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=Causes of nausea=
There are a number of major categories of causal factors.
* '''Infection'''. One of the most common causes of nausea is gastrointestinal disease.* '''Diabetes'''. While studies are a little equivocal, there is some evidence that both type I and type II diabetics report more nausea than non-diabetic subjects. The mechanism for this is unclear.
* Anxiety. People that are suffering from anxiety have an abnormally low threshold for any type of potential threat. This may cause them to also have a lower threshold for the detection of toxins, and make them more susceptible to nausea.
* '''Pregnancy'''. During the first trimester of pregnancy nausea becomes more common, and this is often referred to as "morning sickness", or Nausea and and Vomiting of Pregnancy (NVP). NVP is believed to be a hyper-sensitization of the sensors that detect toxins as a survival mechanism to protect the fetus.* '''Surgery'''. Nausea is a common complication of surgery and anesthesia, and is termed Post-Operative Nausea and Vomiting (PONV).* '''Chemotherapy'''. Studies indicate anywhere from 25% to 75% of patients undergoing chemotherapy suffer from nausea. Chemotherapy related nausea can be divided into three categories.** '''Anticipatory nausea'''. This is nausea that occurs before the chemotherapy treatment. This can be triggered by a smell associated with chemotherapy, or sometimes even the sight of something associated with chemotherapy. Not surprisingly, anticipatory nausea is most common in patients that have previously had nausea from chemotherapy.** '''Acute nausea'''. This is the nausea that occurs during treatment, or within 24 hours of the treatment.** '''Delayed nausea'''. This is where the patient is initially free from nausea, but 18-24 hours after the treatment nausea begins.* '''Motion sickness'''. It is believed that everyone who possesses a working inner either can be made nauseous through a suitably provocative motion. However there are wide individual variations in the level of motion needed to provoke nausea. * '''Space travel'''. Microgravity tends to exacerbate motion sickness, and different space programs reported that 35-66% of astronauts suffer from Space Motion Sickness (SMS). Note that the Mercury and Gemini missions had no SMS, probably because the small capsules restricted motion.
=Taste Aversion=
There is an intrinsic aversion to certain tastes, including sour and bitter taste that tend to be associated with spoiled food or toxins. There is also a powerful learned response, called Conditioned Taste Version Aversion (CTA) that occurs when nausea follows the consumption of a particular food. The food the proceeds the nausea will be avoided, and may even generate anticipatory nausea. This CTA has been shown in animals from the garden slug to humans. While CTA is "conditioned", it is quite unlike the usual classical conditioning. The time period between ingestion and nausea can be much longer than is usual in classical conditioning, and a single bout of nausea is enough to develop CTA. In addition, the CTA appears to last much longer than with classical conditioning. This appears to be a deeply embedded survival mechanism, and it only works between taste and nausea. Using other senses (sight/touch) or using a different negative stimulus (pain) does not result in the rapid, long-lasting conditioning. While CTA is generally around taste, it can often extend to smell. The smell of a food involved in CTA will often trigger nausea.
=Hydration, Nausea and Vasopressin=
Vasopressin is an antidiuretic hormone, which means that it causes the kidneys to retain fluids. Dehydration causes vasopressin to be released, and so prevent fluid loss through the urine. However, there is a relationship between vasopressin and nausea.
* Studies have shown that nausea causes a rise in and vasopressin. From a survival standpoint this makes sense, as fluid losses due to vomiting can cause dehydration. This link is so strong that anti-diuresis was proposed as a measure of motion sickness in 1957. The rise in vasopressin can be seen in subjects experiencing nausea from motion sickness, chemo therapy and substances that cause nausea, such as apomorphine. Generally subjects that do not experience nausea do not have the rise in vasopressin. Note that the vasopressin release occurs without significant changes in blood pressure or blood osmolarity.
* Studies have also shown that vasopressin can cause nausea. In one study the subjects reporting nausea also had abdominal cramping and bloating. It is not clear why vasopressin would cause nausea.
This has some interesting implications for runners.
* A runner that becomes nauseous would have vasopressin released, which could lead to [[Hyponatremia]].
* A runner that becomes dehydrated would have vasopressin released, which could lead to nausea.
* It is conceivable that in mild dehydration could lead to an initial vasopressin release, which in turn leads to nausea, which could lead to further vasopressin release and then to [[Hyponatremia]].
