If you also want to prevent drowsy days, when you're yawning every 10 minutes at work, keep your iron reserves well-stocked. That way, you'll be less likely to skip the gym.
They interfere with your diet and exercise regime, making it difficult to stay on the wagon all season long. Foods rich in Vitamin C: kiwi, strawberries, oranges, etc.
Smoothies are out for the moment. Go for a hot, healthy detox tea or even mulled wine for a nice, comforting change to boost your moral and fend off stress that leads to weight gain.
Don't completely deprive yourself of holiday indulgences. Just be smart about what you do before and after these festive food-and-drink filled occasions.
To get your fill of healthy nutrients and warm you up at the same time, try homemade vegetable soups. That way you have complete control over the ingredients, and you can cut way back on the sodium content.
If it's too cold to go out, don't force yourself. Make yourself a healthy meal, and relax with a good movie. Everyone needs some nights in to recuperate from the harshness of winter
Don't be afraid of biking in the wintertime. Snow might complicate things, but if the streets are clear, just bundle up and get your daily cardio out of the way before and after work.
The days are shorter, and fatigue hits faster at this time of year. So take advantage of your sleepiness, and go get in some good Z's, in order to wake up refreshed and ready to go.