Sneaky winter weight gain traps

When you feel crappy, you're less likely to hit the gym, obvs. But there's more: Ad36, a virus similar to those that cause the common cold, pinkeye and diarrhea, may increase body fat, according to a study from Louisiana State University.

Trim the fat: Experts aren't sure why Ad36 is so evil, and they're trying to develop a vaccine for it. In the meantime, wash your hands frequently to avoid it and other bugs that can make you sick, says lead study author Nikhil Dhurandhar, Ph.D.

Your credit cards are still exhausted from the workout they got over the holidays. Now the aftermath—bills!—could cause a rapid rise in the stress hormone cortisol, which makes you hungrier and triggers your body to store fat, a study in the American Journal of Epidemiology reveals.

Trim the fat: Stress eaters, try replacing the junk food with something else that takes the edge off, says lead study author Jason Block, M.D. Kickboxing class, venting session with a bud—whatever works for you.

Your body burns more calories when it's cold because it uses energy to stay warm. But going from heated home to heated car to heated office means you rarely feel the frosty air that could help you maintain a healthy weight, says Fiona Johnson, Ph.D., research psychologist of epidemiology at University College London.

Trim the fat: If you can tough it out, get your exercise outdoors. If not, at least set the temperature in your house as low as you can handle it. Your body and the environment will thank you.

Mac 'n' cheese makes winter blahs more bearable—plain and simple. And it doesn't help matters that your body naturally wants you to eat more when the temp drops, because digestion generates body heat, says Rallie McAllister, M.D., author of Healthy Lunchbox.

Trim the fat: You can get all the food love of Mom's kitchen without the mega calories. Skip the carb-and-fat fest and warm up your insides with a bowl of tomato soup or a mug of hot chocolate made with skim milk.

We get a lot less sunlight in winter, and that affects the serotonin activity in the brain that helps make us feel full and content, says Judith Wurtman, Ph.D., coauthor of The Serotonin Power Diet.

Trim the fat: Fill up on protein at breakfast (nonfat Greek yogurt) and lunch (grilled chicken). At dinner, include carbs (sweet potatoes, pasta), which will boost serotonin production. Still hungry? Switch on a sun lamp (try NatureBright SunTouch Plus, $72; amazon.com) for half an hour every morning to soak up serotonin-generating rays.