Even when getting lots of sun, I still had vitamin D deficiency. Both my husband and I were tested, & I am periodically due to age, meds, and an abundance of caution, I suppose. Both of us are taking daily D supplements due to deficiency. D is needed to absorb Calcium which becomes very important as we age, particularly in post menopausal women. But for men as well, so don't forget the guys when considering D supplements as it plays a role in a healthy Prostate, and who wants their loved one to have to face prostate cancer?
And for anyone with chronic kidney disease, even at the very low level, D is important.
Also certain prescription drugs lead to D deficiencies, so when you are taking them, D levels should be monitored, although I know that doesn't always happen.
And although sun-screen is strongly encouraged to prevent skin cancer, it does reduce absorption of D through the skin. Other factors, such as where we live (where we get our sun exposure) because the UV rays are different in different areas of the earth, how much melanin is in our skin (it varies with different people & skin color) as melanin reduces UVR available for conversion to D.
But even so, as a woman with fair skin, who rarely wears sunscreen, I still am D deficient without a supplement. And I do spend time outdoors. Rarely enough to get a sunburn, but supposedly enough to get enough D. But I don't.
Some references:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2912737/https://www.health.harvard.edu/stay...r-health-breaking-old-rules-raising-new-hopeshttps://www.medicalnewstoday.com/articles/161618#benefitshttps://my.clevelandclinic.org/health/articles/15050-vitamin-d--vitamin-d-deficiencyhttps://www.ncbi.nlm.nih.gov/pmc/articles/PMC5946242/