Grassrootshealth net
It depends on what your purpose is.
If you are just wanting to meet your daily needs then above 40ng/ml would be adequate BUT aiming a bit higher 45ng/ml would give a margin for error.
If you think it's a smarter idea to have some vitamin D3 actually stored in your cells so it's anti oxidant protection can be available to the mitochondria in those cells than above 50ng/ml is better and 60ng/ml probably ideal.
For nursing mothers human breast milk is vitamin d3 replete at that level.
If you have a cancer diagnosis there is some evidence that vitamin d can act directly on those cancer cells at around 100ng/ml.
Mercola, who spends his winter working from the tropics and his summers at Latitude 42, keeps his around 100ng/ml. and he looks pretty fit, and I don't think he needs to take vitamin D supplements because not only does he have access to the sunlamps he sells but his time in the tropics will fill his vitamin D tank.
However for me at latitude 52 and no winter in the tropics I keep mine at 60ng/ml with regular UVB exposure in winter + 5000iu/daily throughout the year and full body sun exposure WHENEVER possible cloud/wind permitting from April through to September.
I test my 25(OH)D twice yearly. I suggest you may need a bit more testing initially to be sure your levels are moving in the right direction. But after you have had a 3 yrs being able to predict your 25(OH)D result so you are sure you know how your body responds to D3/SUNLIGHT then you could drop down to one test every other year HOWEVER don't give up on testing even if you think you know what you are doing.
Things change, your skin's ability to make D3 is associated with you TC cholesterol level. If you cholesterol drops it's likely your ability to generate vitamin D has also dropped.
It's also possible urban and even rural pollution levels change over time and while you probably wouldn't notice slightly higher ozone levels in the atmosphere they will block UVB and that reduces the amount of UVB reaching your skin and the D3 creation.
UVA degrades vitamin D near the skin surface. Changing your office so you spend time by a sunny window every day as only UVA passes through glass this new sunny window time would be reducing your Vitamin D status while you worked.
So both the environment and your body may change in ways that may reduce the amount of vitamin d you are making or losing so getting checked from time to time even after you are sure you've got it sussed will still be needed.
I have mind done twice yearly and I think I know what I'm doing. I usually am able to predict the result within 5ng/ml. but you never know what changes my have occurred to upset our plans so better safe than sorry.