Web edition: January 26, 2006
If you happen to have an unhealthy penchant for salt, it may not be solely your fault. Mom may have to share some of the blame. Or so suggests a small but intriguing body of research.
The newest of these studies finds that among normal-weight newborns, the smaller a baby is at birth, the more likely it is that he or she will show an early and strong preference for salty fare. As have other studies, the new research hints that some sort of biological programming can occur in the womb that may foster a preference for salty foods later in life.
"The [newest] findings should be viewed as preliminary and hypothesis-generating," note Leslie J. Stein and her coauthors at the Monell Chemical Senses Center in Philadelphia. However, Stein adds, the data fit with results of past studies of salt intake by animals and small groups of people. The idea of people being preprogrammed to crave salt is worth pursuing in follow-up studies, she argues.
The Monell center is a nonprofit research institution focusing on the biological mechanisms underlying taste, smell, and irritation from chemicals.
"It's pretty amazingalmost staggeringthat in this day and age, we still don't understand much about how people detect salty tastes and the factors influencing how much we like salt," says Stein. Many people overuse this condiment, a contributor to high blood pressure, and thus a major risk factor for heart disease. Better understanding of salt's siren song might suggest ways people can resist it, adds Stein.
Stein and her colleagues tested the acceptability of salty water to 80 slightly hungry, 2-month-old babies. All had weighed at least 2,500 grams (5.5 pounds) at birth, the cutoff for normal-weight, full-term babies.
The researchers offered each infant a chance to drink from bottles of plain water, a saline concentration that was roughly as salty as a commercial chicken-noodle soup, and another saline solution that was twice that salty. The bottles were presented in random order with a 30-second pause between bottles. Babies were allowed to drink up to 30 milliliters from any bottle. Shortly after the third bottle had been taken away, each baby was offered the bottles again, this time in the reverse order. Afterward, a researcher who didn't know the bottles' contents recorded how much of each liquid the baby drank.
In an upcoming issue of the European Journal of Clinical Nutrition, Stein's team reports that while some infants rejected the salty solutions, others preferred them to the plain water. The trend followed a clear pattern, the researchers noted, with the preference for salty solutions being higher in babies born small.
The same test was administered to all the babies 4 months later. At this point, the infants' consumption of the three drinks was roughly comparable. Stein notes that this is a time when babies typically are being introduced to solid foods, so all those in the study may have been more willing to try novel tastes than they had been earlier.
The researchers then let about 3 years pass before bringing half the children back to the lab for more taste-preference tests. This time, each child was asked to sample seven foods. Four were salty (potato chips, pretzels, bacon bits, and dill pickles) and three weren't (shortbread cookies, chocolate candy, and dried banana chips). Each child was asked to sample the food and then give it to a Sesame Street doll to eat or throw awayeither to Big Bird for eating or to Oscar the Grouch for disposal in his trashcan. The researchers presumed that the decision as to whether a food should be eaten by a character would reflect a child's preference for that food.
"We did this," Stein explains, "because it doesn't put any demands on the kids to try and please us." Often, kids comply with what they think an adult expects, or they refuse just to be contrary. By employing the Sesame Street characters, she says, "the kids viewed it as a game," which took the researchers' expectations out of the equation.
During this test, birth weight reemerged as a determinant of preference. The smaller the child had been at birth, the more likely he or she was to give Big Bird the salty foods. Birth weight played no role in the apparent palatability of the nonsalty snacks.
Earlier studies had shown that babies of women who experienced morning sickness, frequent vomiting during early pregnancy, tend to be slightly smaller at birth than babies born to moms who avoided such illness. And at least two of those studies suggested that the offspring of women suffering from considerable morning sickness have a penchant for salty items. Researchers from the University of Washington published both studies.
In the more recent one, Ilene Bernstein and Sue Crystal tested 16-week-old babies. The mothers of 14 infants had reported frequent vomiting during the first trimester of pregnancy. Mothers of the 15 other babies reported few if any bouts of morning sickness.
Using a syringe, the researchers alternately delivered a small quantify of distilled water or slightly or heavily salted water into a baby's mouth. The infant's expression after tasting the liquid was videotaped to record any positive reaction, such as licking or sucking, or negative sign, such as a wrinkling of the nose.
Following this test, the babies were offered bottles of distilled or salty water, and a researcher removed a bottle after 1 minute or signs that the child was rejecting the nipple. The unsalted and salty liquids were offered to each child in the same order as in the syringe-administered taste tests. As before, a 30-second pacifier break separated each tasting. Five minutes after the sequence was completed, the researchers offered each baby the bottles in the opposite order.
Each bottle's contents were measured to assess how much of its liquid a child had consumed in both rounds.
Bernstein and Crystal found that babies whose moms had experienced moderate-to-severe morning sickness were significantly less likely than the other babies were to show aversive facial reactions to the salty liquids during the syringe tests. The morning sickness group also drank about three times as much of the saltiest water as they did of either the low-salt or the no-salt solutions. The nomorning sickness group drank roughly the same amount of all three liquidsa quantity comparable to the distilled-water consumption by the other group.
This trend may persist over decades. A few years ago, Bernstein and Crystal found that young adults whose mothers had suffered from moderate-to-severe morning sickness salt their food more heavily and have a greater preference for salty snacks than do adults whose moms had experienced little morning sickness.
When the body loses salt, such as after bouts of vomiting, hormonal signals kick in to foster a craving to replenish the vital chemical. Principally, a person's blood concentrations of aldosterone and angiotensin increase.
It's possible that if women vomit substantially during early pregnancy, Bernstein and Crystal say, the increase of these hormones in their bodies might somehow program the developing fetus to later crave salty foods. That certainly happens in lab animals.
Stein and her colleagues have some evidence that programmed salt craving might occur in people. Her group at the Monell center identified teens that had been given a saltfree formula as infants. The deficiency of the formula triggered a metabolic syndrome in these children that upregulated their bodies' production of aldosterone and angiotensin.
To see if that early hormonal response to salt deprivation had any lasting sensory consequences, Stein's group tested the teens' preference for salty foods. In an attempt to account for any family influence on eating patterns, the researchers tested the salt preferences of a parent, and a sibling who was living at home and close in age to the teen who had received the saltfree infant formula. Both the parents and siblings proved less likely to prefer salty foods than were the children who had the hormonal imbalance as babies.
The effects observed in all of these studies have been subtle, Stein says. Clearly, many factors contribute to a penchant for salty foods or any other sensory preference, she notes. However, she says that it appears that low birth weight and exposure in the womb to excess aldosterone and angiotensin might program the palate to appreciate salt.
Not all data fit this pattern, however. For instance, researchers at Harvard and Brown Universities reported 4 years ago that the higher an infant's birth weight, the more likely he or she was to exhibit a preference for salt. A grandparent with high blood pressure also portended higher blood pressure in these infants.
Stephen H. Zinner of Harvard, who led the study, says that the design of his experiments was so different from those of the Monell and Washington tests that comparisons between findings are difficult.
He admits that among the babies he tested, none appeared to like the taste of a salty solution. However, he told Science News Online, "some rejected it less, and that's what we termed a preferential response to a salt taste."
In two experiments, conducted a month apart, 283 newborns of various birth weights within the normal range were allowed to taste a few drops of plain water and each of two salty solutions. After each taste, Zinner's team measured the infant's sucking responseas measured by an instrumented nipple. This sucking didn't deliver any additional liquid.
The babies also were administered a sugary solution. This triggered the biggest sucking responseone dramatically higher than for plain water and unrelated to birth weight or blood pressure. The salty solutions triggered less sucking than did either the plain or the sugared water. However, sucking responses by babies with the highest blood pressures at birthand a family history of high blood pressureindicated that they were least turned off by the taste of a salty liquid.
What Zinner would now like to know is whether that innate acceptability for salt in some babies causes them to consume saltier diets and develop ever-higher blood pressure throughout lifeor whether these kids will maintain high blood pressure regardless of their salt consumption.
In fact, the Harvard researcher might be able to test those hypotheses soon, since the subjects in his study are now young adults 20 to 24 years old. "My guess is that we could track down at least 50 to 60 percent of the subjects," he says. Indeed, when he moved to Harvard from Brown University a few years back, he brought all the files on the children with him.
"It's become one of my passions to see if such early tests have any predictive value over 10 or 20 years. When I think of all of the work that I've done over the last 35 years, this has always seemed the most interesting and important," he says.
Leslie J. Stein
Monell Chemical Senses Center
3500 Market Street
Philadelphia, PA 19104-3308
Ilene L. Bernstein
Department of Psychology
University of Washington
Seattle, WA 98105
Stephen H. Zinner
Harvard Medical School
Department of Medicine
Mount Auburn Hospital
330 Mount Auburn Street
Cambridge, MA 02138
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