Sit down with a group of pregnant women and it’s just a matter of time before the conversation turns to food: one only wants to eat ice cream, another craves liver, a third has to cross the street whenever she sees a Starbucks to avoid the smell.
New research from the Fijian island of Yasawa supports the idea that food cravings and aversions are an adaptive strategy to guarantee the health of the fetus. Women avoid foods with potential toxins and pathogens, and crave foods that contain nutrients the developing fetus needs.
The research also illuminates how societal taboos on dangerous foods interact with biological cravings and aversions.
“Along with our previous studies on the island, this research suggests that the women of Yasawa cope with the challenges posed by pregnancy through both genetic and cultural adaptations. The work shows the usefulness of studying human behavior through the lens of both genetics and culture,” says Joseph Henrich (Harvard University), a senior fellow in the Institutions, Organizations & Growth program.
His paper, coauthored by Mark Collard and Luseadra McKerracher, was published in the journal Human Nature.
Among the Yasawan women they studied, 71 per cent reported developing a food aversion during pregnancy, most commonly to fish and cassava. Another 70 per cent reported cravings, most commonly for bananas and plantain. Most foods that were craved were not aversive and vice versa. The notable exception was fish, which 81 per cent of women found aversive but 20 per cent craved.
The study supports the idea that aversions are intended to protect the embryo. This is especially true of the aversion to fish; women unconsciously steer away from foods likely to contain pathogens, because their immune systems, suppressed in order to accept foreign fetal tissue, cannot risk the possibility of pathogenic insult. Because Yasawan villages are without refrigeration, this risk is particularly acute.
The fact that women also crave animal foods suggests that, given the nutritional demands being made by the fetus, their bodies need these rich sources of protein and micronutrients and therefore seek them elsewhere in their diet – in the form specially prepared fish (cooked to a minimum temperature and/or with antimicrobial spices), meat, dairy or shellfish.
The aversion to starchy plant-based foods is less easy to explain, but it seems likely that Yasawan women are unconsciously responding to cyanide-producing compounds found in cassava that could endanger fetal development. Another hypothesis is that pregnant women will avoid foods with high glycemic indexes, such as cassava, which increase the risk of gestational diabetes and preeclampsia. As is the case with fish, avoidance of starchy vegetables that are staples of the Yasawa diet results in energetic deficits that in turn create cravings for similar foods.
Henrich’s previous research had looked into the role of food taboos on the island, which place certain foods off limits for pregnant women. That work showed that the taboo foods contained toxins that were more likely to harm the developing fetus. Obeying the taboos cut the chances of fish poisoning by 30 per cent during pregnancy and 60 per cent during breastfeeding.