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Comment by rich_sasha | Hacker News Reader

I keep reading about these “IU”s on HN, what on earth are they? Here in the uk you just buy “Vitamin D” and take one pill per day. No fine-tuning, unless you need specialised therapy perhaps.

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International units. OTC quantities tend to be quite low, e.g 300-400 IUs, which aren't sufficient unless you're also getting daily sunlight not through a window. 1000-2000 IU is better - widely regarded as safe and no cases I've ever read of of hypervitaminosis, but it's above usual OTC doses. 4000+ daily is pushing it and I have read of cases of toxicity above that, but if you're struggling to get serum levels up then perhaps it's needed.

Honestly, OTC quantities vary depending on what is legal where you live. It has been 8-9 years since I lived in the US but nevertheless: At the time, you could buy 1000-2000iu strengths. I worked at a large pharmacy chain and stocked vitamins weekly.

Then I moved to Norway. One strength is sold in most stores - 400iu per tablet. You might be able to buy stronger in the pharmacy, but I had to have a prescription for a higher strength when I had low levels.

And to be fair, 2 of those store tablets keeps my levels high enough. So while 400 might be inadequate, 800 certainly isn't. Even without enough sunlight: I literally cannot get enough during the winter because the sun simply isn't strong enough when it happens to be up.

It would depend on individual ability to metabolize D3 and amount of outdoors sunlight and the extent that local food is fortified with D3. So everyone's situation is unique.

I'm confident that 2,000 IU daily is safe, though.

Not just fortified with vitamin D, but also the things that contain it naturally. Like seafood.

It would depend on individual ability to metabolize D3 and amount of outdoors sunlight and the extent that local food is fortified with D3

The issue is that I fairly far north. You can miss the entirety of daylight in December if you work first shift at a factory (there will be a glow of twilight as you go home). The sunlight we do get in the winter is of poor quality. I'm not even above the arctic circle - it gets worse up there with no direct sunlight. I'm not sure of the extent of food being fortified: I can't see where they've added it to the milk (for example). I could be wrong about this last bit.

There is a lot of fish in the diet, but nonetheless, they recommend the general population take some vitamin D in the wintertime.

Is it possible they sell a lower strength because the nordic diet tends to include a lot of seafood which has vitamin D? Or is it just a legal thing about selling weaker supplements?

I don't know the answer to this. If I were to guess, though, it is more about selling reasonable strengths to folks self-treating. First, supplements are generally recommended to everyone living here as part of being healthy, and truly stressed for immigrants. Even eating fish just isn't always enough, especially if you do not have the very fair skin that is common here. Second.. well, melatonin was prescription when I moved here.

One thing to be careful about in selecting a supplement is the reputation/testing of the brand since they are loosely regulated. Specifically, how accurate and even are the doses in each batch and capsule? 50 micrograms is small quantity to measure accurately and consistently unless the right equipment and process is followed. There have been a number of supplements and manufacturers over the years who's doses from batch to batch or even capsule to capsule vary wildly.

Here in UK, like everywhere else, Vitamin D dosage is measured in IU.

You probably never noticed it, or your pills have low-strength dosages (400 IU), which are quite common to find in pharmacies, and personally I don't even bother with those.

On my bottle it just says "Vitamin D: 15 μg" on the bottle.

Apparently 1 μg = 40 IU for vitamin D, but different per vitamin.

Some figures to provide some perspective on the wild numbers thrown around in other comments. Recommended daily doses for adults for some vitamins are:

• Vitamin A: 5,000 IU (upper limit of 10,000 IU)

• Vitamin D: 600 IU

• Vitamin E: 22.4 IU

Eh as someone also in the UK this seems false. If you check the packaging it will (in my experience) always give you the dose is IU. I know someone recently prescribed Vitamin D, and again, it was a specific dose. If you take it as part of a multi-vitamin dosage may not be made as clear.

The NHS provides some guidance on doses here: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-...

IUs are used in the UK - my prescription is given as "Colecalciferol 800unit capsules" and many supplements list it. Some only list the μg value though.

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