“If you ever cook meat in this house again, I’m leaving.” Those were not the words of a vegetarian but of a woman eight weeks pregnant with a whiff of steak up her nose — my nose.
Usually, I was happy to chow down on a hunk of meat. But suddenly, the smell of broiling cow had me heading for the toilet. Of all the things I could have hated, why meats?
New mothers, guided by their doctors, have attributed such aversions to the hormonal storm brewing inside them, or to the roller-coaster emotions that attend pregnancy.
But in the late 1980s, biologists such as Margie Profet of the University of California at Berkeley offered a new explanation: Morning sickness, she said, could be an evolutionary adaptation, one that protects a vulnerable fetus from natural toxins. By avoiding certain foods during pregnancy, women may improve their chances of having healthy children — and so pass on the same aversion to future generations.
Last summer that idea became science when Cornell University graduate student Samuel Flaxman and his adviser, neurobiologist Paul Sherman, published the results of a cross-cultural study in The Quarterly Review of Biology. Their data confirmed the outlines of the evolutionary theory, but the details came as a surprise.
Whereas Profet and others suggested that primarily vegetables and spices trigger morning sickness, the new study fingered meat, fish, poultry and eggs. Many women avoid these foods early in pregnancy when fetal organs are forming, Flaxman and Sherman found. And the evolutionary reason seems clear: Meat was once the food most likely to carry parasites and pathogens that could harm a fetus, as well as put a mother at risk.
“Morning sickness is a misnomer,” Sherman explained recently, over lunch at a Vietnamese restaurant on the edge of the Cornell campus. “It’s not always in the morning but happens all day long for most women. And it’s not a real sickness but really something positive.”
Getting sick makes sense early in pregnancy, Sherman says. When the body is invaded by something — be it a virus or a ball of foreign cells called an embryo — nausea often results. Estrogen, progestin and other hormones clearly mediate this response, and studies show that women who have some form of sickness are less likely to miscarry.
But contrary to conventional wisdom, high hormone levels don’t necessarily translate into severe morning sickness. In fact, women with morning sickness tend to have no more estrogen and progestin than women without it.
Flaxman and Sherman looked at the results of 56 morning sickness studies covering 79,000 pregnancies in 16 countries. Sixty-six percent of pregnant women, they found, reported some sort of illness in weeks 6 through 14 of their pregnancy.
More interesting, about 65 percent of the women had an aversion to at least one food — far too many for the aversions to be merely whimsical. Among those women, 16 percent avoided caffeinated drinks, 8 percent avoided such strong-tasting vegetables as broccoli and cabbage, and 4 percent avoided spicy ethnic food — all of which contain natural toxins, called secondary compounds, that protect plants from pests and pathogens.
A full 28 percent, however, couldn’t stomach animal products, especially meat, poultry, eggs or fish.
The Cornell team found further proof of this pattern by comparing anthropological reports from 27 societies where the early symptoms of pregnancy were discussed. In seven of those societies, morning sickness was all but unknown. In 20 others, it was common. Societies in the first group rarely ate animal products. Most in the second group drank milk or ate fish or meat.
Pregnant women in meat-eating societies seem to be caught in a double bind. Their fetuses could use the protein in meat, but the pathogens it might carry are too dangerous to ingest, hence morning sickness.
In a previous study, another Cornell graduate student, Jennifer Billing, worked with Sherman to show that spices such as chili pepper, allspice and oregano kill parasites and pathogens in food.
“When you go to the local grocery store and look at the spice rack, it’s an apothecary,” Sherman says. “We’re using those compounds to protect ourselves.”
Especially in hot climates, where meat spoils quickly, people traditionally use spices to disinfect their food. Unfortunately, pregnant mothers don’t have that option: Spices contain natural toxins too. If spices are out, then pregnant women are better off avoiding foods that spoil altogether.
To most women, morning sickness seems to begin and end arbitrarily, yet its timing makes sense: A fetus’ major organs develop between 6 and 14 weeks after conception. That’s also when a mother’s immune response temporarily weakens, giving the embryo time to burrow into the uterine wall. As a result, pregnant women are especially susceptible to bacteria, viruses and tumor cells during those weeks.
For example, spoiled food commonly contains toxoplasma, a protozoan parasite. Toxoplasma is usually harmless, but early in pregnancy it can cause maternal infection and possible miscarriage. Once the fetus is less vulnerable — after the first trimester, say — the nutritional value of meat outweighs the risk, and the aversion usually subsides.
