1. Climate

Most people are not getting enough sunny days in the climate that they live in to maintain a tan year-round. Unless you are able to get full body exposure to the sun at least 4-5 days of the week you are probably not able to achieve optimal Vitamin D3 levels.

2. Sun Strength

Even if you live in an area with more sunnier days than not, the sun might not be strong enough most days of the year to be effective. Pollutants in the air can also block UVB rays from reaching their maximum potential.

3. Other Factors

Factors such as skin pigmentation, age, and weight can all play a role in how your body process vitamin D3 and whether you are getting enough.