Having your air conditioner running during those hot summer days helps keep you cool and comfortable, but did you ever think that it could make you fat?
A review of data on possible causes of obesity states that it just may be. How?
Modern technologies -- such as air conditioning and heating -- help keep us in "the thermoneutral zone," a temperature range where we do not have to regulate our body temperature, a report suggests.
When our bodies are above or below this zone, we increase the amount of energy we spend, which "decreases energy stores," such as fat, the study's authors say.
This is just one of many potential factors that could be driving America's obesity epidemic, said David Allison, director of the Clinical Nutrition Research Center at the University of Alabama in Birmingham.
Besides the usual suspects such as fast food and a lack of exercise, he argues that any number of things could be making the problem worse, such as sleep deprivation, taking certain medications, or even quitting smoking -- and snacking instead.
Case in point: Michael Littman, a 37-year-old jeweler from New Jersey who smoked on and off for about 20 years before he quit.
"In the past, every time I quit, I definitely would gain a lot of weight," Littman said. "Part of that is just finding something else to do with your craving."
Gaining weight from quitting smoking is an important side effect to understand, Allison said.
"There are many other factors [not commonly considered] that may also be quite important" to exacerbate weight gain, said Allison, the lead author of the review of obesity studies, published in the International Journal of Obesity.
For example, chronic sleep deprivation also was considered a possible factor.
Many Americans don't get enough sleep, and when tired, people do not have energy to exercise and are tempted to eat, or overeat, in order to gain energy, said David Jenkins, Canada research chair in nutrition and metabolism.
Also, several popular medications, such as anti-diabetic drugs and antihistamines -- anti-allergy drugs -- contribute to weight gain.
So can genetic factors, such as the heritability of a person's body mass index. The study looked at various data from previous studies, which showed that BMI had a heritability of about 65 percent.
Other possible causes: Chemicals known as endocrine disruptors (typically medications that block the body's normal functions, such as birth control or other hormone-based drugs), a decrease in smoking, older women giving birth, and people having children with people who are like themselves (in this case, both overweight).
Not all experts agree with the claims.
"Since people stay thin in all different climates, it is unlikely [air conditioning] plays much of a role," said Darwin Deen, a professor of clinical family and social medicine at Albert Einstein College of Medicine in the Bronx, N.Y.
Experts also have concerns about attributing obesity to genetics.
"Approximately 30 percent of the risk for obesity is genetic," said Terrill Bravender, director of adolescent medicine at Duke University Medical Center. "However, the increases we have seen in obesity have occurred over the past 25 years -- clearly not enough time to see significant genetic drift in our population."