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The taste of kale is so repulsive to fetuses that it causes them to screw up their faces in disgust while in the womb, a new imaging study reveals. Researchers used 4D ultrasound to observe the reactions of 100 fetuses to the flavors ingested by their mothers, and found that they respond favorably to carrot but dislike leafy greens.
Though scientists have long suspected that human fetuses experience taste by ingesting nutrients dissolved in the amniotic fluid, no previous study had ever demonstrated this directly. Rather, the taste preferences of babies after birth has generally been considered indirect evidence of predilections developed in-utero.
“A number of studies have suggested that babies can taste and smell in the womb, but they are based on post-birth outcomes while our study is the first to see these reactions prior to birth,” explained study author Beyza Ustun in a statement.
“As a result, we think that this repeated exposure to flavors before birth could help to establish food preferences post-birth, which could be important when thinking about messaging around healthy eating and the potential for avoiding ‘food-fussiness’ when weaning.”
The researchers gave capsules containing either carrot or kale powder to pregnant women at both 32 and 36 weeks of gestation. Scans conducted shortly after ingestion revealed that fetuses were more likely to make “laughter-faces” when exposed to the taste of carrot and “cry-faces” after encountering the delights of kale.
“This effect occurs within a relatively short time: Around 30 min after maternal ingestion of the flavor capsules, we detected observable facial reactions in the fetuses,” write the study authors. “Thus, in this short time, the flavor content of the capsules undergoes digestion, absorption into the mothers’ bloodstream, metabolization and circulation through the placenta and fetus, collection in the amniotic fluid, and fetal chemoreceptors.”
The researchers also noted that the grossed-out faces elicited by kale tend to develop later in pregnancy than the happy carrot faces. This, they say, is due to the fact that looks of disgust require more complex facial muscle contortions than contented expressions.
Ultimately, the results of this study may help to shed light on the development of human taste and smell receptors, and could facilitate the development of healthy eating habits by familiarizing babies with certain foods before they are born.
“It could be argued that repeated prenatal flavor exposures may lead to preferences for those flavors experienced postnatally,” said study author Jackie Blissett. “In other words, exposing the fetus to less ‘liked’ flavors, such as kale, might mean they get used to those flavors in utero.”
“The next step is to examine whether fetuses show less ‘negative’ responses to these flavors over time, resulting in greater acceptance of those flavors when babies first taste them outside of the womb.”
Accordingly, the researchers are now conducting a follow-up study to determine how this experiment has affected the food preferences of these same babies after birth.
The study was published in the journal Psychological Science.
