When exposed to sunlight, your skin produces vitamin D. How much you create depends on your skin type, time in the sun, and how much skin is exposed. We can estimate this using two simple steps. First, we calculate how much vitamin D your skin makes per square centimeter per minute based on UV radiation and daylight hours. Then, we adjust that number for how long you’re outside, how much skin is exposed (like arms, legs, and face), and your skin type. For example, exposing 5,000 $cm^{2}$ of skin to the sun for 30 minutes will produce more vitamin D if done around noon when UV rays are strongest. Always avoid sunburn!

[Vitamin D](vitamin-d), also known as cholecalciferol (aka Calciol), is a hormone that our skin makes when exposed to sunlight, specifically UVB rays. For this to happen, we need enough of a molecule called 7-dehydrocholesterol in our skin. If we don't get enough sunlight, whether it's because we stay indoors, wear too much clothing, or use sunscreen, we can't produce enough vitamin D. When vitamin D enters our blood, it can either float freely or bind to a protein called DBP. Once in the bloodstream, vitamin D can be taken up by fat and muscle cells for storage. Our body can also convert vitamin D into different forms, like calcifediol and calcitriol, which are more active in the body. The names of these forms refer to the number of hydroxyl groups (OH) added to the molecule—one for vitamin D, two for calcifediol, and three for calcitriol. Studies measuring vitamin D levels in people have shown differences over time. For example, a 1984 study found lower levels in healthy subjects (around 3-5 ng/ml), while a 2013 study reported higher levels (around 37-45 ng/ml). These variations might be due to changes in vitamin D supplementation or other factors. However, these levels don’t necessarily have clear biological or medical significance. read more