Vitamin D toxicity is rare but occurs with extremely high doses.
It usually develops over time since extra vitamin D can build up in the body.
Nearly all vitamin D overdoses result from high amounts of vitamin D supplements.
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This is a detailed article about vitamin D toxicity and how much it is considered too much.
Vitamin D toxicity: How does it happen?
Vitamin D toxicity implies that vitamin D levels in the body are so high that they cause harm.
It’s 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.
However, we know that the active form of vitamin D functions similarly to 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 signaling processes affected by vitamin D.
One of the main signaling 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 or elevated calcium levels 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.
Summary: 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.
Blood levels of vitamin D: Optimal vs. excessive
Vitamin D is an essential vitamin, and almost every cell in your body has a receptor for it.
It’s produced in the skin when it’s exposed to the sun.
The main dietary sources of vitamin D are fish liver oils and fatty fish.
Vitamin D supplements are important for people who don’t get enough sunlight.
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 1,000–4,000 IU (25–100 mcg) should be enough to ensure optimal blood levels for most people.
Summary: Blood levels in the range of 20–30 ng/mL are usually considered sufficient. The safe upper limit is about 60 ng/mL, but people with symptoms of toxicity usually have levels above 150 ng/mL.
How much vitamin D is too much?
Since little is known about vitamin D toxicity, it’s hard to define an exact threshold for safe or toxic vitamin D intake.
According to the National Academy of Medicine, formerly known as the Institute of Medicine, 4,000 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 increase reported cases.
Suggested read: How much vitamin D should you take for optimal health?
A daily intake ranging from 40,000–100,000 IU (1,000–2,500 mcg) for 1 to several months has been shown to cause toxicity in humans.
In repeated doses, this is 10–25 times the recommended upper limit. 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 100–4,000 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.
Summary: The safe upper intake limit is set at 4,000 IU per day. Intake in the range of 40,000–100,000 IU per day (10–25 times the recommended upper limit) has been linked with toxicity in humans.
Symptoms and treatment of vitamin D toxicity
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 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.
A doctor may also correct your calcium levels with increased salt and fluids, often by intravenous saline.
Summary: 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 without symptoms of toxicity
Large doses of vitamin D can be harmful, even though there may not be immediate symptoms of toxicity.
Suggested read: Fat-soluble vitamins: A comprehensive overview
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 challenging 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, do not exceed the 4,000 IU (100 mcg) upper limit without consulting a doctor or dietitian.
Summary: 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.
Does the intake of other fat-soluble vitamins change the tolerance for vitamin D?
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 by sparing the vitamin K stores.
Another nutrient that may be important is magnesium. It’s one of the nutrients needed for improved bone health.
Taking vitamin A, K, and magnesium with vitamin D may 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 ensure you get enough of these nutrients if you supplement with vitamin D.
Summary: If you’re supplementing with vitamin D, it may be important also to ensure sufficient vitamin A, vitamin K, and magnesium intake. These may reduce the risk of adverse effects from a higher vitamin D intake.
Summary
People respond very differently to high doses of vitamin D. Therefore, it’s 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’s not recommended to exceed the upper limit of safe intake, which is 4,000 IU (100 mcg) per day.
Larger doses have not been linked with any additional health benefits and may be unnecessary.
An occasional high dose of vitamin D is sometimes used to treat a deficiency but always consult a doctor or dietitian before taking a large dose.
As with many other things in nutrition, more does not always equal better.
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