I recall this graphic every winter.

Vitamin D is a top vitamin deficiency for adults during winter. Your skin synthesizes Vitamin D when exposed to UV radiation in sunlight. During Fall and Winter, the sun's angle prevents sufficient UV exposure if you live above 37 degrees latitude. 

Where do you live on the map? 

Vitamin D is vital to runners because it absorbs calcium to grow and repair bones. Without sufficient Vitamin D, bones become thin, brittle, or misshapen. Your immune system also needs Vitamin D to work well. There are interesting studies about Vitamin D deficiency and increased risk of a severe or critical case of COVID.

Our Runner Multivitamin contains Vitamin D in adequate quantity to match runners' higher outside activity levels. A few foods naturally contain sufficient Vitamin D, including salmon, eggs, and shellfish. Vegan sources include mushrooms, and many nut milks and cereal products are commonly fortified with Vitamin D.

Best wishes on chasing your running goals...and Vitamin D!