Most people generally do not get enough sun exposure to produce adequate levels of vitamin D especially in colder climates. 

Vitamin D enhances the barrier function of the cells lining the inside your mouth.  And vitamin D enhances the bacterial killing functions of white blood cells that defend against gum disease. 

The Recommended Dietary Allowance (RDA) for vitamin D is 600–800 IU  per day ( for bone health). The Endocrine Society recommends 1500–2000 IU  to maintain a minimum of serum 25(OH)D concentration of 30 ng/mL.  Even higher daily doses may be required to correct, treat, and prevent vitamin D deficiency.