Sitting down to a meal can be daunting for an expectant mother weighing conflicting recommendations about which foods are good for her unborn child -- and which might subject that child to long-term ills like allergies to peanuts and other foods.
"We have a tendency to beat pregnant women over the head with 'do this, don't do this,'" said obstetrician-gynecologist Dr. Laura Riley, medical director of labor and delivery at the Harvard-affiliated Massachusetts General Hospital. "We always want people to have a balanced diet that includes protein, fat and carbohydrates. We don't think pregnant women should take any one thing out of their diet."
But just as dietary advice for weight loss changes as new scientific information becomes available, so, too, does dietary advice for moms-to-be about how their eating shapes their future sons' and daughters' health.
The advice has been particularly tricky with respect to peanut allergy, a potentially fatal condition that affects an estimated 1 percent to 2 percent of children. The incidence has gone up in the last decade, although scientists can't say why.
From 1998 to 2000, the American Academy of Pediatrics and the British Committee on Toxicology recommended that in families where parents or siblings have allergies, women avoid peanuts during pregnancy and breast-feeding. But the data for these recommendations was scant and scientific studies yielded conflicting findings: Some said early exposure might be protective, others, harmful.
In 2008, the AAP reversed its position. Similarly, the European panel reversed its recommendation to stay away from peanuts during pregnancy and breastfeeding.
It now appears that in families with lots of allergies, it makes some sense for mothers-to-be to go easy on the peanuts, because of new research suggesting heavy consumption, particularly late in pregnancy, might set the stage for peanut allergies.
But for most families, doctors say there's no evidence that pregnant moms' peanut eating will produce an allergic baby -- or that avoiding peanuts will guarantee a healthier one.
To help clarify the issues, the Consortium of Food Allergy Research studied the relationship between maternal diet and childhood allergies. The researchers, led by Dr. Scott H. Sicherer of the Jaffe Food Allergy Institute at Mount Sinai School of Medicine in New York, followed 512 infants with food allergies to see if they became allergic to peanuts over time.
The investigators from Mount Sinai, Duke University in Durham, N.C., Johns Hopkins in Baltimore, National Jewish Health in Denver and Arkansas Children's Hospital in Little Rock, also asked the mothers about their prenatal eating.
In results published online Oct. 29 in the Journal of Allergy and Clinical Immunology, which will appear in the December print issue, they reported that the more that a mom consumed peanuts in the third trimester of her pregnancy, the greater the chances her infant would test positive for sensitivity to peanuts.
However, sensitivity doesn't equate to peanut allergy, "just an increased risk," Sicherer said.
At enrollment, the children were ages 3 months to 15 months, too young for food challenges typically used to establish allergies. The study authors said the youngsters would have to be monitored over time to determine if they developed peanut allergies later.
Sicherer also qualified the findings by saying the study involved only families with allergy histories, so the findings might not apply to the general population. Also, he said the study is observational, which means it doesn't prove cause and effect.
Nevertheless, several experts called the results significant, but not practice-changing.
"This is an area that deserves more investigation and more studies," said Dr. Neeta Ogden, an adult and pediatric allergist in private practice in Closter, N.J. "I've suspected in my own clinical practice, based on anecdotal evidence, that there has been some association. A study like this puts it on our radar for pregnant women that perhaps they should be consuming peanuts with caution, especially when they're in their third trimester."
While it's clear that a mother's diet influences the health of her unborn child, there's a lack of evidence that eating specific foods can prevent certain illnesses and conditions in her child.
Said Dr. Stephen Wasserman, an asthma and allergy specialist at UC San Diego, "There is not going to be one diet or one set of behaviors for all people. People bring their genetics to the table and one person's good is another person's bad."
Doctors agree that pregnant women should aim for a well-balanced diet and make sure they get sufficient protein, which is the basis for a growing fetus' cells and tissues. But some go beyond that:
Some obstetricians and others urge mothers to eat fish rich in the omega-3 fatty acids thought to promote development of a healthy nervous system in the fetus. Most include the caveat that they need to avoid going overboard, because fatty fish also tend to have higher levels of mercury, which is toxic to immature brains.
A limited number of studies suggest a diet rich in vitamin D, the sunshine vitamin, may protect against allergies. Still, said Dr. Christian Pettker, medical director of labor and birth at Yale-New Haven Hospital, "we are actually learning that more and more women have vitamin D deficiency than we previously suspected For these reasons, more obstetricians are advocating a diet that accounts for a healthy amount of vitamin D, and if vitamin D deficiency is suspected, this should be looked into." However, said Dr. Harvey Leo, a pediatric allergist at the University of Michigan's Center for Managing Chronic Disease, because vitamin D is predominantly made by sun exposure, "a mother's diet may not make a difference at all. It may be the mother's sun exposure."
Both Omega-3 fatty acids and vitamin D decrease inflammation, and asthma and allergies are inflammatory disorders, so Dr. Katherine Sherif, Director of the Center for Women's Health and associate professor of internal medicine at Drexel University College of Medicine in Philadelphia, recommended pregnant women take Omega-3 supplements and vitamin D supplements daily.
Some studies suggest a Mediterranean diet rich in fresh fruits, vegetables, legumes and lean protein could protect against allergies. However, physicians who are sticklers for evidence, rather than anecdotes, say the benefits to the unborn child aren't proven. "The Mediterranean diet is sort of like vitamin C: It's probably not bad, but probably as many studies say it's beneficial as say it's not," said Dr. Dana Wallace, an allergist in Fort Lauderdale and the incoming president of the American College of Allergy, Asthma and Immunology.
Probiotics, which encourage good bacteria to take up residence in the digestive system, may discourage the development of allergies, said Dr. Leonard Bielory, an allergist and immunologist now at the Center for Environmental Prediction at Rutgers University in New Jersey. Bielory was the primary investigator in a meta-analysis that suggested children may not be as allergic "if they have probiotics introduced into the diet or the mother's diet while pregnant."
Doctors are of one mind when it comes to pregnant women avoiding unpasteurized milk products, deli meats, raw fish and raw meats, which can lead to infections that could permanently affect the development of the unborn baby "and in some cases can be life-threatening," said Dr. Ari Brown, a pediatrician in Austin, Texas, and co-author of "Baby 411."
Beyond that, doctors tend to be more selective. Leo, for example, asks pregnant mothers who have a child with a documented food allergy "to avoid peanuts and tree nuts during the last trimester of pregnancy and during nursing if it's feasible. The data is still variable on this, but I feel this is a reasonable precaution for now in light of current studies."
Bielory said there are well-done studies coming down on both sides of whether early exposure to peanuts protects against allergies or makes children more vulnerable. He cited a recent study that showed in Israel, where "there's a lot of peanut-based products introduced early in childhood, they have a lower prevalence of peanut allergy, where as in the U.K., where they did adopt a ban on ingestion of peanut-based products in infants and children, there was interestingly a higher prevalence of peanut-associated allergic responses."
"It is hard to know what is right or if there is any definitive influence," Sicherer said.
He added he's had mothers say they ate lots of peanuts and thought it caused their child's allergies, while other mothers who avoided peanuts are stumped as to why their child is allergic to peanuts.
"I think that we, unfortunately, have to say we do not yet know a certain answer," he said
Most of all, Sicherer said he doesn't want his study to make mothers feel guilty about their past eating decisions.
"Our study says we need to look more carefully," he said, "but we are not prepared to change public health recommendations."