About ten years ago, Ilene Bernstein, a behavioral neuroscientist at the University of Washington in Seattle, came across an interesting paper. Researchers had given pregnant rats a diuretic, which made them lose fluids and sodium. The offspring of those rats showed a distinct preference for salt. "In the paper," says Bernstein, "the authors speculated, I thought rather wildly, that this research might suggest a link between a mother's morning sickness and her offspring's taste for salt. I later met one of the authors, and he said his own mother had had morning sickness and he was crazy about salt. And I thought, ëThis is the silliest thing I've ever heard of.'"
More recently, while teaching an undergraduate psychology class, Bernstein thought she'd use the study to help her students understand how to test a hypothesis. She surveyed the students about their taste for salt and had them ask their mothers about how severe their morning sickness had been. "I fully expected negative results," says Bernstein, "but lo and behold, those whose moms said they vomited a lot seemed to like salt more."
Susan Crystal, a former graduate student now at the Monell Chemical Senses Center in Philadelphia, decided to find more concrete proof by studying 16-week-old babies fed only breast milk or formula. To find out if the infants liked salt, Crystal dripped a few drops of salty solution into their mouths and videotaped their expressions. Some babies licked up the last drop of the salty solution, while others grimaced, pursing their tiny lips and screwing up their noses. Crystal also had the babies drink from bottles containing plain water, slightly salty water, or more salty water and recorded how much they drank. Babies whose mothers had vomited frequently in the first few weeks of pregnancy liked the saltiest solution more than did those babies whose mothers had little or no morning sickness.
Bernstein suspects that the mother's vomiting helps determine an infant's taste for salt because dehydration and the loss of sodium spur the release of hormones that regulate salt intake. These hormones can cross the placenta and affect the developing brain of the fetus. The fetus might also churn out its own hormones in response to the mother's dehydration. "We know very little about the things that contribute to variation in taste preference," says Bernstein. "It's awfully interesting to have found a prenatal effect that can have a lifelong influence."
