1 by Evangielyn Luma

1. It Really Is Genetic 
When scientists first discovered it in certain chubby mice, they called it simply the fatso gene. Years later, when they scoured the human genome for markers that increased vulnerability to type 2 diabetes, the fatso gene (now more politely called FTO) showed up there too. Turns out, people with two copies of the gene were 40 percent more likely to have diabetes and 60 percent more likely to be obese than those without it. Those with only one copy of the gene weighed more too. 

Scientists now suspect that there are lots of fat genes. "There could be as many as 100 of them," says Claude Bouchard, PhD, executive director of the Pennington Biomedical Research Center at Louisiana State University System, "each adding a couple of pounds here and a pound or two there. That's a noticeable difference when it comes to how much more fat we need to burn off." 

As much as 16 percent of the population has two copies of the FTO gene, and half of us have one copy. So far, scientists suspect that the other possible obesity-promoting genes have a small effect compared with FTO. The good news? "A genetic predisposition isn't necessarily a life sentence," says Bouchard. Exercising regularly can offset the risk. 

PLUS: 19 Weight Loss Secrets From Around the World 

2. Some People Just Have More Fat Cells 
And the range is enormous, with some people having twice as many fat cells as others have, says Kirsty Spalding, PhD, of the Karolinska Institute in Stockholm. Even if you've lost a few pounds (or gained some), your fat-cell count remains, holding tight to the fat already inside and forever thirsting to be filled up with more. (To add insult to injury, the fat cells of overweight and obese people hold more fat too.) 

New fat cells emerge during childhood but seem to stop by adolescence. Those of us destined to have a lot of these cells probably start producing them as young as age two. The cells' rate of growth may be faster, too-even if kids cut way back on calories. 

Strangers have written to Spalding, telling her how depressed they are by her research. But she says her news isn't all bleak. You're better off with more fat cells, she says, than with fewer fat cells that become overstuffed and enlarged. (New research suggests that the overstuffed group are more vulnerable to obesity-related health complications.) So while you can't reduce your total number of fat cells, there are things you can do to keep them small. (See next point.) 

3. You Can Change Your Metabolism 
Another Scandinavian team looked into what happens at the cellular level when you gain weight. Kirsi Pietiläinen, PhD, an assistant professor of nutrition at Helsinki University Central Hospital, studied sets of twins where one was fat and the other thin, and learned that fat cells in heavier twins underwent metabolic changes that make it more difficult to burn fat. Pietiläinen's team suspects that gaining as little as 11 pounds can slow metabolism and send you spiraling into a vicious cycle: As you gain more fat, it becomes harder to lose it. 

How to get back on track? "The more I learn on the job, the more I'm convinced we need physical activity," Pietiläinen says. Once a chubby child herself, she now runs regularly and is at a healthy weight. 

4. Stress Fattens You Up 
The most direct route is the food-in-mouth syndrome: Stressful circumstances (your bank account, your boss) spark cravings for carbohydrate-rich snack foods, which in turn calm stress hormones. (When researchers in one study took away high-carb food from stressed mice, their stress hormones surged.) 

Stress hormones also ramp up fat storage. For our prehistoric ancestors, stress meant drought or approaching tigers, and a rapid-storage process made sense; we needed the extra energy to survive food shortages or do battle. Today we take our stress sitting down-and the unused calories accumulate in our midsection. 

To whittle yourself back down to size, in addition to your usual workout routine, make time for stress relief-whether it's a yoga class or quality time with family. 

PLUS: 11 Healthy Ways to