The amount of D3 one takes is based on your Vitamin D, 25-Hydroxy, Total, Immunoassay lab test, which costs about $40 if you pay out of pocket, but your insurance or Medicare should pay for it. There is no agreement on exactly what your vitamin D level should be, but at least 30 ng/ml is fairly standard. Many doctors use rates similar to the ones in this table. As I am considered high risk and have heart disease, I keep mine around 70 ng/ml. I had a cancer scare several years ago, and pushed it up to 100 ng/ml.
I have genetic snps that result in my body not efficiently converting sunlight to vitamin D, so I have to supplement at a high level. The only way to know how much vitamin d3 a person needs to take, is to test. I take 10,000 IUs in the summer and 15,000 IUs in the winter. That is more than most people need or should take, but the only way to know is to test.
Vitamin D frees up calcium, which is both good and bad. You need to take Vitamin K with vitamin d3 to route the free calcium to your teeth and bones, otherwise it could collect in your heart and arteries.
Vitamin D should be taken with fat, so take it with a meal. Many people also take magnesium with vitamin d.

I have genetic snps that result in my body not efficiently converting sunlight to vitamin D, so I have to supplement at a high level. The only way to know how much vitamin d3 a person needs to take, is to test. I take 10,000 IUs in the summer and 15,000 IUs in the winter. That is more than most people need or should take, but the only way to know is to test.
Vitamin D frees up calcium, which is both good and bad. You need to take Vitamin K with vitamin d3 to route the free calcium to your teeth and bones, otherwise it could collect in your heart and arteries.
Vitamin D should be taken with fat, so take it with a meal. Many people also take magnesium with vitamin d.