The vitamin D that our skin makes must be converted by the liver to 25(OH) D3, which is what is usually measured when you get your vitamin D levels tested by a doctor. Then it's converted by the kidneys to calcitriol, which is the active hormone form of the vitamin.
Unbeknownst to many, the enzymes that perform both of these conversions rely on magnesium. Unfortunately, 50% of Americans do not consume adequate magnesium, which could lead to vitamin D staying stored and inactive for a huge proportion of people. Thankfully, magnesium is found in foods like dark leafy greens, almonds, pumpkin seeds, full-fat dairy yogurt, and dark chocolate.