Your whole life you’ve been told that diet and exercise are the keys to a healthy weight. And while that still holds true, "calories in, calories out" is only part of the equation.
Here are five surprising influences that may be tipping the scales in your favor -- or not.
Friends and Family
Your expanding waistline is one more thing you can blame on your mom. While you’re at it, blame your best friend and coworkers too.
In their new book, Thinfluence, two Harvard experts speculate that your friends, family and environment have just as much influence on your body weight as what you eat and how much you move. Your chances of becoming obese increases by 35 percent if your spouse is obese, 40 percent if your sibling is obese and 57 percent if a close friend is obese.
Dr. Walter Willet and Dr. Malissa Wood, along with Dan Childs, director of the ABC News medical unit, claim that people and places help inform your behavior to such an extent you don’t even realize you can change them. Take those twice-weekly birthday celebrations at the office, for example. They become so much a part of your life it might never occur to you that they’re sabotaging your weight loss efforts.
A recent study at New York-Presbyterian Hospital/Weill Cornell Medical Center found that medication may play a part in about 5 percent of obesity cases. That's not a staggering percentage but with millions of Americans popping prescription pills, it could be that the buttons on their clothing are popping right along with them.
Some of the worst offenders, the study found, are drugs used to treat allergies, infections, heart disease, diabetes, depression and cancer. In other words, some of the most commonly used medications.
While the reason for medication-induced weight gain isn’t entirely understood, scientists speculate that drugs can cause fluid retention, disrupt the mechanics of metabolism and fat, or mess with the enzymes that drive overeating.
More than three dozen environmental chemicals have been identified as "obesogenic." Studies have linked exposure to pollutants found in some paints, plastics, wallpaper, textiles and floor coverings to weight gain and higher insulin levels. Other persistent environmental baddies -- such as insecticide DDT, dioxin and PCBs -- have been identified as possible candidates for upping the risk of both obesity and type II diabetes.
Why might pollution contribute to weight gain? The chemicals that flood our air, soil and drinking water may somehow affect the bacteria that inhabit our digestive tracts by the millions. Scientists believe these gut bacteria play a key role in weight and insulin control and by changing their composition, it somehow hampers the body’s ability to fight fat.
Though studies can only prove an association rather than a direct link, getting too little sleep appears to add up to large weight gains. One review of the sleep habits by Columbia University in New York found those who got by on less than four hours of sleep a night were 73 percent more likely to be obese than people who slumbered seven to nine hours nightly. Even those who caught up to six hours of shuteye nightly were 23 percent more likely to be obese.
In some studies, overtired subjects experienced a dramatic drop in leptin, a hormone tied to appetite control and fat storage. Fewer Z’s has also been shown to give rise to insulin sensitivity, a symptom connected with both obesity and type 2 diabetes.
In the late 1980s, Dr. Nikhil Dhurandhar, a physician and researcher, noticed that chickens infected by a specific virus gained weight even when they didn’t overeat. He and others have since discovered nearly a dozen viruses suspected of porking up various other species including rats, cats, dogs and, yes, pigs.
There is some indirect proof that humans may also susceptible to infectobesity. By tracing antibodies, the microscopic calling card all viruses leave behind in the bloodstream of an infected individual, Dhurandhar’s long term studies suggest that people who carry a fat virus gain significantly more weight than the people who remain virus-free.
However, it’s doubtful that a virus is a universal excuse for packing on pounds. In studies, only between 5 and 15 percent of subjects test positive for a fat virus.
Whether you believe your weight is a matter of personal responsibility or something that's beyond your control, join ABC News Health for a tweet chat to discuss your thinfluences today at 1 p.m., ET. We want to know what you think, and what has -- and hasn’t -- worked for you in terms of weight loss.