Vitamin D toxicity: You can have too much of a good thing

·5 min read
'There doesn’t seem to be any real benefit in taking more unless you’re deficient' says Dr Mellor
'There doesn’t seem to be any real benefit in taking more unless you’re deficient' says Dr Mellor

Vitamin D: it’s known as the sunshine vitamin and is essential for healthy bones and muscles, and our immune system. But the case of a man who overdosed on vitamin D supplements, and spent eight days in hospital with kidney failure, is a cautionary tale of how the dose makes the poison.

According to the new research in the British Medical Journal’s Case Reports, the man took a cocktail of vitamins in a drive for good health, including 1250 micrograms (mcg) of vitamin D – 125 times the 10mcg daily dose recommended in the UK. According to the report, the man’s symptoms started a month after taking the supplements. The study’s authors said such cases of ‘hypervitaminosis D’ were on the rise globally.

In some respects, confusion about vitamin D isn’t surprising. Many of us are deficient and the government recommends we all take a daily vitamin D supplement between October and March. Unlike other vitamins which come from our diet, we make vitamin D in our skin using the sun’s ultraviolet B (UBV) rays. During the dark autumn and winter months, our vitamin D stores decline. The National Diet and Nutrition Survey shows about 1 in 6 UK adults have low levels in their blood.

Most doctors and nutritionists support the recommended level of supplementation of 10mcg a day as it’s difficult to get enough vitamin D from food (although oily fish, cod liver oil and some fortified foods contain good amounts, and small quantities are found in egg yolks, liver, and wild mushrooms).

Some nutritionists, however, are prescribing higher doses touting vitamin D supplements as a near-miracle cure for a range of conditions – from multiple sclerosis and dementia to depression and cancer – even though good supporting evidence is lacking. Sales boomed during the pandemic, when vitamin D was widely mentioned as a possible cure for Covid-19 or a means to prevent severe symptoms, although no robust evidence has yet emerged to support this.

“Some nutritionists do prescribe very high doses of vitamin D, but I don’t feel it’s a good idea because it’s very easy to become toxic,” says Dr Sarah Brewer, a GP and qualified nutritionist, and author of The Essential Guide to Vitamins, Minerals and Herbal Supplements. 

Several years ago, one UK laboratory found that over 3 per cent of blood samples contained toxic levels of vitamin D. “Most of those were from people who had been taking liquid supplements of vitamin D, which are very highly concentrated,” says Dr Brewer. “Some people actually swig them from the bottle. I think there’s a perception that all vitamins are good for you and that more is better than less, which absolutely isn’t the case.”

Dr Brewer believes some groups benefit from higher doses than the officially recommended 10mcg per day, including those aged over 50, for whom she prescribes 50mcg. However, very high doses are potentially dangerous because vitamin D is fat soluble, and the body has no satisfactory mechanism for excreting any excess.

Dr Duane Mellor, a senior lecturer at Aston Medical School, recommends most people stick to the recommended 10mcg, although a maximum of 100mcg per day is generally considered safe. Some people with a diagnosed deficiency might need more but should only be taken under medical supervision.

“There doesn’t seem to be any real benefit in taking more unless you’re deficient,” Dr Mellor says. He says there’s no real way of knowing whether you’re lacking, except by having a blood test; you could ask your GP for this if you feel you have symptoms, or tests are available privately at some medical clinics. Symptoms of deficiency including stomach cramps, low mood and bone pain tend to develop over time.

“Long term, very low vitamin D levels affect your bone health because you don’t absorb as much calcium from your food and your body balances that by drawing it from your bones.” Dr Mellor says. This can lead to conditions such as rickets in children and bone pain caused by a condition called osteomalacia in adults.

But anyone who takes vitamin D supplements needs to be sure they’re not accidentally consuming too much, as the way doses are measured and printed on packaging can be confusing. The word microgram is sometimes written with the Greek symbol μ followed by the letter g (μg). And sometimes, the strength of vitamin D is stated in international units (IU) rather than micrograms. Dr Mellor warns against getting the two mixed up: 1mcg = 40 IU, so the recommended daily intake in the UK is 400IU or 10mcg.

The maximum recommended daily limit in the UK – from diet and supplements combined – is 100mcg for adults and children 11 – 17; 50mcg for children aged 1 – 10 y; and 25mcg for infants. Be sure to factor in any foods such as breakfast cereals and milk that are sometimes fortified with vitamin D.

If you choose to take supplements, vitamin D3 (cholecalciferol) is more effective than D2 (ergocalciferol), according to the NHS. And some studies have shown that taking a vitamin D supplement with the largest meal of the day improves absorption.

Dr Brewer advises caution when taking other supplements too, notably iron, vitamin A (particularly for pregnant women) and selenium. “For these, there’s a narrow window between a dose that’s desirable and a dose that’s toxic, and it can be easy to take too much.”

In other words, when it comes to vitamins, you certainly can have too much of a good thing.

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