Enter dminder, an app for your iPhone/iPad that tracks the Vitamin D you get from the sun.
off on your purchase.
The sun is by far the best source of Vitamin D, and dminder allows you to optimize levels based on your location, body type, and time of day, all while making sure you stay safe by warning you of any risk to your skin getting burned. All of these features are available for FREE.
There's also a paid version of dminder ($1.99), that provides the additional feature of letting you track your Vitamin D levels over time.