[baccheion]

Before adding supplemental vitamin D, first ensure nutrient sufficiency and that all RDAs are being met (with just food). For example, low magnesium impairs vitamin D conversion and is a more common problem. Further, hGH/IGF-1 also pushes for conversion of vitamin D. Insufficient potassium, insulin resistance, and some other things clip hGH/IGF-1.

In some cases, fixing diet and sleep (ZMA for males) normalizes vitamin D. After other steps are taken, then add some amount of D3 if necessary. It's been said it's better gotten from red light (one of those special lamps/light) on the skin, as the body is able to regulate/limit absorption.

Either way, some supplementation is likely to be needed with age. Response of the skin to sunlight declines over time, especially due to rising cortisol.