During the holiday season, Americans tend to gain substantial amounts of weight. " Holiday" weight gain can be the result of many things. One factor is poor nutrition. It seems that everyone has 4 holiday parties to go to a night, despite their own family festivities, which are full of sweets, rich food, and alcohol.
Another factor for this seasonal weight gain is due to a decrease in activity. When the weather is brutally cold (or just not comfortable), people tend to stay inside more. This means that the seasonal outdoor athlete isn't engaging in any activity. The runner who trains when it's nice outside is now hibernating! When pairing an unhealthy diet with a lack in physical activity, the result is unhealthy weight gain and a decrease in metabolism.
One way to prevent weight gain and (most importantly) stay fit, is to keep the fitness consistent. Running outside actually isn't that bad once you are moving. If you are the type of person to wake up and go for a run before work, don't let the weather hold you back. Here are some great articles to help you keep the momentum going.
warming up and cooling down