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If you'd like to know more about nausea, I can highly recommend the book Nausea: Mechanisms and Management.

Nausea affects most people at some time in their life, and it can become a problem for runners, especially during longer distance events such as the ultramarathon.

  • Nausea is a difficult to describe feeling, usually being sensed from the stomach and sometimes nausea precedes vomiting. However there is good evidence that nausea and vomiting are separate. For instance, some people receiving chemotherapy and over a quarter of pregnant women experience nausea but no vomiting. Conversely, astronauts can experience vomiting with no sensation of nausea.
  • Much of the research on nausea and vomiting is actually focused on vomiting alone, limiting our understanding of nausea itself.
  • It is not possible to detect or to test for nausea. There are various physiological symptoms which may indicate that the subject is the feeling nauseous, but none of these are definitive.
  • Nausea is generally accepted to be a protective mechanism. When we ingest toxins nausea frequently follows. The sensation of nausea gives a stronger conditioning to avoid whatever food was consumed just prior to the onset of the nausea. This can cause problems for people undergoing chemotherapy, as they frequently develop many food and aversions.
  • Nausea tends to slow the digestive system, probably to reduce the rate of absorption of the toxins.
  • After a fast, even as short as an overnight fast, people will sometimes report a mixture of hunger and nausea. Under exceptional circumstances, nausea can be confused with hunger. This may be an adaptive survival mechanism to prevent a ravenous individual from eating indiscriminately of spoiled food and becoming poisoned.

1 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.
    • Inner ear disturbances. Problems in the inner ear can lead to motion sickness that can in turn lead to 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.
  • Heat Illness. Nausea is a symptom of heat exhaustion (Core temperature over 38°C/100.4°F).

2 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 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.

3 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.