Vitamin D, as we are learning, is not a true vitamin. Ideally, we synthesize our own from natural sunlight, but that is not possible or practical for most of us these days
It seems that almost everyone who has their blood tested lately is diagnosed with a Vitamin D deficiency, and they often receive a prescription for a high dose supplement. It is a little known fact, though it is briefly introduced in Lecture 6, that Vitamin D works best with the form of Vitamin K (menaquinone or K2) that we synthesize if our gut bacteria is in balance. K2 is also present in a very few foods and as a supplement.
Choose either Vitamin D or K2 to kick-off your main post. Discuss risks/benefits of supplementation and deficiency issues. If you choose K2 to start, you might want briefly mention how it differs from K1 (phylloquinone) with regard to food sources and functions, but K1 is not the focus here. Although you are choosing only one of the two vitamins to focus on first, it is appropriate to mention how your vitamin of choice interacts with the other.