Gut

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.