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When most people when they hear the word ‘vitamin,’ they assume more is better.

While it’s generally true with vitamin C and the B–vitamins, it’s not true of every vitamin.

Some vitamins, like the fat-soluble ones, can build-up over time — even with a moderate daily intake.

In fact, some vitamins act as hormones, such as vitamins A and D. 

It’s not unusual for biologists to call them vitamin–hormones.

Retinoids are also hormones – with intracrine activity – because retinol [vitamin A] is transformed in the cells into molecules that bind to and activate specific nuclear receptors, exhibit their function, and are subsequently inactivated.

It’s a powerful growth-promoter that increases skin turnover.

Skin turnover is great for slowing down aging and other skin issues.

In fact, synthetic vitamin A is so strong that you’ve probably heard about it as an acne medication — Accutane.

Or, if you prefer generics, you know it as retinol.
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Speaking of turnover — vitamin A also increases bone turnover.

This time it’s not such a good thing, it makes bones more brittle.

You would think that a vitamin increases your fracture risk factors, but it does.

Vitamin A intake occurs at its highest rate in the world in Sweden, so researchers conducted their study in Sweden.

Researchers looked at vitamin A’s impact on bones.

They examined 2,322 men over the course of 25 years.

Researchers split the men into five groups ranging from the lowest to the highest serum Vitamin A concentrations.

A complementary analytic approach showed that the subjects with retinol levels in the 99th percentile [3.60 μmol per liter] had an overall risk of fracture that was seven times the risk among those with lower levels […] multivariate rate ratio, 7.14.

The results may surprise some people, but not these researchers.

Biologists saw the same effect for decades in mice.

Really, it’s an open secret that the supplement companies don’t usually tell you.

Too much vitamin A interferes with the balance between the different cells in bone metabolism.

Bone metabolism involves two main cells — osteoblasts and osteoclasts.

Osteoclasts break down bone.

They have a Retinoid X Receptor which responds to vitamin A by increasing its activity.

Retinol is released in target cells and converted to retinoic acid, which exerts its effects by binding to specific nuclear receptors. Retinoid receptors have been identified in both osteoblasts and osteoclasts. Retinoic acid suppresses osteoblast activity and stimulates osteoclast formation in vitro.

Found in the cell nucleus, these powerful receptors directly influence gene transcription.

These vitamin A receptors bind directly to the DNA.

But it’s all about balance with vitamin A — too little causes weak bones, too.

It reduces bones’ ability to remodel and repair itself.

These two opposing effects are why we see a ski-shaped tail from the Swedish study graph.

You can see that the ideal serum concentration is near the center:

And it goes exponential on the higher-end of the scale.

What is unusual about the Swedes is that they consume a lot of fish liver oil.

In addition to PUFAs, which are another problem, cod liver oil contains dangerous amounts of vitamin A.

They also vitamin fortify the milk in Nordic countries.

Beta-carotene is a precursor to vitamin A, but it is much safer:

The serum beta carotene level was not associated with the risk of fracture: multivariate rate ratio per [one standard deviation] increase, 0.95.

In fact, not only did beta-carotene not contribute to fractures, it seems slightly protective.

Beta-carotene is like two vitamin A molecules stuck together:

Enzymes in the intestines split beta-carotene, but they do it on demand.

When vitamin A gets high, something called a “negative feedback mechanism” kicks in.

This mechanism tells the enzymes to slow down and stop splitting so much of the beta-carotene.

That mechanism is why beta-carotene never leads to overdose.

On the other hand, your body doesn’t know when to stop absorbing vitamin A. 

It just keeps on doing it, with no feedback.

And that is the big reason why vitamin A has the narrowest therapeutic window out of any vitamin.

It also explains why Nordic countries have the highest rates of osteoporosis — despite their massive milk intake.

All of the excess retinol they consume stimulates bone metabolism too much.

New bone growth can’t keep up, leading to brittle bones.

The problem is so prevalent that Hypervitaminosis A is a recognized medical condition.

And it’s the vitamin with the angriest reviews on Amazon:

I highly recommend that you monitor how much vitamin A you consume.

In fact, I probably wouldn’t take vitamin A supplements and risk getting a too high dose of vitamin A.

Instead, I would supplement with beta-carotene.

That way, my body will stop making vitamin A when it has enough, and I won’t risk getting too much.

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Matt Cook is editor-in-chief of Daily Medical Discoveries. Matt has been a full time health researcher for 26 years. ABC News interviewed Matt on sexual health issues not long ago. Matt is widely quoted on over 1,000,000 websites. He has over 300,000 daily newsletter readers. Daily Medical Discoveries finds hidden, buried or ignored medical studies through the lens of 100 years of proven science. Matt heads up the editorial team of scientists and health researchers. Each discovery is based upon primary studies from peer reviewed science sources following the Daily Medical Discoveries 7 Step Process to ensure accuracy.