One thing to remember about numbers...

When Vit D was discovered, the researcher chose to use IUs for
the doses. Those numbers appear big... like the doses for B12 are large, when taken orally.

But in reality the amount of D3 you are taking is very minute.

1000IU of D3= 25 MICROGRAMS.

So in reality D3 is a very small amount taken daily. 4000IU would be 100 MICROgrams.

As a comparison, some MS treatments today keep the patients at testing levels of 100! This is THEIR goal... because the thinking for MS is that low D factors into that disease some way.

For us, a goal of 50 (or at least 40 which I keep) is acceptable.

I can really feel my D levels drop around late Sept and Oct from all the sun I get in summer. So I take about 10,000 IU every other day in winter. (sometimes less). I use Puritan's brand...very inexpensive.

The 1000IU rule is an approximation. Some people may need more, and some a little less. At 5000 IU daily I tested at 43...so I am one of those who need a little more.