The researchers looked at allergies that produce respiratory and skin symptoms, including dust mites, cats and grass. "Allergic asthma" causes breathing problems, like wheezing and coughing, in the presence of common allergens such as dust.
Researchers from Queen Mary University of London used data gathered from nearly 9,000 mother-child pairs in the Avon Longitudinal Study of Parents and Children, an ongoing research project that tracks the health of families with children born between April 1, 1991, and December 31, 1992.
According to the World Health Organization, 235 million people have asthma, and it's a common disease among children. That number is expected to increase to 400 million by 2025. The American Academy of Allergy, Asthma & Immunology reports that worldwide, sensitivity to one or more allergens among children is approaching 40% or 50%, and 10.6% of children reported respiratory allergies in the previous 12 months in 2012.
"The dramatic 'epidemic' of asthma and allergies in the West in the last 50 years is still largely unexplained -- one potential culprit is a change in diet," said Annabelle Bedard, lead author and a postdoctoral fellow at Queen Mary's Centre for Primary Care and Public Health Blizard Institute. "Intake of free sugar and high fructose corn syrup has increased substantially over this period."
"Free sugars" include those that are naturally present in honey, syrups and unsweetened fruit juices -- though not those in whole vegetables and fruits -- and those that are added to food and beverages by the manufacturer, the cook or the consumer.
"We know that the prenatal period may be crucial for determining risk of asthma and allergies in childhood and recent trials have confirmed that maternal diet in pregnancy is important," Bedard wrote in an email.
To investigate a potential connection between a mom's diet and a child's allergies, Bedard and her colleagues calculated the amount of free sugars consumed by the women during pregnancy based on self-reported estimates in questionnaires.
Next, the team looked at how the moms' sugar consumption compared with allergies and asthma diagnosed in the children beginning age 7.
Nearly 62% of the children did not have any allergic conditions, but the remaining children had one or more maladies or symptoms. About 22% of the kids had a common allergy, 16% had eczema, 12% had asthma, 11% had wheezing with whistling, and 9% had hay fever, the researchers found.
Then, the researchers compared the children of moms who ate the least sugar during pregnancy -- less than 34 grams, or 7 teaspoons, per day -- with the children of moms who ate the most -- between 82 and 345 grams, or 16 and 69 teaspoons, per day.
The children of women with highest sugar intake during pregnancy had a 38% higher risk of allergy diagnosis, the researchers calculated. Kids of the moms in this group had a 73% increased risk of being diagnosed with an allergy to two or more allergens. And the allergic asthma risk increased by 101% for children of moms in the high-sugar consumption group, meaning the risk of developing allergic asthma was double that of kids born to women in the low-sugar group.
Only weak evidence linked sugar consumption during pregnancy to chronic asthma, the researchers say. And there were no ties between a mother eating sweets during pregnancy and either eczema or hay fever.
Bedard and Seif Shaheen, senior researcher of the study and a professor at Queen Mary, emphasized that the research is simply observational and so does not conclude that moms who eat sugar during pregnancy cause allergies in their children.
"Similarly, we cannot rule out the possibility that the main findings arose by chance," Bedard said.
With such strong results, though, the team is continuing to examine the issue and trying to replicate the findings among two other groups of children, Bedard explained.
Sheena Cruickshank, senior lecturer in immunology at the University of Manchester and a representative for the British Society for Immunology, said it is unclear from the study findings how strong the effect is.
"More studies will be needed to work out if there is any causation in this relationship," said Cruickshank, who was not involved in the new research. "Atopic diseases, such as allergy and asthma, are complex and associated with many genetic and environmental effects, including the microbes and pollutants we are exposed to."
Bacteria are commonly found within the digestive tracts of all humans. When it comes to babies, Cruickshank noted that it is well-known that this bacterial content "is influenced by how a baby is delivered, the type of milk the baby is fed as well as their diet as they grow older."
"Future studies should look to take these variables into account so we can understand the full relationship between maternal diet during pregnancy and allergic disease in the offspring," Cruickshank said.
Dr. Shannon M. Clark, an associate professor at University of Texas Medical Branch-Galveston, said the large number of participants was a strength of the study. However, the researchers did not examine each mother's sugar intake until after 32 weeks of pregnancy.
"As a result, the impact of exposure early in pregnancy cannot be assessed," said Clark, who was not involved in the research. "As with any study of this design, additional research would be ideal."
Clark noted that pregnant women need to eat healthy diets to reduce risk of developing gestational diabetes, a form of diabetes that occurs in pregnancy and that is more common among overweight and obese mothers.
"Eating excessive amounts of sugar in pregnancy can put the mother at risk for developing gestational diabetes," she said. "Gestational diabetes increases the risk of having a large baby, cesarean delivery and preeclampsia." And babies born to women with gestational diabetes may have problems with breathing, low glucose levels and jaundice.
For pregnant women, Bedard said, it would be "prudent" to follow general nutritional guidelines cautioning against eating an abundance of sugary products -- "whether or not we subsequently show that a high sugar intake in pregnancy causes allergy in the offspring."