If you take a look at the animal kingdom, you'll notice that aquatic mammals in colder climates naturally develop a layer of blubber to stay warm in the winter. To some degree, we humans also have similar instincts during the winter months.
There are a number of ways that the seasonal changes of a cooling climate are making us fatter. In fact, winter is contributing to our fatness in the following ways:
Keeping warm: Just like the polar bear and the otter, it is possible that humans develop an extra layer of blubber to keep warm. Some studies have shown that children who are born during cold winters are significantly more likely to become obese as adults. If you have a little one on the way, turn the heater up.
Lack of sunlight: Anybody who wakes up to the sunlight in the morning, or who has fallen asleep during a movie when the lights are low knows that sunlight helps to keep you awake and energized. Without that sunlight shining in the evening, we tend to wind down earlier and relax. You know what that means... less activity and less calories burned.
You are under-exposed: People are more likely to get into shape when their bodies are exposed. Going swimming in the summer, wearing a tank top or T-shirt to the mall... those motivations are gone in the winter. You won't have the same pressure to look good for others.
Less quality exercise: Let's face it, you're not going to be spending the same amount of time playing ball in the park when it's cold. When you do finally exercise, you're more likely to use the treadmill or an ab gadget in your nice warm home. Getting out to a gym where you can do some good weight lifting and retain your lean body mass is more difficult. As you lose your lean body mass (muscle), your metabolism slows down.
Less cooking: At first, you would think that less cooking means less weight gain. That would be true if less cooking meant less eating. Again, when the sun sets earlier, it feels later than it really is. Instead of going outside and having a barbeque at 7pm, you are more likely to find yourself sitting in the McDonald's drive through.
S.A.D. (Seasonal Affective Disorder): In other words, you have winter depression from the lack of sunlight. This can be combated with melatonin supplementation, bright light therapy, and exercise, which is also known to lower the symptoms of depression. The only problem is that seasonal depression can be exactly what's holding back your motivation to get into the gym.
Holiday Fatness: So many people tend to put on their holiday 10 between Thanksgiving and Christmas. With renewed vigor, we then vow to undo that damage with a set of New Year's resolutions, only to fail our resolutions by mid-January.
Fortunately for us, the winter battle can be won. It takes a little dedication to a few sound principles of keeping in shape. These may or may not be new to you, but humans tend to retain knowledge that is periodically reinforced. Here's the fix:
Keep your muscle mass in the winter: Important for both men and women. This will most likely require that you do some sort of weight lifting. You will also have to eat enough food to feed the muscle on your body. If you don't maintain your lean body mass or eat enough food, you won't be able to keep your metabolism burning fast.
Expend, even if it's cold: You're less likely to be playing outside, but you still need to use those calories up. Do 30 minutes of weights and follow up with 30 minutes of cardio at the end of your session. Weights and cardio have a synergistic effect, and you will lose more fat by combining the two together. The difference in your physique will become more apparent with every passing month.
Eat a high-protein diet: This is helpful for maintaining muscle and not consuming excess calories. Add in plenty of fibrous vegetables as well as healthy portions of complex carbohydrates. Proper portions of carbs are necessary to help you build muscle and have the energy needed for a killer workout.
Get on a program for the winter: There's simply no substitute for a good base of knowledge about how to control your body type. A good exercise program can help you to keep in shape by exercising for 3 hours, 3 times a week.
Motivation: Know thyself. If you have a strong reason for exercising, then you will be far more likely to follow through on your commitments. Whether it's to become more attractive, feel more energy, increase cognitive function, build confidence, sleep better, run faster... know it inside of yourself.