KeywordsGamma Radiation Motion Sickness Taste Aversion Adaptive Function Plain Water
An uncomfortable sensation of an imminent need to vomit, usually perceived to originate from the stomach and occurring in waves.
The term nausea stems from the Greek word “naus” (ship) and was originally used in reference to seasickness (Andrews and Sanger 2014). There are, however, a great variety of external and internal stimuli that can provoke nausea in addition to motion sickness. These range from alternate physiological sources, such as food poisoning or pregnancy, to psychological sources, such as anxiety. In terms of diagnosing the presence of nausea, there is a difficulty due to the lack of known biomarkers (biological indicators) of its presence. As a result, nausea can be challenging to detect without self-reports from the patient (Andrews and Sanger 2014).
Nausea is closely associated with emesis (vomiting) as it is often elicited beforehand. Some research suggests these two sensations utilize the same neural pathways and, as such, form a continuum whereby a nausea-evoking stimulus, when present to a larger degree, will trigger an emetic response (Andrews and Sanger 2014). However, this continuum model is contested, as it fails to account for scenarios whereby vomiting occurs in lieu of nausea, as seen in accounts of astronauts vomiting in space (Stern et al. 2011).
Effects on the Body
There are several alternate effects on the body that occur in combination with nausea. These include a rapid heart rate (tachycardia), sweating, blood vessel tightening (vasoconstriction), and yawning (Andrews and Sanger 2014). Additionally, stomach function has been demonstrated to decline while nauseated, which prevents or slows the toxic food from leaving the stomach to be absorbed into the body (Stern et al. 2011).
The primary adaptive function of nausea is considered to be the protection of the body from ingested pathogens and toxins, facilitating both immediate and future avoidance of the contaminated stimuli (Stern et al. 2011). Upon consumption of a pathogenic or toxic source, nausea serves as a warning signal to prevent any further ingestion. Concurrently, the progression of the digestive system is slowed to reduce the amount of the toxin that allowed to be absorbed by the body.
The function of nausea, however, extends beyond facilitating the immediate avoidance of the toxic stimuli by also promoting future aversion of the contaminated food, a process called taste aversion learning (Garcia et al. 1955). This form of classical conditioning forms a long-lasting association between the ingested food and the following nausea or further illness (Bayley et al. 2002). From an evolutionary stance, this adaptation facilitates survival through the promotion of future aversion to toxic substances which may otherwise be ingested recurrently.
The evidence for taste aversion learning primarily stems from animal research on rodents, most famously the experiments on rats by John Garcia. Rats are often used in research concerning nausea due to their lack of emetic reflex (they cannot vomit). As such, it is inferred that they rely on taste aversion to avoid consuming lethal doses of toxic food. Garcia et al. (1955) demonstrated the potency of taste aversion learning in a study into the effect of aversion resulting from gamma radiation. Rats typically prefer a solution of water and saccharin, an artificial sweeter, over plain water; however, after the rats were conditioned to associate the solution with gamma radiation, the rats became aversive to the solution, preferring the plain water.
This taste aversion is specific to food consumption, as demonstrated by Garcia’s later experiment with Koelling (1966). In this study, rats were given a water source which emitted an audio-visual stimulus (light and a clicking sound) as the rats consumed the water. When the rats were subject to an electric shock while drinking, the aversive reaction was directed towards the audiovisual stimulus – the rats associated the light and the clicking sound, not the water, with the shock. When the negative stimulus was replaced with an x-ray or a toxin, which elicit nausea in humans, the reaction of the rats was directed towards the water and not the audio-visual stimulus. This suggests that nausea as an adaptive function has evolved to reinforce the avoidance of ingested stimuli and, when there are multiple potential causes for the nausea, to direct the aversion to the ingested stimuli. This is further evidenced by nausea resulting from chemotherapy treatments for cancer patients. While the patients know that the nausea and vomiting are a consequence of the treatment they are receiving, they can still grow averse to the food they consume, associating it with the nausea (Garcia and Koelling 1966).
A further adaptive function of nausea, interlinked with the ingestion of toxins, is “morning sickness” during pregnancy. This refers to the nausea and vomiting experienced by roughly 66 % of pregnant women that is most prevalent between the 8th and 12th week of pregnancy (Sherman and Flaxman 2002). Though not technically occurring more frequently in the mornings, this elicitation of nausea has the evolutionary function of protecting the mother and fetus from ingested toxins and pathogens. As aforementioned, nausea here also serves to prevent the ingestion of further amounts of consumed toxins and pathogens as well as to avoid the substance that hosted them in the future. This can be seen in the cravings and commonly avoided foods during pregnancy. This idea has been encapsulated by the maternal and embryo protection hypothesis (Hook 1976; Profet 1992), which suggests that the primary toxins that are avoided during pregnancy are teratogens (chemicals that lead to birth defects) and abortifacients (chemicals leading to abortion of the fetus). Recent research has supported this assertion of aversion, specifically for pregnant women, towards strong-tasting vegetables and caffeinated drinks which host the chemicals. Flaxman and Sherman (2000), however, suggest that aversion to these chemicals were less common than aversions to meats, fish, poultry, and eggs. They go on to suggest that, from an evolutionary standpoint, this could be an adaptive response to the parasites and pathogens these foods can often contain. This would be especially critical for our evolutionary ancestors who were unable to preserve food for extended periods of time or, more recently, that heavily salted their meat which would also be dangerous for the fetus if consumed by the mother.
Motion sickness, at first glance, provides a challenge from an evolutionary perspective as there appears to be little adaptive value in nausea and vomiting as a result of motion. Additionally, it can occur in the absence of recently ingested food which would suggest that the immediate expulsion of toxins may not be the sole cause of nausea and that its etiology may be more complex. Treisman (1977), however, suggested that motion sickness is the result of inconsistencies in the spatial representation of at least one of three inputs: visual, vestibular (representation of the position of the head, primarily from the inner ear), and proprioceptive (position of the body, input received from the main body and limbs). Treisman argued that one or more of these inputs being discrepant is a signal of an ingested neurotoxin and, as such, nausea is triggered even in the absence of recently ingested food as a warning signal that a previously ingested food may be altering the sensory inputs of the body. This theory has received little empirical support however, and the usefulness of regurgitating the remaining stomach contents after the toxins have already been absorbed would not rid the system of the toxins.
Nausea is an evolutionary adaption which has evolved to protect the individual from ingested toxins and pathogens. The immediate function of nausea is the prevention of consumption of further amounts of the toxic substance and to slow the digestive function of the stomach in order to limit the amount of toxins that are absorbed by the body. Furthermore, it serves to create an aversion to the substance containing the pathogen or toxin via taste aversive learning.
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