Vitamin D

Vitamin D works with Calcium to help develop strong bones and teeth. If you're not getting enough Vitamin D every day, you may not be able to adequately absorb Calcium from the foods you eat, and your body will have to take Calcium from your bones. Getting your daily recommended amounts of Calcium and Vitamin D can help you do this.
If you can have sunlight your body can produce vitamin D but at winter you can't.
So you need to get vitamin D supplement to take.