Women who smoke can lose a few kilos and can also gain less weight during pregnancy than they would otherwise, but new research shows that their children tend to develop overweight. The new study emphasises that the association is independent of the mother’s pre-pregnancy body mass index (BMI) and genetic predisposition to overweight. Although the children born to women smokers are smaller at birth, their fat deposits increase more during early life, leading to overweight.
Twenty percent of children 5–19 years old in Denmark have overweight. Genetics and environment influence overweight, but how these interact is still unclear. For many years, smoking by women during pregnancy has been considered an important risk factor for child overweight. Nevertheless, whether the effects result from the smoking itself or the mother’s genes has been debated. A new study examined health data for 1668 children and their mothers from the Danish National Birth Cohort and distinguished genetic predisposition from the environmental effect of smoking for the first time.
“The study shows that smoking while pregnant is clearly associated with child overweight independent of the mother’s pre-pregnancy BMI and genetic predisposition to overweight. So the mother’s genes do not aggravate the smoking-induced risk. Mothers who avoid smoking during pregnancy reduce the risk of their children becoming overweight. The reasons for the association have not been confirmed, but we have some good ideas for explanations,” explains Camilla Schmidt Morgen, who during the study was Senior Researcher, National Institute of Public Health, University of Southern Denmark, Copenhagen and affiliated with the Novo Nordisk Center for Basic Metabolic Research, University of Copenhagen.
Absolutely no interaction
The researchers examined data from a selected group of 1668 of the 100,000 mother–child pairs from the Danish National Birth Cohort. The data included how much the mothers smoked during pregnancy and the BMI of the mothers before pregnancy and the children from birth until 7 years old.
“Smoking while pregnant was associated with higher child BMI and higher odds of child overweight in a dose–response relationship. The more the mothers smoked, the greater the risk. For mothers who smoked more than 10 cigarettes per day, the risk increased by 142%. The children of mothers who smoked during pregnancy were smaller at birth, but this difference was offset in a few years and instead they became overweight,” says Camilla Schmidt Morgen.
Numerous studies have shown that mothers smoking during pregnancy is associated with child overweight. However, this association could be spurious, since the children of mothers with overweight are genetically predisposed to overweight, and the mothers with overweight may have tried to keep their body weight down by smoking, with smoking appearing to have induced child overweight.
“To investigate this possibility, we examined the effects of the mothers’ pre-pregnancy BMI and the genetic predisposition to overweight based on 941 common genetic variants associated with BMI. The association between smoking and child overweight did not change when corrected for genetic predisposition. So the marked increase in the risk of child overweight apparently results from smoking and does not appear to be either spurious or amplified by the maternally transmitted genes,” explains Camilla Schmidt Morgen.
Theresia Schnurr, Postdoc at the Novo Nordisk Foundation Center for Basic Metabolic Research, adds:
“We had the genetic information on the 941 genetic variants for both the mothers and their children. This enabled us to distinguish between the genetic risk variants that the mother transmitted to her child and the genetic risk variants that the mother carries but did not pass on to her child. Partitioning the genetic risk factors did not change our results, meaning no interaction between genetic predisposition – transmitted or not transmitted from the mother to her child – and smoking on child overweight.”
The body compensates for the missing weight
The new study thus provides yet another really good reason to stop smoking and not just for women’s own health. According to the researchers, women who smoke to reduce their BMI might think again, because this negatively affects their health and that of their children.
“Smoking is extremely harmful for mothers, but it also affects their children for life, including in other ways than increasing the risk of overweight. Although we do not yet fully understand what causes these children’s BMI to increase, they seem to be smaller at birth and have less muscle tissue. The body appears to compensate by storing fat, and this could indicate a shift in the balance in the metabolism of fat tissue – something that these children will have to combat throughout life,” says Thorkild I.A. Sørensen, Professor, Section for Epidemiology, Department of Public Health and Novo Nordisk Foundation Center for Basic Metabolic Research, University of Copenhagen.
Obesity is a major risk factor for developing chronic cardiometabolic disorders such as type 2 diabetes and cardiovascular disease. Although the new study clearly associates maternal smoking and child overweight, researchers are still seeking to understand exactly what goes wrong in children’s metabolism to improve prevention and treatment. The researchers still hope to discover a genetic needle in the haystack.
“Although we did not establish that genetic predisposition explained the association between maternal smoking and the increased risk of child overweight, there are clearly strong genetic risk factors for overweight. The 941 genetic variants we included in our study explain only about 6% of the genetic variation for adults, so genes not yet discovered must be essential for understanding how overweight develops,” concludes Thorkild I.A. Sørensen.