Yes, vitamin D is well-known to be harmful in excess [0] and to be fat-soluble, meaning that it can build up gradually over time. You might not hear of such conditions very often, though, since getting excess vitamin D usually takes effort.

[0] https://en.wikipedia.org/wiki/Vitamin_D#Excess

It takes a LOT of effort. Like in excess of 50K UI a day for months. And then it needs co-factors to do harm, like excess calcium intake along with a K2-MK7 deficiency and/or a potassium deficiency (both of which are, unfortunately, very common). All in all, it's a very complex system that interacts with the body's buffering mechanisms.

From Wikipedia: “according to some research, the tolerable upper intake level (UL) is 4,000 IU/day for ages 9–71 (100 µg/day). Other research concludes that, in healthy adults, sustained intake of more than 1250 μg/day (50,000 IU) can produce overt toxicity after several months”

Of course literally everything is harmful in excess, including essentials such as water and oxygen. The harmful dose being very different to the normal/helpful dose is as good as it gets.

Certain vitamins and minerals. Fat soluble vitamins can build up to the point of causing problems. Excess of iron and magnesium may have negative effect AFAIK.

For most people, excess magnesium levels would be pretty hard to reach. Unless you count loose stools as harmful, which some forms of magnesium supplements can cause.

Combining calcium and vitamin D can be seriously problematic. And high doses of calcium in general can make you more likely to end up with kidney stones -- those are not fun to pass!

Some supplements can even compete with each other for absorption (calcium & magnesium come to mind), so taking them together in a multivitamin is probably not the way to go.

But if I’m mindful of my iron intake etc. should have included that assumption

Vitamin D is fat soluble, but high doses needed for overdose.

It all depends on the context.

There is no reason why your body should use the supplement vitamins or minerals if your diet is normal (not deficient). For example, a lot of people eat a bunch of animal products rich in calcium, D3, protein etc. and end up with diseases like osteoporosis, sarcopenia etc. Even with real food their bodies did not do anything with it.

Intervention studies with supplements on these kinds of people show that they rarely help or help very little (not long-term).

You need to make your body use the stuff you give it.

I read up on this because I had a really bad sinus infection a couple months ago and found myself taking "Emergen-C" every morning, which advertises "1000% your daily value of Vitamin C". Immediately after my sinus infection cleared up the virus stuff started, so I kept taking it for a while.

From what I read, at least with Vitamin C, you have to get into the 2000% range before there start being negative side-effects.

I take as much as 20 grams a day with no side-effects. It can take your stomach awhile to get used to that much but once it has adjusted its fine.

The other thing I've always heard is that taking more than the recommended amount is pointless, because your kidneys filter it out. I definitely wouldn't take the time to adjust to an even higher dose. I was mainly only taking the dose I was because I didn't want to try and subdivide the little packets.

Men do have to worry about retaining too much iron. Women before menopause typically don't have to worry as they lose a bit of blood every month.

https://www.winchesterhospital.org/health-library/article?id...

If you're a man, you can give blood to get this effect. I believe there was a study linking this to a decline in diabetes risk (or something like that).

If you're a man you'd use leeches.

Or periodic woodworking accident.

Or combat.

Excess iron appears to be more of a risk factor for heart disease than diabetes. This is suspected as a reason why pre-menopausal women have a lower risk of heart attack than men and post-menopausal women. This is still an area of active research and not really proven.

Yes, that, too. I was thinking of this, but I think they're all closely related. And hey, even if the science doesn't pan out, you've still probably helped someone out with your donation.

https://www.nature.com/articles/d42473-018-00382-9

I am male and have the opposite issue. I become anemic without supplements. There seems to be some correlation with autism and low iron/b12.

I take Floradix, which is a German-made liquid made from vegetables and yeast. It has probably saved my life, because without it I become lethargic then severely depressed.

I joke that I'm a vampire and it is my human blood substitute that I take so I don't have to feed on people.

For this reason men's multivitamins typically don't include iron.