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The vitamin D basics

Das Vitamin D Einmaleins

Vitamin D is completely different from most other vitamins. Strictly speaking, this vitamin is a steroid hormone that is produced from cholesterol when your skin is exposed to the sun's rays. For this reason, vitamin D is sometimes referred to as the "sunshine vitamin".

However, sunshine alone rarely ensures an adequate supply of vitamin D, making it necessary to obtain this vitamin through food or supplements. Unfortunately, only a handful of foods contain significant amounts of this vitamin, which is why vitamin D deficiency is very common (1, 2, 3). In the USA, for example, a study found that around 41.6% of the population suffers from a vitamin D deficiency (4).

This article will tell you everything you need to know about vitamin D.

What is vitamin D?

Vitamin D is a fat-soluble vitamin, which means that it dissolves in fats and oils and can be stored in the body for a long period of time.

There are two forms of vitamin D found in food:

  • Vitamin D3 (cholecalciferol): this form of vitamin D is found in some animal foods such as oily fish and egg yolks.
  • Vitamin D2 (ergocalciferol): This form is found in some plants, fungi and yeasts.

Of these two forms, D3 (cholecalciferol) appears to be almost twice as effective as vitamin D2 (ergocalciferol) in increasing vitamin D levels in the blood (6, 7).

Summary: Vitamin D is a fat-soluble vitamin that the body can store over a long period of time. Of the two main forms - D2 and D3 - the latter is more effective when it comes to increasing vitamin D levels in the blood.

What does vitamin D do in the body?

Vitamin D has to go through two conversion steps before it becomes active (8, 9). First, it is converted to calcidiol, or 25(OH)D, in the liver. This is the storage form of vitamin D.

Next, calcidiol is converted to calcitriol, or 1,25(OH)2D, mainly in the kidneys. This is the active steroid hormone form of vitamin D. Calcitriol interacts with the vitamin D receptor, which is found in every single cell in the body (10, 11).

When the active form of vitamin D binds to this receptor, it switches genes on or off, leading to changes in the cells. This is similar to the way most other steroid hormones work (12, 13).

Vitamin D affects various cells associated with bone health. For example, it promotes the absorption of calcium and phosphorus from the digestive tract (14). Scientists have recently discovered that vitamin D also plays important roles in other areas of health, such as immune function and protection against cancer (15).

Summary: Vitamin D is first converted in the body into calcidiol - the storage form of vitamin D - which is then converted into the active steroid form calcitriol. Calcitriol binds to the vitamin D receptor within the cell and thereby activates or deactivates certain genes.

Sun exposure is an effective way to obtain vitamin D

Vitamin D can be synthesized in the skin from cholesterol when the skin is exposed to ultraviolet radiation (UVB) from the sun (16). People who live in regions with plenty of sunshine can get all the vitamin D their body needs by sunbathing a few times a week.

You should keep in mind that you need to expose a larger part of your body to the sun. If only your face and hands are exposed to the sun, then your body will not produce much vitamin D. Sunscreen and other sun protection products will also severely limit or completely prevent vitamin D production (17). Nevertheless, you should of course use sun protection if you are going to be in the sun for a longer period of time. Sunshine is healthy, but sunburn can lead to premature skin ageing and increase the risk of skin cancer (18, 19).

If you are going to be in the sun for a long time, depending on your skin's sensitivity, you may consider spending the first 10 to 30 minutes in the sun without sunscreen and then applying sunscreen before you get sunburned.

As vitamin D can be stored in the body for weeks or months, you only need occasional sunbathing to keep your blood vitamin D levels in an adequate range.

However, if you live in an area without adequate sun exposure, then vitamin D intake through food or supplements is essential, especially during the winter.

Summary: Sun exposure is an effective way to get vitamin D, but sunscreen products can block vitamin D production. Although sunbathing can help maintain adequate vitamin D levels, many people don't have enough opportunity to get enough sun for most of the year.

The best food sources of vitamin D

Here is the vitamin D content of some of the best sources of vitamin D3 (20):



% of daily requirement

Cod liver oil, 1 tablespoon (15 ml)

1,360 IU / 34 mcg


85 grams of cooked salmon

447 IU / 11 mcg


85 grams of canned tuna

154 IU / 4 mcg


85 grams of beef liver

42 IU / 1 mcg


1 large whole egg (vitamin D is in the egg yolk)

41 IU / 1 mcg


1 sardine, in oil from the can, drained

23 IU / 0.6 mcg


Although fatty fish such as salmon, mackerel, swordfish, tuna and sardines are viable sources, you would need to eat these almost every day to get adequate amounts of vitamin D. The only excellent food source of vitamin D is fish liver oil such as cod liver oil, which contains up to twice the recommended daily intake in one tablespoon (15 ml).

In addition, some dairy products and breakfast cereals are fortified with vitamin D (21). Some rare mushrooms also contain vitamin D and egg yolk also contains small amounts.

Summary: Cod liver oil is by far the best source of vitamin D. Oily fish is also a good source, but you would need to eat it regularly to get enough vitamin D.

Symptoms of vitamin D deficiency

Vitamin D deficiency is one of the most common nutritional deficiencies. Some people have a higher risk of this than others. In the USA, for example, 41.6% of the population suffers from vitamin D deficiency, with differences between groups of different ethnic backgrounds (4). The figures are similar in Europe and Germany.

Older adults are also at a higher risk of developing vitamin D deficiency (22). In addition, certain diseases can also increase the risk of vitamin D deficiency. For example, one study showed that 96% of people who had suffered heart attacks had low vitamin D levels (23).

All in all, vitamin D deficiency has the quality of a silent epidemic. The symptoms are subtle and can take years or even decades to become apparent, The best known symptom of vitamin D deficiency is rickets - a disease of the bones that is common in children in developing countries. This disease has been almost completely eliminated in the Western world by fortifying foods with vitamin D (24).

Vitamin D deficiency is also associated with osteoporosis, reduced bone mineral density and an increased risk of falls and fractures in older people (25).

In addition, studies show that people with low vitamin D levels have a higher risk of heart disease, type 1 and 2 diabetes, cancer, dementia and autoimmune diseases such as multiple sclerosis (26). Last but not least, vitamin D deficiency is also associated with lower life expectancy (27, 28, 29).

However, it is still unclear whether deficiency contributes directly to these diseases, or whether people with lower vitamin D levels are simply more likely to develop these diseases.

Summary: Vitamin D deficiency is associated with a number of health problems, as well as lower life expectancy.

Potential health benefits

Here are some potential benefits of vitamin D:

  • Reduced risk of osteoporosis, falls and fractures: higher doses of vitamin D may help reduce the risk of osteoporosis, falls and fractures in older adults (30).
  • Increased strength: Vitamin D can increase strength in the limbs of the upper and lower body (31).
  • Cancer prevention: Vitamin D could help prevent cancer. One study found that 1,100 IU of vitamin D per day - together with calcium - can reduce the risk of cancer by 60% (32, 33).
  • Relief from depression: Studies show that vitamin D can reduce symptoms of depression in people suffering from clinical depression (34).
  • Reduced risk of type 1 diabetes: A study conducted with young children linked a daily intake of 2,000 IU of vitamin D to a 78% reduced risk of type 1 diabetes (35).
  • Reduced risk of death: Some studies suggest that vitamin D may reduce the risk of subjects dying during the study period, suggesting that vitamin D may help us live longer (36, 37).

However, many of these results are preliminary. According to a recent study review, further research is needed to confirm many of these potential benefits (38).

Summary: Scientific research suggests that vitamin D may have numerous benefits in terms of cancer prevention, bone health, mental health and autoimmune disease. However, further studies are needed.

How much vitamin D should you take?

The only way to find out if you have a vitamin D deficiency - and therefore should supplement with vitamin D - is to have your blood tested by a doctor. This test measures the amount of the storage form of vitamin D known as calcifediol. Anything below 12ng/ml is considered a vitamin D deficiency and anything above 20 ng/ml is considered an adequate amount of vitamin D.

The recommended daily intake for vitamin D is as follows (39):

  • 400 IU (10 mcg): Infants, 0-12 months
  • 600 IU (15 mcg): Children and adults, 1-70 years
  • 800 IU (20 mcg): older adults and pregnant or breastfeeding women

Although a vitamin D level of 20 ng/ml is considered adequate, many health experts believe that blood levels above 30 ng/ml should be targeted for optimal health and disease prevention (40).

In addition, many believe that the recommended daily dose is far too low and that much higher amounts are needed to achieve optimal blood levels (41). According to the U.S. National Academy of Medicine, the safe upper limit is 4,000 IU (100 mcg) per day (42).

Vitamin D3 supplements appear to be more effective than vitamin D2 supplements when it comes to increasing vitamin D levels.

Optimizing the remaining nutrients

It is important to keep in mind that nutrients do not usually work in isolation. Many of them are interdependent and increasing the intake of one nutrient may increase the need for another.

Some scientists believe that fat-soluble vitamins work together and that it is crucial to optimize your intake of vitamins A and K while supplementing with vitamin D3 (43, 44). This is especially important with vitamin K2 - another fat-soluble vitamin that most people don't get enough of (45).

Magnesium - another important mineral lacking in the modern diet - may also be important for vitamin D function (46, 47).

Summary: Scientific data suggests that vitamin D interacts with vitamin A, vitamin K and magnesium to promote health.

What are the consequences of vitamin D overdose?

It is a myth that it is easy to overdose on vitamin D. Vitamin D intoxication is very rare and only occurs when very high doses are taken over a long period of time (48).

The primary symptoms of such poisoning include confusion, poor concentration, depression, vomiting, abdominal pain, constipation and high blood pressure (49).


Vitamin D is a fat-soluble vitamin that is important for bone health. If you are deficient in this nutrient, increasing your intake can reduce depression and increase strength.

The skin produces vitamin D when exposed to sunlight. Foods such as oily fish, fish oil and liver also contain vitamin D. The same applies to vitamin D-enriched foods and supplements.

Vitamin D deficiency is common due to a lack of sun exposure of the skin and a poor choice of rich food sources of vitamin D. If you don't spend much time in the sun and rarely eat oily fish, you should consider supplementation.

Getting enough vitamin D is essential for good health.




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