Vitamin D: Side effects and risks

Vitamin D is fundamental in supporting several bodily processes, including:

• the absorption and regulation of calcium, magnesium, and phosphate
• the hardening, growth, and remodeling of bones
• cellular growth and remodeling
• immune function
• nerve and muscle function

There is no set consensus about how much vitamin D is necessary for good health. A person’s daily requirement depends on individual factors, such as age, sex, and health status.

Many people get enough vitamin D from sun exposure.

What are the side effects of vitamin D supplementation?

Doctors tend to consider vitamin D levels to be adequate if there are at least 20 nanograms (ng) of the vitamin per milliliter (mL) of blood. If levels rise above 50 ng/mL, a person may experience adverse side effects.

However, most research indicates that the toxicity threshold for vitamin D is fairly high, around 200–240 ng/mL.

A person with levels of vitamin D in this range usually consumes between 10,000 and 40,000 international units (IUs) of the vitamin per day.

Below are some of the most serious side effects associated with overexposure to vitamin D.

1. Hypercalcemia

Most of the significant side effects associated with vitamin D toxicity are related to hypercalcemia.

Hypercalcemia is an excess of calcium in the blood. This occurs when there are more than 10,4000 ng/mL of calcium in the blood.

Researchers often cite this threshold as 10.4 milligrams (mg) of calcium per deciliter of blood or as 0.104 mg/mL.

There is a strong relationship between vitamin D and calcium. When there are high levels of vitamin D metabolites in the blood, this increases the amount of calcium that the intestines absorb.

High levels of vitamin D metabolites can also promote the release of calcium from the bones into the bloodstream.

Having too much calcium in the blood can lead to a wide range of complications and symptoms. Some of the most serious include:

• a loss of appetite
• diarrhea and constipation
• confusion, disorientation, or trouble thinking
• nausea and vomiting
• joint and muscle pain
• continuous headaches
• irritability and anxiety
• unexplained exhaustion
• muscle weakness
• increased thirst and more frequent urination
• an irregular heartbeat
• reduced reflexes
• a metallic taste in the mouth
• high blood pressure

2. Kidney damage

Excess calcium in the bloodstream can bind with phosphate and form crystals that deposit in soft body tissues.

These crystals can cause tissue damage and eventually organ damage, depending on their location, number, and size.

The kidney is especially vulnerable to calcium deposits because of its role as a filter and its many small passageways.

When calcium deposits get stuck in kidney tissues, nephrocalcinosis can occur. If this condition is severe, it can cause permanent kidney damage and, eventually, kidney failure.

Symptoms of nephrocalcinosis include:

• nausea and vomiting
• fever and chills
• severe pain in the stomach, the sides of the back, or the groin area, including the testicles in men

Authors of a 2015 review considered the effects of vitamin D3 toxicity.

On average, the people affected had taken a total of 3,600,000 IU of the vitamin in fewer than 3 months. They experienced several side effects, including acute kidney damage.

Elevated levels of calcium in the blood can harm the kidney’s ability to concentrate urine.

This can result in a person producing and passing unusually large quantities of urine, which is called polyuria.

Researchers are not entirely sure why this occurs. One theory is that high calcium levels reduce the effects of antidiuretic hormones.

These hormones encourage the kidneys to retain water, and fewer hormones may result in larger quantities of diluted urine.

When a person passes a lot of diluted urine, they lose large quantities of water and electrolytes. For this reason, a person with vitamin D toxicity may be prone to dehydration.

Symptoms of moderate dehydration include:

• a dry mouth and tongue
• restlessness
• irritability
• sunken eyes
• increased thirst
• decreased urinary output
• decreased tear production and dry eyes
• skin that is slow to return to normal after being pinched

Severe dehydration can result in life-threatening conditions. Other signs and symptoms of this condition include:

• a weak or absent pulse
• low blood pressure
• unexplained exhaustion
• very little or no urine output
• unconsciousness

6. Pancreatitis

Hypercalcemia can cause acute pancreatitis, which is the term for inflammation of the pancreas.

In 2017, researchers published a review of the features and treatment of vitamin D toxicity-induced acute kidney injury.

They studied the experiences of 19 people with the toxicity. Each had taken an average of 6,000,000 IU of the vitamin over 1–3 months.

Two of the 19 people experienced acute pancreatitis as a complication.

Signs of pancreatitis include:

• upper stomach pain that extends to the back
• nausea and vomiting
• a rapid pulse
• weight loss
• fever

7. Lung damage

When high levels of calcium and phosphate in the blood bind to form crystals, these deposit in soft tissues.

The crystals are especially likely to deposit in soft-tissue organs that act as filters, such as the lungs.

If many of these calcium salt deposits, or ectopic calcifications, are present in the lungs, they can impair the organ’s function.

Some signs of the resulting lung damage include:

• coughing
• trouble breathing
• chest pain

Most people who have taken too much vitamin D will experience at least two of the following symptoms within a few days of overdosing:

• unexplained exhaustion
• a loss of appetite and weight loss
• constipation
• dry mouth
• skin that is slow to return to normal after being pinched
• increased thirst and frequency of urination
• continuous headaches
• nausea and vomiting
• reduced reflexes
• confusion, disorientation, or trouble thinking
• an irregular heartbeat
• muscle weakness and reduced muscle tone
• changes in gait

People with severe or chronic vitamin D toxicity may develop life-threatening symptoms, including:

• extreme dehydration
• high blood pressure
• slowed growth
• trouble breathing
• temporary losses of consciousness
• heart failure and heart attack
• kidney stones and kidney failure
• hearing loss
• tinnitus (ringing in the ears)
• pancreatitis (inflammation of the pancreas)
• gastric ulcers
• coma

Takeaway

Vitamin D toxicity can cause a wide range of symptoms, and each individual may respond differently.

A person has a higher risk of experiencing toxicity if they have taken supplements over an extended period or taken excessive amounts. This can cause vitamin D to build up in the blood.

The risk of experiencing adverse side effects after absorbing vitamin D from the diet or exposure to the sun is very low.

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