As the article says, D works with other vitamins to do a lot of things. It works with K to help calcium get into your bones. Otherwise you can absorb it and it can hang around in places it's not really supposed to, and that turns out to not be a good thing. You're probably better off with D + K (if you need it) than trying to skimp on D.
(I have celiac [1], and I had to learn about this connection the hard way. The horrid thing is that my normal daily lunch was an enormous bowl of leafy greens, exactly where you're supposed to get vitamin K from, and I still had to supplement it. Calcium in the wrong places was one of the contributing factors to my heart issues in the link. I still have to be a bit careful not to eat too much hard cheese, but only slightly more careful than a normal person who faces potentially blocking themselves up anyhow.)
I see, thank you. I'm wondering whether I should supplement D myself, but I live in a sunny country. I should get some blood tests to check first, I guess.
Doctors usually recommend 1k IU per day but this has been shown to be problematic. 8k-10k IU is much better. Spending a day on the beach will net you many 10ks of vitamin D (given full body exposure to the sun).
(I have celiac [1], and I had to learn about this connection the hard way. The horrid thing is that my normal daily lunch was an enormous bowl of leafy greens, exactly where you're supposed to get vitamin K from, and I still had to supplement it. Calcium in the wrong places was one of the contributing factors to my heart issues in the link. I still have to be a bit careful not to eat too much hard cheese, but only slightly more careful than a normal person who faces potentially blocking themselves up anyhow.)
[1]: https://news.ycombinator.com/item?id=22454099