We are photosynthetic!

With the sun coming out and calcium regulation on Friday's exam, now seems as good a time as any to write about vitamin D. Vitamin D aids in calcium absorption in the gut -- so that when you're eating calcium-rich foods (and secreting parathyroid hormone properly), you actually get the calcium into your system.

Many people know that going out into the sun will increase your vitamin D levels. But why? Does vitamin D come down from the sun? No, silly history major.

We have the precursor molecule (a cholesterol) for vitamin D present in our skin all the time... all we need are a little UV rays and a little heat to convert that molecule to the vitamin D we need.

So how long can we goof off in the sun before it becomes blatant procrastination? The amount of time in the sun varies from person to person, because the lighter the skin, the quicker the UV absorption. According to an NIH fact sheet: "approximately 5–30 minutes of sun exposure between 10 AM and 3 PM at least twice a week to the face, arms, legs, or back without sunscreen" is sufficient. Not nearly enough for my taste.

On the other hand, I drink vitamin D-fortified milk -- but if vitamin D is fat-soluble, how is it dissolved in my skim milk? Anyone?