Vitamin D3: The sunshine vitamin for winter

Vitamin D3 & K2

Vitamin D3 & K2

In Germany, around 60 percent of the population was found to be deficient in vitamin D, and one in five even had a severe deficiency, according to the results of studies by the DGE (German Nutrition Society) and the Max Rubner Institute. To counteract this deficiency, vitamin substitution is recommended, particularly during the winter months. However, it is often said that vitamin D3 should only be taken in combination with vitamin K2. What does that mean exactly? This guide deals with the important vitamin D3 and explains its functions, dosage and connection with vitamin K2. The different forms of preparations are also presented and compared.

What is vitamin D3?

Vitamins are chemical compounds that are vital to humans. However, the body cannot produce them itself; they must be ingested through food. The fat-soluble vitamin D3, however, stands out in this context. In this sense, vitamin D3 is not a vitamin at all, but a hormone. The body can produce it in large quantities from cholesterol in the skin and only needs the UVB rays from the sun to do so. In order to produce enough vitamin D, it is sufficient to regularly expose a few parts of the body to the sun for around 15 minutes during the summer months (for example, forearms, hands, face, legs and feet).

Vitamin D3 is present in 30 organs and types of tissue in our body and is involved in many metabolic processes. Vitamin D3 has an effect on the immune system. It can stop inflammatory processes and strengthen the body's defense cells. It also supports the body in absorbing calcium and phosphorus. These are important for bone health. Vitamin D can therefore strengthen bones and promote tooth formation in children. The vitamin also has a positive effect on muscle strength.

Target group: Who should take vitamin D supplements?

In people who eat a healthy, balanced diet and regularly go outside into the fresh air, a vitamin D deficiency can usually be ruled out. However, if there is not enough time for a lot of sun because of work or other reasons, vitamin D supplements should be used. People with darker skin are also more likely to suffer from a deficiency because they cannot produce as much vitamin D as people with lighter skin. Nevertheless, you should also be careful if you have lighter skin: sun protection prevents UVB rays from reaching the skin, which means that the body cannot absorb them for vitamin D production. However, since protection against skin cancer with sunscreen is also very important, it can also be substituted here if necessary. In addition, for seniors over 65 or people who are already ill, taking vitamin D can support bone health and strengthen the immune system.

Vitamin D deficiency:

A D deficiency usually occurs when the affected person is not exposed to enough sun. Typical symptoms are varied:

  • Hair loss
  • Headache
  • Muscle weakness, pain in limbs
  • Muscle cramps
  • Disturbed bone mineralization

Vitamin D levels

Serum 25-hydroxyvitamin D (ng/ml)

Vitamin D status

>10 Severe vitamin deficiency
>10-20 Vitamin D deficiency
>21-29 mild vitamin D deficiency
>30 sufficient vitamin D
<150 Vitamin D toxicity

A long-term vitamin D deficiency can have serious consequences. In addition to the symptoms mentioned above, there are also some serious illnesses that can be caused by a lack of vitamin D:

  • Depressive moods (especially winter depression)

  • Cardiovascular diseases

  • Respiratory diseases

  • Autoimmune diseases

Especially in older people, a vitamin D deficiency can increase the risk of bone fractures, loss of strength or balance disorders.

Winter also poses a high risk of low vitamin D levels. This can sometimes lead to a type of “winter depression.” Those affected often feel depressed, tired and listless, even if life is generally going well.

Vitamin K2

In general, vitamin K is responsible for blood clotting. But it also has many other positive effects on our health. In nature, vitamin K can be found in two forms: vitamin K1 and vitamin K2. However, K2 can be absorbed much better by our bodies. It is found naturally in fermented foods or certain dairy products, and is also produced by microorganisms such as the E. coli bacteria that live in our intestines. The vitamin is absorbed into the blood via the intestines and transported from there to the liver. K2 works throughout the body and supports the activation of important proteins. It is also important that vitamin K2 can be divided into different forms. MK7 (also menaquinone-7) is considered the most effective form of vitamin K2. It has the highest bioavailability, is more fat-soluble, circulates in the blood the longest and thus has the best effect.

Why should I only take vitamin D3 in conjunction with vitamin K2?

Both vitamins D3 and K2 play a fundamental role in the human body. Their interaction is particularly important. Vitamin D3 promotes calcium absorption in the body, which can lead to an increased calcium content in the blood. To prevent health risks, vitamins D3 and K2 should be substituted together. K2 activates the proteins osteocalcin and MGP, which are responsible for calcium utilization in the blood. This prevents calcium from being deposited in the blood vessels. In addition, a vitamin D3 deficiency usually automatically leads to a severe vitamin K2 deficiency, which is another reason for a combination preparation.

The different forms of vitamin D3 – preparations with advantages and disadvantages

If you have a vitamin D3 deficiency, it is a good idea to cover it by exposing your skin to sunlight and eating food. However, this is not always possible, for example if you have a long day at work in the office. In these cases, it is a good idea to take the vitamin in supplements. The advantages of this are that it is quick and convenient to take, the dosage is usually high and you can also take it in winter. There are several ways to take vitamin D supplements: in tablet form, in capsules or as drops.


Advantages: The preparations in tablet form are certainly the best known form of intake. If you have difficulty swallowing, they can easily be dissolved in water and taken diluted. The tablets are also usually quite affordable.

Disadvantages: However, tablets also have some disadvantages. Unfortunately, they contain many additives or colorings and flavorings that should not be part of a vitamin preparation. These additives can also limit the absorption of the vitamin.


Advantages: Vitamin D3 capsules are rubbery soft capsules containing the vitamin. This is usually already dissolved in oil. In addition, there are significantly fewer additives in the capsules compared to tablets.

Disadvantages: The problem is the gelatine used for the shell, which makes the product unsuitable for vegetarians and vegans. In addition, people who have difficulty swallowing should not use capsules, as they are usually quite large and do not dissolve in water.


Advantages: The preparations in the form of drops are characterized by the fact that they do not contain any avoidable additives; the vitamin is dissolved directly in a natural oil. This makes them easy to take and the amount of drops can be individually dosed. In addition, studies have shown that the effect of the drops is the greatest compared to the other preparations: with the same dose, the drops show the highest increase in vitamin D.

Disadvantages: The only disadvantage is that the drops have a relatively limited shelf life. Therefore, when buying them, you should make sure that you choose a pack size that suits your needs. Vitamin D drops are often a little more expensive, but the investment is worthwhile due to the advantages mentioned above compared to capsules and tablets.

FürstenMED's vitamin D3/K2 is also available in the form of drops and is made exclusively with natural coconut oil. It contains no colorings, fillers or flavor enhancers. It is also naturally gluten- and lactose-free and is produced without genetic engineering. The K2 MK7 vitamin it contains is also a high-quality branded raw material from the Kappa Bioscience brand.


It is almost impossible to overdose naturally through strong sunlight or above-average consumption of foods rich in vitamin D. This is different when taking supplements in excess. The result of this can be kidney stones forming or kidney calcification. You can also consult a doctor to see whether a supplement is advisable for you.

If vitamin D intake and production are low, the amounts indicated below should be substituted:


Quantity in IU

Babies 400 IU
Children from 1 year of age 800 IU
Teenagers and adults 800 IU
Pregnant and breastfeeding women 800 – 1000 IU

German Nutrition Society. Vitamin D (calciferols). 2020.


Vitamins D3 and K2 have a variety of important functions in the human body and also work together to support each other.

Vitamin D3

Vitamin K2

Is produced through the skin Known as “menaquinone”
For the normal and proper functioning of the immune system Important for the utilization of calcium in the body
Strengthening bones and muscles Supports the activation of various proteins that are used in different processes in the body

Since more than half of the people in Germany suffer from a vitamin D deficiency and absorption through the sun is difficult, especially in the winter months, it is advisable to take the vitamin as a supplement. It is available in the following forms:

  • tablet

  • capsule

  • drops

When comparing them, it turned out that the advantages of the drops outweigh the disadvantages, which is why we recommend taking vitamins in drop form if you are willing to invest a little more money and value quality.

Make sure that you take a vitamin D3 supplement in combination with K2, as this will prevent possible side effects and ensure that both vitamins have an optimal effect.

Important: This guide is intended as a source of information only. Always consult your doctor if you have acute problems. Under no circumstances should you change the dosage of your medication or stop taking it yourself.


  1. Matthias Wacker, Michael F. Holick (2013) Sunlight and Vitamin D3

  2. Rathish Nair, Arun Maseeh (2012) The properties of vitamin D3

  3. Leon J Schurgers 1, Kirsten JF Teunissen, Karly Hamulyák, Marjo HJ Knapen, Hogne Vik, Cees Vermeer (2007) Vitamin K-containing food supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7.

  4. Maresz, Katarzyna, Proper Calcium Use: Vitamin K2 as a Promoter of Bone and Cardiovascular Health, 2015, Integrative Medicine: A Clinician's Journal

  5. van Ballegooijen, Adriana J., The Synergistic Interplay between Vitamins D and K for Bone and Cardiovascular Health: A Narrative Review, 2017, International Journal of Endocrinology

  6. German Nutrition Society. Vitamin D (calciferols). 2020.

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