iStock/ThinkstockBY: DR. AMITHA KALAICHANDRAN

(NEW YORK) — Why are more children becoming obese by medical standards and what effect could their moms have? That’s what researchers in a new study wanted to find out.

Nearly 1 in 5 children in the U.S. younger than 19 has a body mass index defined as obese, according to the U.S. Centers for Disease Control and Prevention.

Researchers in the study looked back at surveys showing some of the lifestyle habits of moms and their children over several years to find out what could be contributing to childhood obesity.

Their findings, published in the British Medical Journal, suggest that mothers who follow five specific habits may reduce the risk of obesity in their offspring.

Researchers at the Harvard School of Public Health analyzed data of just over 24,000 children ages 9 to 18 enrolled in the Growing Up Today Study. They were the offspring of nurses who had been enrolled in the Nurses’ Health Study II in the early 1990s.

As part of the NHS II, the nurses filled out questionnaires about their lifestyle habits, including alcohol use, smoking habits, exercise habits and diet.

Of the children included, 5.3 percent, or 1,282, developed obesity during the follow-up period. When researchers looked back at the risk factors and compared them to the non-obese children, two factors were connected to better outcomes for kids: A healthy body weight in a mom was associated with a 56 percent lower risk of obesity in her children, and being a non-smoker was associated with a 31 percent decrease.

An even larger decrease in obesity risk — 75 percent — was found when the mothers followed five healthy habits in combination: Maintaining a healthy body weight, never smoking, 150 minutes of moderate to vigorous exercise a week, consuming a healthy diet and low to moderate alcohol drinking — even compared to those who abstained completely.

While some of these lifestyle choices have been known to contribute to obesity, finding this combination of influences on children’s bodies was new.

“Currently there isn’t much data specifically on maternal behaviors after giving birth and associations with childhood obesity,” lead author Dr. Qi Sun, an assistant professor in the department of nutrition at the Harvard School of Public Health, told ABC News. “There are many risk factors that can be modifiable. Previous research has shown, for instance, the risk of maternal smoking on the development of childhood obesity, but we haven’t found data on which combination of specific lifestyle factors, if followed by mom, could decrease the risk of obesity in their children.”

In the study, the combination of the five health behaviors seemed to matter more than any individual factor.

For instance, it didn’t matter as much what mom ate — maternal diet alone was not associated with childhood obesity risk. The researchers believe this could be because children’s diets are also influenced by what they eat at school and among friends. Diet can also be a difficult factor to control.

“Programs that seek to change a child’s behavior into one of healthy eating are very difficult, but our work suggests that some strategies may involve intervening at the mother’s level,” Sun said.

Neither a specific sex nor age of the child were associated with obesity. But it’s hard to say whether these results could be generalized across socioeconomic or racial groups, since the mothers involved were medium- to high-income nurses and mostly Caucasian.

Another limitation of this study: Both a mom’s body weight and her dietary history were based entirely on self-reported data. But he notes that the study findings verify some previously known theories.

“Unfortunately people don’t usually report their dietary patterns accurately,” Dr. Robert Lustig, a pediatric endocrinologist and professor emeritus at the University of California San Francisco, who is a national expert in pediatric obesity, told ABC News. “But the association with smoking makes a lot of sense. Especially if the mother smoked during pregnancy, we know that it can lead to insulin resistance in the baby due to inflammation. And we have good data that suggests this insulin resistance can manifest in the newborn and continue throughout their life.”

In addition to the five lifestyle factors noted in the Harvard study, he said a wide body of research on childhood obesity indicates a few more risk factor, as well.

“In my view, the most important thing for mothers to do, to impact the risk of obesity in their child, is to reduce their own stress levels — and that’s not that easy to do,” Lustig said. “For children themselves, [it’s important] to limit the food items that increase insulin resistance. So, for instance, decreasing or eliminating their consumption of sugar-sweetened beverages — juice is probably the single-worst culprit.”

The study researchers said they aren’t done analyzing all the information from the survey. The next step is to continue to study the children in the GUTS program to see if there are any longer-term effects on their body mass that show up after age 18 and could be related to their moms’ lifestyle choices.

Amitha Kalaichandran, M.D., is a pediatrics resident working in the ABC News Medical Unit.

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