New research has turned up another reason for women to avoid smoking during pregnancy. A recent study published in the journal Gut Microbes has found an association between mothers smoking during pregnancy and a higher risk of their children becoming overweight or obese.

Childhood obesity is a growing concern globally, affecting more than 18% of children and adolescents aged 5 to 19 in 2020, a startling increase from just 4% back in 1975.

And it’s a big problem. The study notes childhood obesity is linked to such negative outcomes as poorer health and lower self-esteem, and an increased likelihood of being bullied.

Anita Kozyrskyj, a microbiome epidemiologist and professor of pediatrics at the University of Alberta, is the co-senior author of the study. Kozyrskyj, who is a member of the Women and Children’s Health Research Institute, has done a lot of research on early life factors that affect infant gut bacteria. These factors include how infants are born, how they’re fed, the health of the mother and stress during her pregnancy.

Kozyrskyj says other researchers have shown that women who smoke during pregnancy tend to have babies who become overweight in childhood. But she says her group’s study represents new ground because they show an association that might indicate the cause. And it’s in those microbes found in the gut.

“We’ve known that for a long time. We just didn’t know how it happened,” Kozyrskyj says. “There may be many ways, but in our study, we showed one way is by changing the gut bacteria in the infant.”

Kozyrskyj and the team used data from more than 1,500 children from the Canadian Healthy Infants Longitudinal Development (CHILD) Cohort. The team collected data from families that told about the mothers’ environmental and lifestyle factors during pregnancy and those of their children from birth to age 3. Weight outcomes were measured at 1 and 3 years of age. Stool samples were collected at 3 and 12 months of age and analyzed to get a profile of the bacteria in them.

The researchers found that the increased risk of children being overweight and obese was associated with the amount and diversity of a phylum of bacteria known as Firmicutes. It was discovered that maternal smoking during pregnancy significantly increased the abundance of Firmicutes bacteria in the gut.

Breastfeeding helps

Kozyrskyj hastens to point out that Firmicutes are not a villain here. “It’s not so much that these Firmicutes are bad bacteria, because they’re required—a normal part of our gut bacteria.” What’s different is that they are present in higher amounts in the guts of infants whose mothers smoked during pregnancy than in those of mothers who did not.

She adds, “These Firmicutes increase from soon after birth to later infancy. It’s just that the increase (in infants of smoking mothers) occurred too soon. The amounts were too high.”

Kozyrskyj explains that the excessive levels of Firmicutes can result in excessive production of butyrate, a short chain fatty acid.

“It’s not like butyrate is poisonous or anything like that. It’s a natural byproduct. We have butyrate levels in our intestines all the time,” she says. But again, her team observed earlier and higher levels of butyrate among infants of smoking moms.

This study doesn’t indicate a direct cause and effect between too much butyrate in infants and them becoming overweight, but rather an association. In an interesting twist, the association applied to those children even if their mothers reduced their intake or quit smoking part way through pregnancy.

The association was not present if mothers quit smoking before pregnancy. Something that ameliorated the smoking effect, however, was breastfeeding exclusively for the first 3 months of the baby’s life.

Since the findings of the study are still associative, other studies would need to establish causality. But the implications are important, emphasizing the need for public health initiatives aimed at encouraging women to stop smoking before becoming pregnant.