I am mid 50’s, in the sun maybe hours per day, and very active outdoors. However, my D3 is low in the normal range if that makes sense. If you look at normal ranges I am the lowest point in that range. How that can happen with all the sun exposure is beyond me, but it’s a fact. Since being Dx’d with Prostate Cancer in 07, I have taken a 2000iu D3 supplement daily.
How far north do you live and how dark is your skin? If you live in the higher lititudes, you may never get enough UV light to make Vitamin D even you are out all day.
It’s been a long time since I sat in a biochemistry or a physiology class but I do remember that there is a maximum dose of Vitamin D that your body will make regardless of your skin color or UV content of the light. The body modulates the ammount of precursors available in the skin. Your thermostat may be set low.
Strange that you run low, you are definately a good candidate for supplementation. What do you run with 2000IU/day?