Tuesday, May 8, 2012

Morning sickness: Animal vs. Plant Foods

Some investigators have reported that morning sickness is less common or even unknown among people who eat plant-based diets, but common among people who eat animal-rich diets.  These authors report that women typically have greater aversions to animal products than to any other foods, and propose that morning sickness functions to reduce exposure to food-borne pathogens.

Morning sickness: a mechanism for protecting moth... [Q Rev Biol. 2000] - PubMed - NCBI


Approximately two-thirds of women experience nausea or vomiting during the first trimester of pregnancy. These symptoms are commonly known as morning sickness. Hook (1976) and Profet (1988) hypothesized that morning sickness protects the embryo by causing pregnant women to physically expel and subsequently avoid foods that contain teratogenic and abortifacient chemicals, especially toxic chemicals in strong-tasting vegetables, caffeinated beverages and alcohol. We examined this hypothesis by comprehensively reviewing the relevant medical, psychological and anthropological literature. In its support, (i) symptoms peak when embryonic organogenesis is most susceptible to chemical disruption (weeks 6-18), (ii) women who experience morning sickness are significantly less likely to miscarry than women who do not (9 of 9 studies), (iii) women who vomit suffer fewer miscarriages than those who experience nausea alone, and (iv) many pregnant women have aversions to alcoholic and nonalcoholic (mostly caffeinated) beverages and strong-tasting vegetables, especially during the first trimester. Surprisingly, however, the greatest aversions are to meats, fish, poultry, and eggs. A cross-cultural analysis using the Human Relations Area Files revealed 20 traditional societies in which morning sickness has been observed and seven in which it has never been observed. The latter were significantly less likely to have animal products as dietary staples and significantly more likely to have only plants (primarily corn) as staples than the 20 societies in which morning sickness occurred. Animal products may be dangerous to pregnant women and their embryos because they often contain parasites and pathogens, especially when stored at room temperatures in warm climates. Avoiding foodborne microorganisms is particularly important to pregnant women because they are immunosuppressed, presumably to reduce the chances of rejecting tissues of their own offspring (Haig 1993). As a result, pregnant women are more vulnerable to serious, often deadly infections. We hypothesize that morning sickness causes women to avoid foods that might be dangerous to themselves or their embryos, especially foods that, prior to widespread refrigeration, were likely to be heavily laden with microorganisms and their toxins. The alternative hypotheses that morning sickness is (i) an epiphenomenon of mother-offspring genetic conflict or hormones associated with viable pregnancies, or (ii) an indicator to potential sexual partners and kin that the woman is pregnant, resulting in reduced sexual behavior and increased nepotistic aid, were not well supported. Available data are most consistent with the hypothesis that morning sickness serves an adaptive, prophylactic function.

According to the CDC:

What foods are most associated with foodborne illness?

  • Raw foods of animal origin are the most likely to be contaminated; that is, raw meat and poultry, raw eggs, unpasteurized milk, and raw shellfish.
  • Because filter-feeding shellfish strain microbes from the sea over many months, they are particularly likely to be contaminated if there are any pathogens in the seawater.
  • Foods that mingle the products of many individual animals, such as bulk raw milk, pooled raw eggs, or ground beef, are particularly hazardous because a pathogen present in any one of the animals may contaminate the whole batch.
    • A single hamburger may contain meat from hundreds of animals.
    • A single restaurant omelet may contain eggs from hundreds of chickens.
    • A glass of raw milk may contain milk from hundreds of cows.
    • A broiler chicken carcass can be exposed to the drippings and juices of many thousands of other birds that went through the same cold water tank after slaughter.
  • Fruits and vegetables consumed raw are a particular concern. Washing can decrease but not eliminate contamination, so the consumers can do little to protect themselves.
    • Recently, a number of outbreaks have been traced to fresh fruits and vegetables that were processed under less than sanitary conditions. These outbreaks show that the quality of the water used for washing and chilling the produce after it is harvested is critical. Using water that is not clean can contaminate many boxes of produce.
    • Fresh manure used to fertilize vegetables can also contaminate them. Alfalfa sprouts and other raw sprouts pose a particular challenge, as the conditions under which they are sprouted are ideal for growing microbes as well as sprouts, and because they are eaten without further cooking. That means that a few bacteria present on the seeds can grow to high numbers of pathogens on the sprouts.
    • Unpasteurized fruit juice can also be contaminated if there are pathogens in or on the fruit that is used to make it.
 Notice that fruits and vegetables carry pathogens only if processed under less than sanitary conditions or contaminated with manure (an animal product).  Otherwise, the types of microbes that infect plants typically do not infect animals like ourselves.

This seems to provide evidence against the idea that modern humans evolved in dependence on uncooked (or cooked) meat as a source of energy to support our large brain.  

If humans spent millions of years adapting to a meat-based diet, eaten raw, as proposed by some authors, then it seems strange that pregnant women would be unable to stomach such a diet during the critical phases of pregnancy. 

Since it is certainly better for a woman to be able to ingest foods during the early critical stages of pregnancy that to be unable to do so, if an increasingly meat-based diet dominated human evolutionary time, natural selection would have favored the reproduction of women who did not get sick at the sight, smell, or ingestion of animal foods (perhaps because of much higher production of stomach acid, different odor tolerances, etc.), and eventually, it would have produced a human species that had no problems eating a diet rich in raw or cooked meat during the critical phase of pregnancy.

But instead we have a human species in which the pregnant females have the strongest aversions to animal products, and women eating plant-based diets have the lowest incidence (as low as zero) of morning sickness.