I’m not sure how correct that is. Particularly in certain countries and especially in winter. I get out for more than 30mins every day and my levels are deficient when I tested.

It also depends on your skin color and latitude.

it varies greatly person to person.

+1. I was deficient, despite averaging 7000 steps per day (80 min of walking). I also live in seattle and stay bundled up

Copenhagen in the winter will have you sit outside all day. All 6 hours of it.

In order to produce vitamin D, your skin needs continuous time in the sun to fully react with sunlight.

30 minutes of continuous chemical reaction will have vastly different effect than a reaction that stops & starts repeatedly, for a sum total of 30 minutes.

You actually are going to synthesize _more_ vitamin D if you do fractionated exposure.

That's because vitamin D synthesis is an equilibrium reaction, UV drives backward and forward reactions. So you'll get more vitamin D if you expose your skin to the sun for a few minutes, then let the synthesized vitamin to diffuse out of the skin layer, and then expose yourself again.

That makes sense. Thanks for explaining.