Vitamin D: Key to bones, immune system and wellbeing

Can levels be too high?

Current studies show that a vitamin D level that is too high can be just as harmful as one that is too low. In general, an optimal vitamin D level sits between 50 and 80 ng/ml. Lower vitamin D levels are associated with weaker immune defence. High vitamin D levels can suppress the immune system, which can be useful in the short term in autoimmune disease or aggressive inflammatory disease. If an infection is driving an illness, however, vitamin D levels should stay more moderate so the immune system can fight the infection. It is worth remembering at this point that vitamin D is a steroid.

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What is vitamin D?

Vitamin D, actually a prohormone, is stored in the body — in the liver and in adipose tissue. It plays a central role in calcium and phosphate metabolism and is involved in processes such as blood-pressure regulation, immune function and cell growth. Deficiency is associated with conditions such as rickets, osteoporosis, multiple sclerosis and cancer (Holick, 2007).

The strong seasonal swings in vitamin D levels help explain why we are more prone to infections in the winter months, when levels are at their lowest. The link between low levels and the winter flu wave is well documented (Sabetta et al., 2010; Grant et al., 2007). Sabetta et al. (2010) showed that a vitamin D level above 38 ng/ml offers markedly better protection against infections.

Thresholds and effects

The following overview summarises health states that meta-analyses and systematic reviews have associated with vitamin D level. Superscript numbers point to the study list at the end of the article.

Natural levels

Our reference for natural vitamin D levels comes from studies of indigenous populations in equatorial regions. That research shows that traditional peoples such as the Maasai and Hadzabe in East Africa have average 25(OH)D levels of about 46 ng/ml (115 nmol/l), which can be read as evolutionarily “normal” values. These levels are reached through regular but not excessive sun exposure, which suggests they could serve as natural target values for humans (Luxwolda et al., 2012).

Upper limits

To understand this fully, it helps to know that vitamin D toxicity is extremely rare and would almost have to be produced on purpose. With regular blood tests the risk is practically ruled out. The real concern should be the widespread vitamin D deficiency we face in Europe. That is captured in the title of a 2016 systematic review: “Vitamin D deficiency in Europe. Pandemic?” The authors conclude: “Vitamin D deficiency is widespread across the European population, with worrying prevalence rates that, from a public-health perspective, require action” (Cashman et al., 2016).

Can you overdose on vitamin D?

The most reliable way to rule out toxicity is a blood analysis. What is measured is the storage value 25-OH vitamin D, which is the gold standard.

A Mayo Clinic cohort study analysed more than 20,000 measurements over 10 years (Dudenkov et al., 2015). Only 1% of people had values above 100 ng/ml. The only documented toxicity case occurred at 364 ng/ml, after 50,000 IU daily for several months.

Below 100 ng/ml there were no signs of hypercalcaemia. Slightly elevated values are not a concern as long as they are confirmed by measurement.

The findings confirm that vitamin D toxicity usually occurs only after extremely high, long-lasting doses of supplements (Dudenkov et al., 2015). According to the National Institutes of Health (NIH), vitamin D toxicity typically appears at serum 25(OH) levels well above 150 ng/ml. It mainly shows up as hypercalcaemia, with symptoms such as nausea, vomiting, muscle weakness and, in extreme cases, kidney failure and soft-tissue calcification. Such high values are reached almost exclusively through excessive intake of vitamin D supplements, often at doses far above 10,000 IU a day. Sun exposure alone does not cause toxicity, because surplus vitamin D is converted in the body into inactive forms (NIH, 2020). These data show that regular blood analyses are a safe way to monitor vitamin D level and avoid toxicity.

Frequently asked questions

How much vitamin D should I take each day?

That depends on your current level. The German Nutrition Society recommends 800 IU daily, but with a confirmed deficiency, medically supervised doses of 2,000 to 4,000 IU can make sense. The only way to find the right dose is a blood analysis, then a follow-up measurement after 2 to 3 months.

Can I cover my vitamin D level through food?

Only to a very limited extent. Oily fish such as salmon, herring and mackerel contain relevant amounts, as do egg yolk and mushrooms. Even with an optimal diet you can cover only about 10 to 20 percent of the requirement. Most of it has to come from sunlight or supplementation.

When is the best time for a vitamin D test?

A measurement in late winter (February/March), when the body’s stores are lowest, is the most informative. That is how you see your true low point. A second measurement in late summer shows how well your body builds vitamin D from the sun. The marker is the storage value 25-OH vitamin D in blood.

Is it possible to take too much vitamin D?

Yes, but toxicity is extremely rare and typically appears only at levels above 150 ng/ml. That usually requires doses well above 10,000 IU daily over several months. Sun exposure alone cannot cause an overdose. Regular blood tests practically rule the risk out.

Why is vitamin D not measured as standard at the GP?

Vitamin D is not part of the routine lab panel covered by statutory health insurance. It is only funded as a standard benefit when there is a well-founded suspicion of deficiency. As an out-of-pocket (IGeL) test at the GP it costs about 20 to 30 euros. Deficiency is still widespread in Germany: according to the Robert Koch Institute, around 60 percent of the population have suboptimal values.

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