The study offers new insights into a connection that has only become evident to researchers in the past decade: that children born to mothers who smoked during pregnancy are more likely to be overweight or obese. In the 1960s and 1970s, close to 40% of U.S. and Canadian women smoked during pregnancy, suggesting that tobacco may be one factor in the dramatic run-up of obesity in North America during the past three decades. (Other prenatal factors that may predispose one toward obesity are poorer maternal nutrition, maternal obesity and closely spaced pregnancies.)
But the latest research, published “Online First” in the Archives of General Psychiatry on Monday, suggests that smoking may influence the development of the fetal brain in ways that will predispose the affected baby to choose fatty foods over foods less dense in fat. It appears to do so by suppressing the size of the amygdala, the almond-shaped structure at the heart of the brain that, among other functions, helps regulate appetite and other reward-seeking behavior.
The study recruited 378 Canadian high school students between 13 and 19 years of age — 180 of them exposed to cigarette smoke while in utero. All were at a late stage in their pubertal transition, a period during which obesity related to mother’s tobacco use systematically crops up. Each adolescent had his or her body-mass index and body-fat composition measured, and brain scans assessed the size and structure of several brain regions thought to play a role in appetite and reward-seeking. The teens were also asked about their use of illicit drugs, alcohol or tobacco, and parents filled out questionnaires relating to a mother’s smoking during pregnancy, breastfeeding and the family’s socioeconomic status.
Compared to kids whose mothers did not smoke during pregnancy, those exposed to cigarette smoke in utero had smaller amygdalea. Their food diaries indicated their diets were higher in fats, representing 33.1% of the daily caloric intake versus 30.4% for nonexposed children.
The average body weight and BMI of the tobacco-exposed group was slightly higher than that of the group whose mothers had not smoked. But their body compositions reflected substantial differences: The tobacco exposed group had on average 15% more body fat than did the group of kids whose moms did not smoke during pregnancy.
Kids whose moms smoked during pregnancy were also significantly more likely to have experimented with illicit drugs and alcohol than did those whose fetal experience was not bathed in nicotine. Curiously, neither group of children was more likely to use tobacco.
“Diets high in fat are considered highly rewarding,” the authors write. And that sense of reward that comes with, say, biting into a doughnut, registers in the same places in the brain — the amygdala among them — that are activated by drugs of abuse. Past research has found that having an amygdala that is either small or not highly responsive to addictive pleasures is highly correlated to addictive behavior.