When it comes to our bodies, we are what we eat—or so the adage goes.
“Conventionally, obesity research focuses more on diets, physical activity, and a sedentary lifestyle,” Xiaozhong Wen, associate professor at the University of Buffalo, told EHN.
But to explain the childhood obesity epidemic, researchers are increasingly looking beyond the usual culprits, like fast foods, for more ubiquitous and insidious causes in the environment.
A team of international researchers has found that exposure to certain indoor air pollutants such as particulate matter and nitrogen dioxide and some heavy metals is linked to child obesity. The study, published today in Environmental Health Perspectives, is the first of its kind to test for a wide range of environmental exposures that could cause obesity. The results reinforce previous studies linking air pollution and smoking during pregnancy to obesity, and offer a new model for evaluating the complex environmental influences on health.
Obesity plagues U.S. children and teens—roughly 18 percent of youth aged 2 to 19 years old are obese, according to the Centers for Disease Control and Prevention. Obese children worldwide face an increased risk of type 2 diabetes, cardiovascular disease, and cancer as adults, and that trajectory sets in early.
While diet and lifestyle are still major factors in children’s weight, studies have shown that common chemical contaminants in plastics, tobacco smoke, pesticides and cosmetics can interfere with hormones during early development and in the womb. Meanwhile, the layout of neighborhoods and cities may discourage walking, or lack green spaces for children to play in.
These myriad variables contribute to an individual’s “exposome,” a concept originally conceived to describe the various environmental causes of cancer.
The new study attempts to piece together the exposomes of children across six countries in Europe, and identify environmental factors associated with an increased risk of obesity. The HELIX (Human Early-Life Exposome) project assembled 1,300 children between the ages of 6 and 11, and their mothers, from long-term population studies in France, Greece, Lithuania, Norway, Spain, and the U.K. Nearly a third of the children in the cohort were overweight or obese.
Among the environmental exposures they compiled were road traffic noise, access to green space, UV radiation, and chemicals used in pesticides and water disinfectants.
“Usually we only look at one or a few exposures in one study,” said Wen, who was not involved in the HELIX study. “This one is pretty ambitious.”
Twenty-seven of the 96 childhood exposures the researchers tested for turned out to have positive associations with obesity. Backing up decades of evidence, the researchers observed that children whose mothers smoked during pregnancy had increased rates of obesity. While chemicals commonly used in pesticides weren’t implicated in weight gain, indoor pollutants like particulate matter and nitrogen dioxide were. These can arise from household heating, secondhand smoke, and car exhaust.
Unlike many adverse environmental exposures, which disproportionately afflict low-income families, indoor exposure to nitrogen dioxide was a heightened factor in obesity among children in high-income households.
With the COVID-19 crisis keeping children cooped up at home, such indoor “obesogens” —chemical compounds that are linked to obesity—are even more important to investigate.
Children growing up in urban, densely-populated areas also had higher body mass indexes, a measurement of body fat based on height and weight.
One aim of the exposome approach is to cast a wide net and explore exposures that haven’t yet been studied in relation to obesity. One unexpected finding: the researchers saw high levels of the heavy metals copper and cesium in the blood of obese children, a finding they say merits further study.
“We took 100 exposures, but there are probably thousands in our environment that could be of concern,” said Martine Vrijheid, Research Professor at the Institute for Global Health, Barcelona and lead author of the HELIX study. “There’s much more to be done in looking at how the different exposures interact together.”
The study doesn’t prove the exposures cause obesity; it’s possible that some, like the heavy metals, could accumulate in body fat rather than cause obesity. But the researchers hope that their holistic model will open up new avenues of research, and eventually inform policy that considers broader aspects of children’s health. “It’s a start,” said Vrijheid.
In the U.S., it’s projected that by 2030 half of American adults will be obese. Yet we know “startlingly little” about how environmental stresses affect health during development, Yeyi Zhu, research scientist at Kaiser Permanente Northern California Division of Research, told EHN.
“This is a rapidly growing field of increasing interest in the scientific community,” she said.
In 2016, the National Institutes of Health launched its Environmental Influences on Child Health Outcomes (ECHO) program, which will map the exposomes of more than 50,000 children in the U.S. and Puerto Rico.
The children who took part in the HELIX study are now teenagers. “It’s now six, seven years since we last examined the children,” said Vrijheid, adding that HELIX has received funding to check back in with the original cohorts to evaluate the effects of their exposures over time. “We’re very excited about that.”