distension and abdominal pain. So in space, without gravity, will this pathological reflux be aggravated? This is not something to worry about, because there is generally a one-way valve between the various segments of the digestive tract to prevent excessive reflux from occurring. Between the esophagus and stomach, there is a valve formed by the sphincter high-pressure band, diaphragm and the bottom of the esophagus angle, usually closed most of the time, only when swallowing open to prevent the contents of the stomach back into the esophagus; stomach and small intestine separated by the pylorus, usually has been in the open and closed repeat cycle, but open time is often because the stomach is contracting, the stomach pressure is higher, so basically also The small intestine and large intestine are separated by the ileocecal valve, which is also a check valve. There is another very important valve in the duodenum, which is in the wall of the duodenum, through which you can access the bile duct, liver duct or pancreas. This valve is also unidirectional, allowing only bile and pancreatic fluid to enter the small intestine. If there is a backflow in this area, the consequences can be very serious, such as acute pancreatitis. A thousand words, theory is better than practice. Although the physiological and anatomical common sense that eating in space will not be random reflux, but also more convincing than practice. Looking at the good spirits and health of the astronauts, I think the above theory is right.