Gut

. If it weren’t for this combat readiness—learned through training by our gut bacteria—there would be no blood groups and any donor could give blood to any recipient. For newborn babies, who do not yet have many bacteria in their guts, this is indeed the case. They can theoretically receive transfusions of blood from any group without any incompatibility effects. (As a precaution, hospitals give babies blood from the mother’s blood group, since antibodies from the mother can find their way into the baby’s bloodstream.) As soon as babies begin to develop a rudimentary immune system and gut flora, they can only tolerate blood from their own group.

However, we cannot receive blood from donors whose blood cells have a different blood-group marker on their surface. It would immediately remind our immune system of bacteria, and since the immune system knows that bacteria have no business being in the bloodstream, it would consider the donated blood cells an enemy and cause the cells to form clumps

Some bacteria have structures on their surface that bear a close resemblance to those on the surface of our own cells. This is the reason why, for example, scarlet fever should be treated immediately with antibiotics. If it is not treated quickly, the immune system can begin to mistake the cells of the joints or other organs for the bacteria that cause scarlet fever and attack them. It might suddenly think our knee is a nasty sore-throat germ hiding out in our leg. It happens rarely—but it does happen

Anyone who suffers from anxiety or depression should remember that an unhappy gut can be the cause of an unhappy mind. Sometimes, the gut has a perfect right to be unhappy—if it is dealing with an undetected food intolerance, for example. We should not always blame depression on the brain or on our life circumstances—there is much more to us than that

Until scientists have filled those gaps in their knowledge, we can make use of the facts we already know to improve gut health. It starts with the little things like mealtimes, for example, which should be enjoyed without pressure, at a leisurely pace. The dinner table should be a stress-free zone, with no place for scolding or pronouncements like “You will remain at the table until you’ve finished the food on your plate!” and without constant television channel hopping. This is important for adults, but it is vital for small children, whose gut brain develops in parallel with their head brain

One theory proposed by research bacteriologists is that stress is unhygienic. The altered circumstances stress creates in the gut allow different bacteria to survive there than in periods of low stress.

When the brain is stressed, vomiting expels partly digested food in order to save the energy required to complete the digestive process. The brain can then use that energy to solve the problems at hand. When the gut is stressed, partly digested food is ejected either because it is toxic or because the gut is currently not in a position to digest it properly. In both cases, it can make good sense to press the eject button. There is simply no time for gentle, comfortable digestion. When people throw up from nerves, it is simply their digestive tract trying to do its best to help.

Vomiting can also be caused by intense feelings such as emotional strain, stress, or anxiety. Under normal circumstances, we synthesize the stress-response hormone CRF (corticotropin-releasing factor) in the morning, creating a supply to help face the challenges of the day. CRF helps us tap into energy reserves, prevents the immune system from overreacting, and helps our skin tan as a protective response to stress from sunlight. The brain can also inject an extra portion of CRF into the bloodstream if we find ourselves in a particularly upsetting situation.

However, CRF is synthesized not only by brain cells, but also by gastrointestinal cells. Here, too, the signal is—stress and threat! When gastrointestinal cells register large amounts of CRF, irrespective of where they originate (in the brain or in the gut), the information that one of the two is overwhelmed by the outside world is enough for the body to react with diarrhea, nausea, or vomiting

Currently, the best explanation of motion sickness is this: when the information sent to the brain from the eyes is at odds with that sent by the ears, the brain cannot understand what is going on and slams on every emergency brake at its disposal.

When a passenger reads a book in a moving car or train, their eyes register “hardly any motion,” while the balance sensors in the ears say “lots of motion.” It’s the same, but opposite, effect as when you watch the trees whizz by when driving through a forest. If you move your head a little as well, it looks as if the trees are rushing by faster than you are actually moving—and that, too, confuses the brain. On an evolutionary scale, our brains are familiar with such mismatches between eyes and balance sensors as signs of poisoning. Anyone who has ever drunk too much or taken drugs will have felt the room spinning, even when they are not moving at all.

bits of food is almost certain to have originated from the stomach and not from the small intestine. The smaller the particles, the more bitter the taste, and the more yellow the color, the more likely it is to be a salutation from the small intestine.