Vitamin D is a best-selling dietary supplement. It is mainly offered in combination with calcium. Both together can strengthen the bone structure and thus offer protection against osteoporosis.

But vitamin D fulfills far more tasks in the body: it is involved in the release of hormones and neurotransmitters and thus in a wide variety of metabolic processes, blood pressure regulation and nerve and muscle function.

This suggests the reverse conclusion that low vitamin D levels could increase the risk of various diseases. And indeed: countless large observational studies have shown a connection, for example, with various cardiovascular diseases, various forms of cancer as well as diabetes and depression.

Recently, the information that people with low vitamin D levels also suffered more severe courses of Covid-19 caused a sensation. So, could vitamin D be an omnipotent patron saint in pill form?

Unfortunately it’s not that easy. “A statistical association does not yet prove a causal connection,” explains Stephan Scharla from the German Society for Endocrinology in an interview with NetDoktor. For this purpose, so-called controlled intervention studies are necessary. Group A receives vitamin D for a longer period of time, group B a placebo. At the end of the study, physicians and participants learn who received what. And then it’s counted.

“The results were almost always disappointing,” reports the doctor. For example, those who had taken vitamin D developed coronary heart disease, heart failure, depression or type 2 diabetes about as often as participants who did not take vitamin D.

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The explanation: people with low vitamin D levels are usually not in top shape overall. For example, those who move little have a higher risk of heart disease and diabetes – and are often out and about in the sun less often.

“Of course, they also have correspondingly low vitamin D levels,” says the resident endocrinologist with a practice in Bad Reichenhall. Sunlight is required as a catalyst so that the vitamin can form in the skin. The body produces 90 percent of the vitamin D it needs itself.

Likewise, old people are usually less in the fresh air. “In addition, their skin is less able to produce vitamin D anyway,” explains the endocrinologist. Overweight people, on the other hand, need more of the fat-soluble sun vitamin overall, since larger amounts are stored in the fatty tissue, so that less is available for the current metabolism.

A low vitamin D level is therefore usually not the cause of health problems, but develops in parallel. Studies on men and women who have genetically low vitamin D levels were also illuminating in this context. They are no more likely to develop cardiovascular disease or diabetes than the rest of the population.

In addition to bone formation, vitamin D also seems to have a strong influence on another important health factor: the body’s immune system. “Vitamin D is undoubtedly a modulator of the immune system. It influences the function of various immune cells, the regulatory T cells, for example, but also cytokine secretion,” explains Scharla. The latter are messenger substances that trigger inflammatory reactions.

This could explain why the vitamin can have a beneficial effect on autoimmune diseases, among other things: it has a positive effect on the course of rheumatoid arthritis and could possibly delay the onset of type 1 diabetes. In the orchestra of various factors, vitamin D probably plays a subordinate role, says the endocrinologist, but: “For people who have a predisposition to autoimmune diseases, it could be worthwhile to compensate for low levels of vitamin D.”

The positive influence of the vitamin on the course of Covid-19 could also be explained by immune regulation. Because in many cases it is not the virus itself that dramatically worsens the course of the disease, but the violent immune reaction. “Vitamin D can downregulate excessive immune reactions again,” says Scharla. However, vitamin D alone will not get the pandemic under control.

However, all of these positive effects only occur if the vitamin D level is actually too low. If you are already sufficiently supplied, an additional gift cannot help you.

Taking dietary supplements without a specific deficiency is not a good idea anyway. “We already know that from vitamin E,” reports the endocrinologist. Years ago, large studies had shown that the consumption of appropriate dietary supplements shortens life instead of lengthening it.

Therefore, a lot does not bring much – in fact, it can even be problematic. “An overdose is counterproductive,” explains the doctor. “The body has its own protective mechanism against overdose for many biological systems – this also applies to the vitamin D system.”

In the event of an overdose, the body automatically counteracts it. It downregulates the active form of vitamin D, calcitriol. “Then the likelihood of broken bones increases.” In the case of a severe overdose, the high calcium levels in the blood can also damage the kidneys, and serious cardiovascular problems occur.

Anyone who actually needs to compensate for a vitamin D deficiency should therefore take vitamin D every day rather than relying on high doses on individual days.

“The daily dose should be 1000, 2000, a maximum of 3000 units, but not more than that,” says Scharla. Exceptions are people whose absorption of the vitamin is disturbed, for example due to chronic inflammatory gastrointestinal diseases.

The goal is not too much and not too little vitamin D. But exactly how high the target value in the blood should be is more difficult to determine than one would think. “The dissent starts with the normal value,” says the doctor. The specialist societies have currently agreed on a value of 50 nmol/l for 25-hydroxyvitamin D – the storage form of vitamin D. Although less is not considered a real deficiency, it is classified as insufficiency.

However, this value reflects the actual individual vitamin supply only insufficiently. Here, too, the human genetic make-up plays a role. For example, it influences the sensitivity of the vitamin D receptors on the body cells. “If they are very sensitive, lower vitamin D levels are sufficient for the supply,” explains Scharla.

Other values ​​provide a more precise picture of the vitamin D balance – for example, the level of the so-called parathyroid hormone, which regulates calcium metabolism, or that of alkaline phosphatase. They show how the metabolic situation actually is. But these values ​​are rarely determined.

And that is rarely necessary: ​​”In general, healthy adults under the age of 60 with normal lifestyle habits rarely have a biologically relevant vitamin D deficiency,” explains the doctor. Even if the level can be lower at the end of winter, this usually has no physical consequences. In spring, the stores of the fat-soluble vitamin fill up again.

The actual risk of developing a vitamin D deficiency therefore only exists in healthy people from a certain age, as well as in younger people who, for whatever reason, are unable to spend enough time in the sunlight.

But people with dark skin and women who cover larger parts of their body for religious reasons also tend to produce less vitamin D. They are also at higher risk of developing a deficiency. Scharla says: “A check of the values ​​can actually make sense for these risk groups.”

The original to this post “Overdose damages bones and kidneys: when vitamin D really makes sense” comes from NetDoktor.