More research is needed to strengthen the link between air pollution and obesity. But for people with poor diets and sedentary lives, prenatal exposure to air pollution may make them even more susceptible, Rundle said.
The human digestive tract is a lush ecosystem containing microorganisms that play a crucial role in digestion. But research shows that changing the delicate mix of bacteria may lead to weight gain.
Antibiotics, which wipe out both harmful and helpful bacteria, are one of the most common ways to upset the balance.
Studies suggest that microbes in the intestines affect the way the body metabolizes food. Early exposure to antibiotics may kill off healthy bacteria that influence the absorption of nutrients and help keep people lean, said Dr. Leonardo Trasande, an associate professor in pediatrics, environmental medicine and health policy at the New York University School of Medicine.
Trasande's team recently looked at more than 10,000 children and found that infants who had received antibiotics in their first six months were more likely to be overweight or obese as toddlers than other babies. The timing mattered; children given antibiotics between six months and 14 months of age didn't show a similar effect, according to the research, published last month in the International Journal of Obesity.
Another study, published last month in Nature, found that mice treated with antibiotics packed on up to 15 percent more body fat than those who weren't. The antibiotics seemed to shift the balance of certain gut microbes and affected hormones related to metabolism, the researchers found.
The microbes found in mice that ate antibiotics produced more short-chain fatty acids, a type of fat that cells use for energy, said lead author Ilseung Cho, assistant professor of medicine at the NYU's School of Medicine. "Ultimately, we were able to affect body composition and development in young mice by changing their gut microbiome through (antibiotic) exposure," he said.
The fattening effect parallels what is already seen with livestock. Farmers commonly use small doses of antibiotics to plump up cattle, pigs and chickens without increasing calories.
But antibiotics might not be the only thing changing the body’s microbiome. According to a study published last month in Obesity Reviews, overconsumption of fructose can affect microbes in the gastrointestinal tract and may contribute to metabolic disorders and obesity.
Fructose, a simple sugar, is normally well absorbed by the body when contained in natural foods, including honey, fruits and root vegetables.
"But when too much fructose in consumed, such as in soft drinks, it could affect the diversity of the bacteria," said study coauthor Christophe Lacroix of the Institute of Food, Nutrition and and Health in Zurich.
Sleeping less than six hours a night doesn't just make people irritable. It can mess up the systems in the brain that regulate metabolism, said the University of Chicago's Kristen Knutson, an assistant professor of medicine who recently reviewed published research on the topic in the American Journal of Human Biology.
Sleep deprivation can alter the circulating levels of the hormones that control hunger. "For example, after a short sleep, ghrelin, which stimulates appetite, is higher, and leptin, which signals satiety or fullness, is lower," Knutson said.
Sleep-restricted people also report greater levels of hunger, particularly for calorie-dense foods, Knutson said.
A lack of sleep may also decrease resting metabolic rates, or the number of calories burned while doing nothing, said sleep loss expert Dr. Orfeu Buxton, a neuroscientist at Brigham and Women's Hospital in Boston and assistant professor in the division of sleep medicine at Harvard Medical School.
Buxton's latest research, published in April in the journal Science Translational Medicine, showed that getting less sleep — five or six hours a night — and having disrupted circadian rhythms also increased the glucose concentration in the blood after meals. High blood sugar increases a person's risk of Type 2 diabetes, a condition closely correlated with obesity.
During the nine days that volunteers received recovery sleep and had their body clocks realigned, their metabolism normalized.
Sleep loss could also lead to weight gain in simpler ways: It gives people more time to eat, and it can make people tired and lethargic, reducing physical activity.
"Diet, sleep and exercise are the three pillars of health," Buxton said. "Start with diet to maintain or lose weight. Sleep supports a healthy diet — if you're rested, a salad will appeal more than the stale doughnut with the pink frosting in the break room. Once that's stable, then you can increase physical activity."