I used to take a supplement with 10k vit D units. Made by a reputable company here in Europe.

I did a blood test for vit D metabolites (oh 25 or something). It turned out I was deficient.

The doc gave me 8k units as a prescription drug. In theory weaker. After 2 months I was no longer defiecient (kinda borderline). After 4 months the doc decided to go down to 4k iu.

Now I wonder why some experts are telling people to take 10k iu? Is it because the supplements are crap and contain 10% of what they claim? (I've been taking vit K as well)

I think the general idea is that the stats labs use to decide what is normal is not necessarily ideal, and like everything else will vary per person.

Too much Vitamin D3 can be toxic, so doctors are reluctant to tell you to go over published guidelines. but if you don't have other issues, and stick to what you can get OTC from a normal vitamin brand and follow the instructions, you're probably safe. There was a story a while ago about a woman who died from too much D3, but it turned out she was taking a whole bottle's worth every day for years.

> Now I wonder why some experts are telling people to take 10k iu?

Taking 10K per day is a bad idea. I went into the excess Vitamin D range with half that dose even in winter with an indoor job.

More is not always better.