Liz Neporent is a diet and fitness expert and author of 12 fitness bestsellers. She regularly appears on national TV programs and is the president of Wellness 360, a New-York based wellness provider.

When you step on the bathroom scale and it registers a higher number than it should, you probably chide yourself for eating too much and moving too little. And, while I am certainly the last person to let you off the hook for flagrantly abusing the first law of thermodynamics, I do think it's important to understand that physical activity and diet – what researchers refer to as "The Big Two" – are probably not entirely the reason more than 60 percent of Americans have trouble zipping up their pants.

Scientists increasingly suspect that the worldwide epidemic of excess poundage has other contributing factors. Thought I'd share three of the more surprising ones with you this week, just to give you a sense of how complicated this issue truly is. Once you read this, it's OK to stop judging yourself so harshly, but promise me you'll eat a piece of fruit and go for a brisk walk anyway!

Heavy Sleepers

Lack of shut-eye appears to be strongly linked to obesity, probably because it has an effect on the hormones that control metabolism. Numerous studies tie short sleep to larger size. In one decade-long study of 740 people, Laval University researchers in Quebec found that men who reported getting seven to eight hours of slumber per night were 16.5 pounds lighter than those who said they only caught five or six hours of sleep nightly. Women who got a full night's rest were more than 11 pounds thinner than those who didn't. So it seems likely that getting a good night's sleep will help your weight loss efforts. At the very least, you'll be less groggy when choosing between oatmeal and ice cream dippers for breakfast.

Chubby Bugs

One theory says it's possible to gain weight the same way you catch a common cold. Scientists have long known that viruses can cause obesity in animals such as chickens, dogs and monkeys. Now they're hot on the trail of one that causes obesity in humans. Adenovirus 36, a pesky microbe responsible for respiratory and eye infections, is thought to be the most likely candidate. The mechanism by which "AD-36" and other possible fat bugs work isn't fully understood, but recent studies have demonstrated that they're capable of transforming stem cells, the so-called master cells, into fat cells with a Herculean ability for gathering and storing body fat. The virus may also trigger immature fat cells to multiply more quickly than normal and put the brakes on metabolism so the body burns calories more slowly. Like any other virus, AD-36 could potentially be transmitted through sneezes, coughs or direct contact. Right now, it's all just speculation, but some researchers believe that they may one day be able to create a vaccine that cures certain types of obesity.

Fair Weather Fat

In the past few decades, the number of homes with heat and air conditioning has risen right along with obesity rates and some scientists think it's no coincidence. In the southern states -- where obesity rates tend to be highest -- the number of houses with air conditioning is 70 percent, up from 37 percent in 1978. What's the possible connection? As warm blooded animals, we work at keeping our body temperature in steady state. That means sweating when it's hot and shivering when it's cold. Since these physical acts burn calories, spending all your time in comfy climes requires less energy. In one small study, women who lived in a climate bubble where the thermostat was set to 80 degrees (Fahrenheit) versus 72 degrees, burned 239 fewer calories a day. Far from conclusive, but interesting. Certainly worth further investigation.

What do you think of these theories? You can read more about them in New Scientist and the The International Journal of Obesity. Got any of your own? Share please.