Vitamin D toxicity is extremely rare, but does occur with extreme doses.
It usually develops over time, since extra vitamin D can build up in the body.
Nearly all vitamin D overdoses result from taking high amounts of vitamin D supplements.
It is almost impossible to get too much vitamin D from sunlight or food.
This is a detailed article about vitamin D toxicity and how much of it is considered to be too much.
Vitamin D toxicity implies that vitamin D levels in the body are so high that they cause harm.
It is also termed hypervitaminosis D.
Vitamin D is a fat-soluble vitamin. In contrast to water-soluble vitamins, the body has no easy way of getting rid of fat-soluble vitamins.
For this reason, excessive amounts may build up inside the body.
The exact mechanism behind vitamin D toxicity is complicated and isn't fully understood at this point.
However, we know that the active form of vitamin D functions in a similar way as a steroid hormone.
It travels inside cells, telling them to turn genes on or off.
Usually, most of the body's vitamin D is in storage, bound to either vitamin D receptors or carrier proteins. Very little "free" vitamin D is available (, ).
However, when vitamin D intake is extreme, the levels can become so high that there isn't any room left on the receptors or carrier proteins.
This may lead to elevated levels of "free" vitamin D in the body, which may travel inside cells and overwhelm the signalling processes affected by vitamin D.
One of the main signalling processes has to do with increasing the absorption of calcium from the digestive system ().
As a result, the main symptom of vitamin D toxicity is hypercalcemia - elevated levels of calcium in the blood (, ).
High calcium levels can cause various symptoms, and the calcium can also bind to other tissues and damage them. This includes the kidneys.
Bottom Line: Vitamin D toxicity is also termed hypervitaminosis D. It implies that vitamin D levels in the body are so high that they cause harm, leading to hypercalcemia and other symptoms.
Vitamin D is an essential vitamin, and almost every cell in your body has a receptor for it ().
It is produced in the skin when it is exposed to sun.
The main dietary sources of vitamin D are fish liver oils and fatty fish.
For people who don't get enough sunlight, vitamin D supplements can be important.
Vitamin D is very important for bone health, and has also been linked with immune function and protection against cancer (, ).
Guidelines for blood levels of vitamin D are as follows (, , , , , ):
- Sufficient: 20–30 ng/ml, or 50–75 nmol/L.
- Safe upper limit: 60 ng/ml, or 150 nmol/L.
- Toxic: Above 150 ng/mL, or 375 nmol/L.
A daily vitamin D intake of 1000–4000 IU (25–100 micrograms) should be enough to ensure optimal blood levels for most people.
Bottom Line: Blood levels in the range of 20–30 ng/ml are usually considered sufficient. The safe upper limit is considered to be about 60 ng/ml, but people with symptoms of toxicity usually have levels above 150 ng/ml.
Since relatively little is known about how vitamin D toxicity works, it is hard to define an exact threshold for safe or toxic vitamin D intake ().
According to the Institute of Medicine, 4000 IU is the safe upper level of daily vitamin D intake. However, doses up to 10,000 IU have not been shown to cause toxicity in healthy individuals (, ).
Vitamin D toxicity is generally caused by excessive doses of vitamin D supplements, not by diet or sun exposure (, ).
Although vitamin D toxicity is a very rare condition, recent increases in supplement use may lead to an increase in reported cases.
A daily intake ranging from 40,000–100,000 IU (1000–2500 micrograms), for one to several months, has been shown to cause toxicity in humans (, , , , ).
This is 10-25 times the recommended upper limit, in repeated doses. Individuals with vitamin D toxicity usually have blood levels above 150 ng/ml (375 nmol/L).
Several cases have also been caused by errors in manufacturing, when the supplements had 100-4000 times higher amounts of vitamin D than stated on the package (, , ).
The blood levels in these cases of toxicity ranged from 257–620 ng/ml, or 644–1549 nmol/L.
Vitamin D toxicity is usually reversible, but severe cases may eventually cause kidney failure and calcification of the arteries (, ).
Bottom Line: The safe upper limit of intake is set at 4000 IU/day. Intake in the range of 40,000–100,000 IU/day (10-25 times the recommended upper limit) has been linked with toxicity in humans.
The main consequence of vitamin D toxicity is a buildup of calcium in the blood, called hypercalcemia ().
Early symptoms of hypercalcemia include nausea, vomiting, diarrhea, constipation and weakness ().
Excessive thirst, an altered level of consciousness, high blood pressure, calcification in the kidney tubes, kidney failure or hearing loss may also develop (, ).
Hypercalcemia caused by regularly taking high amounts of vitamin D supplements may take a few months to resolve. This is because vitamin D accumulates in body fat, and is released into the blood slowly ().
Treating vitamin D intoxication includes avoiding sun exposure and eliminating all dietary and supplemental vitamin D.
Your doctor may also correct your calcium levels with increased salt and fluids, often by an intravenous saline.
Bottom Line: The main consequence of vitamin D toxicity is hypercalcemia, with symptoms including nausea, vomiting, weakness and kidney failure. Treatment involves limiting all vitamin D intake and sun exposure.
Large doses of vitamin D can be harmful, even though there may not be immediate symptoms of toxicity.
Vitamin D is very unlikely to cause severe symptoms of toxicity right away, and symptoms may take months or years to show up.
This is one reason why vitamin D toxicity is so difficult to detect.
There have been reports of people taking very large doses of vitamin D for months without symptoms, yet blood tests revealed severe hypercalcemia and symptoms of kidney failure ().
The harmful effects of vitamin D are very complex. High doses of vitamin D can cause hypercalcemia without toxicity symptoms, but can also cause toxicity symptoms without hypercalcemia ().
To be safe, you should not exceed the 4,000 IU (100 mcg) upper limit without consulting with a doctor or dietitian.
Bottom Line: Vitamin D toxicity usually develops over time, and the harmful effects are very complex. Large doses may cause damage, despite a lack of noticeable symptoms.
It has been hypothesized that two other fat-soluble vitamins, vitamin K and vitamin A, may play important roles in vitamin D toxicity.
Vitamin K helps regulate where calcium ends up in the body, and high amounts of vitamin D may deplete the body's stores of vitamin K (, ).
A higher vitamin A intake may help prevent this from happening by sparing the vitamin K stores.
Another nutrient that may be important is magnesium. It is one of the nutrients needed for improved bone health (, ).
Taking vitamin A, vitamin K and magnesium with vitamin D may therefore improve bone function and reduce the chances of other tissues becoming calcified (, , ).
Keep in mind that these are just hypotheses, but it may be wise to make sure you are getting enough of these nutrients if you are going to supplement with vitamin D.
Bottom Line: If you are supplementing with vitamin D, then it may be important to also ensure sufficient intake of vitamin A, vitamin K and magnesium. These may reduce the risk of adverse effects from a higher vitamin D intake.
People respond very differently to high doses of vitamin D. Therefore, it is hard to evaluate which doses are safe and which are not.
Vitamin D toxicity can have devastating health effects, which may not show up until months or even years after starting to take high doses.
Generally, it is not recommended to exceed the upper limit of safe intake, which is 4000 IU (100 micrograms) per day.
Larger doses have not been linked with any additional health benefits, and may therefore be completely unnecessary.
An occasional high dose of vitamin D is sometimes used to treat a deficiency, but always consult with your doctor or dietitian before taking a large dose.
As with many other things in nutrition, more does not always equal better.
You can find more info about vitamin D on this page: Vitamin D 101 – A Detailed Beginner’s Guide