The best of last June
There is new information from an older idea (from about 2000) by Paul Sherman and colleagues. The idea underlying this research is simple: Symptoms of illnesses may be adaptive. Indeed, this may be true to the extent that we should not call certain things illnesses. Like "morning sickness."
Broadly speaking, there are two different kinds of reasons that a woman may experience nausea in association with pregnancy. 1) This pregnancy thing is a complicated mess with all kinds of hormonal (and other) things going on, so you puke; or 2) a woman who is pregnant feels nauseous for good evolutionary reasons.
From an article in American Naturalist by Flaxman and Sherman:
"Morning sickness" is the common term for nausea and vomiting in early human pregnancy (NVP). Recent interest in why NVP occurs-that is, in the evolutionary costs and benefits of NVP-has spurred the development of two alternative hypotheses. The "prophylaxis," or "maternal and embryonic protection," hypothesis suggests that NVP serves a beneficial function by expelling foods that may contain harmful toxins and microorganisms and triggering aversions to such foods throughout pregnancy. The alternative "by-product" hypothesis suggests that NVP is a nonfunctional by-product of conflict-over resource allocation-between the pregnant woman and the embryo. The critical predictions of the prophylaxis hypothesis have been developed and tested, whereas the by-product hypothesis has not been subjected to similar scrutiny. To address this gap, we developed a graphical model and used it to derive predictions from the by-product hypothesis under two different assumptions, namely, that NVP is either (i) a by-product of current conflict between a pregnant woman and an embryo or (ii) a by-product of honest signals of viability produced by the embryo. Neither version of the by-product hypothesis is fully consistent with available data. By contrast, the timing of NVP, its variation among societies, and associated patterns of food cravings and aversions are consistent with the prophylaxis hypothesis.
In the prophylaxis, or more easily pronounced, "adaptive" hypothesis, a pregnant woman becomes hypersensitive to certain inputs (smell and taste of certain foods) that she should be avoiding for the health of her fetus. There are lots of things that typical adult humans can eat without causing harm that can do bad things to a fetus, especially during the first several weeks of development when organs are forming. Many plant products, for instance, are suspect.
After testing the two dominant theories (one adaptive and the other non-adaptive) for why two-thirds of women around the world -- but seemingly no other mammals -- experience nausea and vomiting in pregnancy, only one holds water, says Paul Sherman, Cornell professor of neurobiology and behavior and a Weiss Presidential Fellow.
"Our study, which tested theories and predictions about the nature of parent-offspring conflict in human pregnancy, shows that nausea and vomiting in pregnancy is beneficial by expelling such foods as meat and strong-tasting vegetables that historically and still may contain harmful toxins and microorganisms that could potentially sicken the woman and damage her fetus just when its organs are developing and are most vulnerable to chemicals," said Sherman, who is an expert in Darwinian medicine -- viewing diseases from an evolutionary perspective.
"Morning sickness" is rare after the first 18 weeks ... the organ forming weeks ... of pregnancy. Severe morning sickness is associated with lower rates of spontaneous abortion. The most pathogen-containing foods (historically, as in, pre-refrigerator) are those that tend to induce nausea in these women. Morning sickness is found more often in societies with risky foods as part of their day to day diet. And, this phenomenon ... morning sickness ... is only known in humans. Humans have a broad and, in terms of toxins, relatively dangerous diet.
This last part could be investigated further, I would think. There must be enough variation out there among omnivores to find some other species with morning sickness. Keep looking guys!
(I would add that slow development is another feature of humans that could be important.)
If the alternative theory that morning sickness is a non-adaptive outcome of an evolutionary tug-of-war between the mother and fetus for resources were correct, then the nausea should peak in the final trimester, when the fetus requires more nutrients and blood than at any other time. But it doesn't. Neither does it occur with every pregnancy. If morning sickness were the result of the fetus signaling its viability to the mother, then all humans and other mammals should experience it.
"All this leads us to suggest that morning sickness is a misnomer," Sherman said. "It doesn't occur just in the morning, and it's not an illness. It can occur any time of day and it appears to be beneficial -- we could call it a form of evolutionary wellness insurance."
Sources:
Flaxman, S., Sherman, P. (2008). Morning Sickness: Adaptive Cause or Nonadaptive Consequence of Embryo Viability?. The American Naturalist, 172(1), 54-62. DOI: 10.1086/588081
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As a male who had severe gallstones, with blockage, I know that when the gall duct is blocked for hours and then unblocked, the bile fluid empties into a usually-empty duodenum, causing profound nausea. (The instant treatment? A tall glass of warm vinegar, which provokes a lengthy belch, giving fast relief.)
I would be surprised if pregnancy had no effect on the functioning of the gall bladder. Has this been investigated?
This is a subject near and dear to my heart--I found great deal of comfort in Margie Profet's "Morning Sickness"--a book that originally laid out this hypothesis. Reading the book gave me the liberty to ignore "What to Expect when You're Expecting" and stop trying to force feed myself broccoli.
There seems some controversy as to whether dogs (the mammals we know most about) can sometimes get morning sickness. I would not be too sure humans are the only mammal to experience this. There are a lot of mammals out there we know little about when it comes to pregnancy.
I heard Sherman talk a few years ago - scientifically it was dreadful. I wouldn't believe anything he says.
Funny you should mention Margie Profet and her unique theories on morning sickness. I just read one of the most interesting stories I've read in a long time -- about her, the latest on her life, as it were:
Margie Profet's Unfinished Symphony
A Promising Scientist Vanishes Without a Trace
http://www.weeklyscientist.com
Rosie: Really? I've seen Paul speak a number of times and I've always been favorably impressed. I would not put too much value in a single experience.
Sailor: No, I doubt that they do. Remember: Dog are always puking. To a dog, puking is just fun. They are obviously using pregnancy as an excuse to puke more...
Rather thin on the foods. Which foods? Name 'em. Then compare among women who experience morning sickness. See if the foods are consistent among a large group of women.
I know this is just one anecdote but my morning sickness WAS in the morning (3 times) - BEFORE I had eaten. All I could force down was dry crackers. Ralphing bile is nasty. Why would my body be rejecting toxic foods before I had eaten ANY foods, even water?
How about a 3rd reason - a non-adaptive side-effect of pregnancy? Not everything is the direct result of evolutionary adaptation - sometimes things happen because of adaption for another reason. Fevers are the bodies way of fighting infection - high fevers make the body less palatable as a growth medium for germs & viruses. But they also make us feel ill and can "get out of control" causing brain damage & death. Hardly the "purpose" of the fever mechanism.
As 6EQUJ5 said, nausea can be a side effect of the body trying to adjust. Pregnancy is a big adjustment and the body may just " get bungled up" during the process and nausea is a side effect.
I also think 6EQUJ5 has a good point - would it not be a better scientific approach to find the physical mechanism first (what is causing the nausea, why it is triggered by pregnancy) rather than all the broad range of assumptions that Sherman had to make about each competing theory
Isn't there a theory that morning sickness is related to the rise in progesterone, which protects against miscarriage? That would fit with Hypatia's Daughter's scenario where morning sickness is the unpleasant by-product of an adaptation, not the adaptation itself.
The time the fetus is most sensitive to teratogens is while differentiation and epigenetic programming is going on and that is mostly in the first trimester. In the formation of the structure of organs, cells have to proliferate and differentiate in specific patterns depending on signals inside and between cells. These developmental processes are extremely complex (a vast understatement). There are many things about the motherâs health that are known to affect the epigenetic programming of the fetus, including stress, diet and illness as well as teratogen exposure.
What is the âpurposeâ of nausea in the first place, among the non-pregnant? We know that nausea evolved, and that many organisms have states that seem to be nausea-like. Presumably nausea didnât suddenly appear with humans.
Nausea does occur during sickness, and reduces food consumption during sickness. The bodyâs need for energy and nutrients is not reduced during sickness, however the bodyâs metabolic capacity is being diverted to sickness fighting, and perhaps nausea is the way to divert metabolic capacity away from food digestion to sickness fighting. If your body can successfully deal with the sickness, then you can regain what ever weight you lost. If it doesnât, then you didnât divert enough resources to fighting it.
During sickness, the body generates high nitric oxide levels through the expression of iNOS. That high NO level is what causes the vasodilation and hypotension of septic shock. NO normally blocks heme enzymes including the cytochrome P450 enzymes which are the major xenobiotic metabolizing enzymes. If the P450 enzyme activity is compromised by high NO, then avoiding consumption of xenobiotic chemicals would be advantageous. Similarly if those P450s are needed to metabolize toxins produced by pathogens, avoiding food might be advantageous.
Early fetal life is pretty hypoxic. Much of the energy is produced by glycolysis which produces far fewer free radicals than oxidative phosphorylation. Early pregnancy does not require additional energy. What is most important is that the fidelity of DNA replication is kept very high. NO is an excellent anti-oxidant, several orders of magnitude better than vitamin E.
What is needed in the first trimester is to expand the blood supply (requires NO to increase Epo), increase mitochondria number (requires NO to trigger mitochondria biogenesis), increase vascularization of the placenta (requires NO for angiogenesis). I suspect that the high NO of sepsis and the high NO of early pregnancy both trigger nausea. I think that trying to counter the nausea of pregnancy is not a good idea.
Daedalus2u, I have to wade through your biology terms (biology is not my forte, I am more into astronomy & physics), but to restate what you said:
You get sick, you produce high NO levels, high NO levels make you nauseous.
"Nausea does occur during sickness, and reduces food consumption during sickness. The bodyâs need for energy and nutrients is not reduced during sickness, however the bodyâs metabolic capacity is being diverted to sickness fighting, and perhaps nausea is the way to divert metabolic capacity away from food digestion to sickness fighting."
But is not the primary function of nausea (& diaherra) during sickness to EXPEL the germs, virus &/or toxins that are making you ill? It may also be its useful in diverting resources during illness, but is that its primary function?
In pregnancy you are not sick but you become nauseous for only an hour two. Then you resume your normal eating patterns. How effective is nausea at making you "avoid... (the) consumption of xenobiotic chemicals" if it only prevents you from eating normally for 2 out of 24 hours in the day?
I suspect that nausea (it is not really "sickness") is a side-effect of pregnancy - something is triggered by the hormones, such as over-production of bile, which then makes you nauseous.
Hypatia' da: Well, this theory is more or less a side effect. Sensitivity goes up. Because sensitivity goes up, the pregnant woman avoids a class of foods. That's the adaptation.
Then she pukes. Side effect.
So, this is an "adaptive explanation" in that it is an explanation that involves an adaptation, but not that the specific symptoms we identify as the syndrome are themselves adaptive.
Anon1 [0]: That was a repost of something that I've not read up on in a year, so I'm not sure so I'm not sure if the rise in progesterone hypothesis is tested and tossed or makes sense or not for some other reasons. But, this is a good point to note the ultimate/proximate issue. In your alternative explanation, the ultimate part is avoiding miscarriage (which presumes something we can't really presume: There is a "thing" called miscarriage that comes along and we want to avoid it) and the rise in progesterone is a proximate mechanism.
If that was shown, it would not eliminate Sherman's hypothesis. I can't think of too many adaptive systems that don't relate to or piggy back on some other existing system. If there is rising progesterone (and there is) it is going to be the signal for any number of other systems that are adaptations related to pregnancy.
d[10]:What is the âpurposeâ of nausea in the first place, among the non-pregnant? We know that nausea evolved, and that many organisms have states that seem to be nausea-like. Presumably nausea didnât suddenly appear with humans.
That might be a good question or it might be a side trip for now. I would not say with confidence that nausea arose in any other species. Offhand I can't think of any research on that at all. Puking, yes, bu nausea, I really don't know.
According to William's hypothesis on disease, nausea would play the role not in diverting or not food related to fighting the dissease, but rather, diverting food from the pathogen. As does (and this may be related functionaly) aenemea.
A reduction in the rate of eating supports an adaptation hypothesis. If the "problem" is the rate of metabolism of xenobiotic chemicals, then spreading out that consumption over a longer period of time better matches metabolic capacity with metabolic needs. Adverse effects are likely concentration dependant, and if the consumption rate is kept low, then the liver may be able to substantially fully metabolize them on the first pass blood flow from the gut.
There are very few pathways in physiology that do only one thing. Virtually all pathways evolved from something else, so they have multiple activities. That is one of the things that makes physiology so difficult to understand, there are so many things going on simultaneously.
Anemia also raises NO levels because hemoglobin is the sink for NO. That has been observed in isovolemic anemia (replacement of blood with plasma or other red blood cell free extenders) in rabbits where exhaled NO increases with anemia. Similarly isovolemic anemia causes vasodilatation and tachycardia, but with no hypoxia. The partial pressure of O2 in the venous return blood is not lower during isovolemic anemia even with ~50% reductions in blood hemoglobin.
NO prevents the formation of bacterial biofilms. I think that is the reason behind the very high NO levels of septic shock, to generate very high NO levels to prevent formation of a biofilm in the vasculature (which would very likely be fatal). Anemia may facilitate that, and may be why anemia during pregnancy occurs and why there are more adverse effects in pregnancies with high hemoglobin levels. Puerperal fever was a major cause of death before antibiotics. Evolution is going to âoptimizeâ survival by trading off death due to anemia, puerperal fever, cephalopelvic disproportion and everything else.