Eating less and exercising more is old advice for shedding pounds. Now contemporary science says that genes, hormonal imbalances, and even viruses, play a role in obesity. Here’s the latest thinking behind size and thighs, fatness and fitness.

1. It Really Is Genetic

When first discovered it was called the fatso gene. Turns out, people with two copies of the more politely named FTO gene were 60 percent more likely to be obese than those without it. Scientists now suspect that there are lots of fat genes.

2. More Fat Cells

Some people have twice as many fat cells as others have, says Kirsty Spalding, PhD, of the Karolinska Karolinska Institute in Stockholm. Gaining or losing pounds will not change your fat-cell count. But, Spalding says you’re better off with more fat cells than with fewer fat cells that become overstuffed and enlarged.

3. You Can Change Your Metabolism

Another Scandinavian team looked at weight gain on the cellular level. Kirsi Pietiläinen, PhD, an assistant professor of nutrition at Hensinki University Central Hospital University Central Hospital, suspects that gaining as little as 11 pounds can slow metabolism making it harder to lose it. Exercise can change that.

4. Stress Fattens You

Stress spark cravings for carbohydrate-rich snack foods, which can calm stress hormones that signal your body to store fat. So, make time for stress relief to help keep the weight off.

5. Mom's Pregnancy Sealed Your Fate

A growing body of science suggests that sugary and fatty foods, consumed even before you're born can make you fat. Moms-to-be, take note: You can give your kids a head start by eating well before they're born.

6. Sleep More, Lose More

University of Chicago researchers reported that sleep deprivation triggers a decrease in leptin (which helps you feel full) while increasing of ghrelin (which triggers hunger). As a result, we think we're hungry when we’re not. Sleep may be the cheapest and easiest obesity treatment.

7. Your Spouse's Weight Matters

A study in the New England Journal of Medicine suggests that if one spouse is obese, the other is 37 percent more likely to become obese too. Obesity seems to spread through social networks.

8. Achoo! The Obesity Virus

Adenoviruses are responsible for a host of ills, from upper respiratory tract problems to gastrointestinal troubles. The link to fat was uncovered when researchers at the University of Wisconsin injected chickens with the viruses and found that certain strains fattened them up.

9. Cookies Are Addictive

When subjects at Monell Chemical Senses Center in Philadelphia were shown the names of foods they liked, the parts of the brain that got excited were the same parts activated in drug addicts. It may have to do with dopamine, the hormone linked to motivation and pleasure.

10. Tainted Taste Buds

Researcher Derek Snyder at the Center for Small and Taste, University of Florida, found that ear infections could damage taste nerve running through the middle ear, lessening the ability to sense sweetness and fattiness, causing sufferers to eat more.

11. Antioxidants Are Also Anti-Fat

Free radicals are now blamed not only for making you look old but weakening the “stop eating” signal. The best way to fight back? Avoid the junk and load up on colorful, antioxidant-rich fruits and vegetables.

12. Pick a Diet, Any Diet

Here’s a secret—all weight loss books have four basic rules to healthy eating:

• Consume carbs in the form of whole grains and fibre.
• Avoid trans and saturated fats.
• Eat lean protein.
• Fill up on fruits and vegetables.

13. You Can Be Fat and Fit

A growing body of literature suggests that size doesn't matter when it comes to your health. A study published in the Archives of Internal Medicine found that 51 percent of the overweight and almost 32 percent of the obese had mostly normal cholesterol, and other measures of good health. It also found that 23.5 percent of trim adults were, in fact, metabolically abnormal and more vulnerable to heart disease.