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For most of my adult life, I’ve been diagnosed with vitamin D deficiency. I take a vitamin D supplement every morning, yet I still never seem to have enough vitamin D. So what gives?

This frustration led me to want to better understand vitamin D: how we get it, what it does in our bodies and how it’s decided that we don’t have enough.

In the video above, I finally pursue answers (and find solutions!) to these — and other — lingering questions about my perennially subpar vitamin D status.

This video is part of a FiveThirtyEight series on our scientific resolutions for 2018. Throughout the week, we’re questioning whether some of our habits and hobbies are based on junk science or real evidence. You can read the full series here.