Health

Vitamin D can be toxic if taken in high doses. Here's how much you should be having

Walk down the pharmacy aisle of any supermarket and you'll find shelves chock full of vitamin supplements.

They may help some people, but is it possible to have too much of a good thing?

In February this year, a middle-aged man in the UK was admitted to hospital with symptoms of vitamin D toxicity — hypervitaminosis D — a rare but serious condition resulting from "overdosing" on the vitamin.

The man had been taking a cocktail of more than 20 over-the-counter supplements every day, including a whopping 150,000 international units (IU) of vitamin D — almost 400 times higher than the recommended daily dose, according to the report published today in BMJ Case Reports.

After one month on the regimen, the man ended up in hospital with vomiting, abdominal pain, leg cramps and increased thirst. He'd also lost close to 13 kilograms.

While the man stopped taking the supplement cocktail as soon as symptoms appeared, they hung around for almost three months.

Alamin Alkundi, lead author and endocrinologist at East Kent Hospitals University NHS Foundation Trust in the UK, said that the man had heard about the benefits of vitamin D supplements on a radio show and wanted to see if it could improve his wellbeing.

This prompted the man to seek out the advice of a nutritionist, who recommended he also take extremely high doses of other vitamins, minerals, nutrients and probiotics, according to the report.

"At that time, he was not feeling great in himself … feeling very low in energy," Dr Alkundi said.

How much vitamin D do I really need?

Also known as the "sunshine vitamin", vitamin D isn't technically a vitamin but a prohormone — a substance your body converts into a hormone.

Vitamin D is converted into calcitriol, a hormone that helps you absorb calcium and maintain healthy bones and muscles.

Getting out in the sun is the best way to give yourself a vitamin D boost, but you can also get a small amount from foods like fatty fish, mushrooms and fortified milk and cereals.

Just how much vitamin D you should be getting each day depends on your age: the older you are, the more you need.

The National Health and Medical Research Council recommends 5-15 micrograms (200-600 IU) of vitamin D each day. 

But vitamin D deficiency is common, affecting nearly one quarter of people in Australia.

"For most vitamins and minerals, people can get them in sufficient quantities from a healthy balanced diet," said Elina Hyppönen, a nutritionist at The University of South Australia who was not involved in the study.

As a result, popping a daily vitamin D pill has become a common way to keep levels healthy, particularly among those who don't spend much time out in the sun.

How much is too much?

But while vitamin D supplements are generally safe, it's possible to take too much.

According to the National Academy of Medicine in the US, the safe upper limit for vitamin D intake is 4,000 IU, or 100 micrograms, though higher doses may sometimes be used to treat health issues like deficiency, cardiovascular disease and diabetes over short periods.

Studies have shown that taking between 40,000 to 100,000 IU (1,000-2,500 micrograms) each day for several months can cause vitamin D toxicity.

This causes a build-up of calcium in the blood, a condition known as hypercalcemia, which leads to a wide variety of symptoms.

Some of these include nausea, vomiting, diarrhoea, fatigue, weight loss and excessive thirst.

Down the track, hypercalcemia can cause kidney problems, bone loss and hardening of the arteries and soft tissues.

When the man was admitted to hospital, he was suffering from vomiting and nausea.

At first, Dr Alkundi and his team initially thought the man had a bug and he was discharged the same day.

But he was back the following day with more severe symptoms like abdominal pain.

Blood tests showed the man's vitamin D levels were seven times higher than normal and his calcium levels were sharply raised.

The tests also indicated that the man's kidneys were failing to filter waste, a condition known as acute kidney injury or failure.

The man spent eight days in hospital, where he was flushed out with intravenous fluids.

He was also treated with anti-nausea medication and bisphosphonates, which are typically used to reduce high levels of calcium in the blood or limit bone loss in osteoporosis patients.

Two months after discharge, the man's calcium levels had returned to normal, but his vitamin D levels were still sky high, Dr Alkundi said.

The man's levels had finally returned to normal when Dr Alkundi and his team assessed him six weeks ago. 

"It took too long for him really," he said.

Why does vitamin D hang around for so long?

Like vitamins A, E and K, vitamin D is fat-soluble and cannot dissolve in water.

Vitamins in this group are more easily absorbed by the body than water-soluble vitamins like C and B, particularly when taken with higher-fat foods.

Once they pass through the gut and bloodstream, fat- or oil-soluble vitamins are stored in fatty tissue.

This gives these types of vitamins a slower turnover than their water-soluble counterparts, said Treasure McGuire, a pharmacologist at The University of Queensland and Bond University.

"It's the intracellular cumulation of oil-soluble vitamins that can become problematic with chronic misuse and overuse, even if people are thinking they're doing it from a therapeutic standpoint," said Dr McGuire, who was not involved in the study.

While vitamin D toxicity is rare, studies have shown that it could become more common as more people rely on supplements as a boost.

A 2017 study on nearly 40,000 people in the US found that the number of people taking 1,000 IU or more of vitamin D had grown between 1999 and 2014.

About 18 per cent of the study participants took more than 1,000 IU of the vitamin each day, while another 3 per cent ingested over 4,000 IU daily.

And in 2019, an elderly woman in the UK died after taking 40,000 IU of vitamin D each day.

Geraldine Moses, a pharmacist at The University of Queensland, suspected that vitamin D toxicity could be even more common than we think.

"Patients are self-medicating so if they run into problems, they're very unlikely to tell anyone," said Dr Moses, who was not involved in the study.

"We always have to make sure we recognise that what gets published is the tip of the iceberg."

For example, a cancer patient Dr Moses recently consulted was taking 5,000 IU of vitamin D each day.

"She's not going to give herself the same toxicity level as this man, but it's still an overdose," Dr Moses said.

How did this happen in the first place?

A major problem with vitamins and supplements is they are not regulated in the same way as over-the-counter and prescription medications, Dr Moses said.

This means that manufacturers are not obligated to warn consumers about the potential risks of a given supplement, such as exceeding the recommended dose and their potential interactions with medications, she said.

"That's what troubles me … people can't make informed and balanced decisions weighing up the risks versus the benefits," Dr Moses said.

"So, it's really vital that the lessons from this case are widely publicised so that people respect that vitamins can cause toxicity."

Dr Alkundi said that it's important to check in with your doctor or general practitioner before taking any new vitamin or other supplement.

"They are the ones that know the patient's history and their medication, so they can advise what suits them," Dr Alkundi said.