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Von der Sonne zum Zellstoffwechsel – der wissenschaftliche Weg des Vitamin D3 im Körper

From the sun to cellular metabolism – the scientific path of vitamin D3 in the body

The “sun vitamin” from a scientific perspective

The term "sunshine vitamin" aptly describes the unique nature of vitamin D3: It is the only "vitamin" that the human body can produce itself – with the help of sunlight. From a biochemical perspective, however, it is not a classic vitamin , but rather a prohormone that is converted into a hormonally active substance in the body.

Interest in vitamin D3 has grown significantly in recent decades. Research teams are investigating its diverse physiological functions – from the regulation of calcium metabolism and cell growth to immunological processes . This has shown that the pathway of vitamin D3 from the skin to the cell is a finely tuned interplay of various organs and regulatory mechanisms.

The aim of this article is to clearly explain this biochemical pathway of vitamin D3 and to show why vitamin K2 plays a crucial role in this system – particularly in controlling calcium distribution in the body.

Synthesis – how the body produces vitamin D3 itself

The role of the sun

The starting point of vitamin D metabolism lies in the epidermis , the outermost layer of the skin. Under the influence of UVB radiation (290–315 nm), the molecule 7-dehydrocholesterol is converted into previtamin D3 , which spontaneously isomerizes into cholecalciferol (vitamin D3) when exposed to heat.

The efficiency of this reaction depends on several factors:

  • Season: In northern latitudes, UVB intensity is too low during the winter months to produce significant amounts of vitamin D3.

  • Skin type: Higher melanin content reduces UVB transmission.

  • Age: With increasing age, the concentration of 7-dehydrocholesterol in the skin decreases.

  • Geographical location and time of day: The steeper the sun's rays hit the earth, the greater the UVB exposure.

The skin only produces vitamin D3 when it is exposed to sufficient UVB radiation – a process that is often limited today by lifestyle and environmental conditions.

The first conversion step in the liver

After formation in the skin, cholecalciferol is transported via the blood to the liver , where it is converted into 25-hydroxyvitamin D (calcidiol) by the enzyme 25-hydroxylase (CYP2R1) .

Calcidiol is the main circulating and storage form of vitamin D and also serves as a diagnostic marker in the blood to determine vitamin D status. This form is still biologically inactive, but has a half-life of several weeks and forms the reservoir for further activation.

Activation in the kidney

The second conversion step takes place in the kidneys . There, the enzyme 1α-hydroxylase (CYP27B1) converts calcidiol into the biologically active form , 1,25-dihydroxyvitamin D (calcitriol) .

Calcitriol acts as a steroid hormone that binds to specific vitamin D receptors (VDRs) in target cells. Its production is precisely regulated:

  • Parathyroid hormone (PTH) stimulates activation when calcium levels are low.

  • High calcium or phosphate levels inhibit the process via negative feedback.

This regulatory system ensures stable calcium homeostasis – a balance that is essential for the function of bones, muscles and nerves.

Cellular effect – how vitamin D3 becomes active in the body

Vitamin D receptors (VDR)

The discovery of vitamin D receptors in virtually all tissues was a turning point in vitamin D research. For a long time, vitamin D3 was considered exclusively a regulator of bone metabolism, but it is now known that VDRs are expressed in over 30 different cell types —including intestinal, muscle, immune, and nerve cells .

When calcitriol binds to the receptor, it forms a complex with the retinoid X receptor (RXR) . Together, they act as transcription factors that activate or inhibit specific genes. In this way, vitamin D3 influences gene expression and thus fundamental cellular functions such as differentiation, division, and apoptosis.

Functions in different body systems

The physiological effect of vitamin D3 can be summarized in three central systems:

  • Bone metabolism: Stimulation of calcium absorption in the intestine and promotion of mineralization via osteoblasts.

  • Muscle: Involvement in calcium-mediated muscle contraction.

  • Immune system: Modulation of innate and adaptive immune responses by influencing T cells and macrophages.

In addition, researchers are investigating how calcitriol affects cardiovascular, endocrine, and neuronal processes . These relationships are complex, but they demonstrate that vitamin D3 is active far beyond traditional bone metabolism.

The role of vitamin K2 in this system

Activation of calcium-binding proteins

While vitamin D3 increases calcium absorption and availability, vitamin K2 is necessary for transporting calcium to the right places. It activates certain proteins that bind calcium and incorporate it into tissues:

  • Osteocalcin – promotes the deposition of calcium into the bone matrix.

  • Matrix Gla protein (MGP) – inhibits calcium deposits in vessel walls and soft tissues.

This activation occurs through carboxylation —an enzymatic process that only functions efficiently when K2 is sufficiently available. Without K2, these proteins remain inactive and cannot fully perform their functions.

Synergy of D3 and K2

From a biochemical point of view, vitamin D3 and K2 form a complementary system :

  • D3 increases calcium absorption and concentration in the blood.

  • K2 ensures the targeted storage of calcium in bones and teeth, while preventing maldistribution in blood vessels.

This calcium balance is the focus of current research. Studies suggest that the combined intake of both vitamins may provide more favorable markers of bone health and vascular elasticity than considering a single vitamin in isolation.

At the same time, open questions remain: What dosage ratios are optimal? How do individual metabolic variants interact? Such topics are the subject of ongoing scientific research.

The balance in the micronutrient system

Research on vitamin D metabolism makes it clear that balance is more important than isolated values . A high D3 level without sufficient K2 availability can be just as unbalanced as a K2 deficiency with insufficient D3 activity.

In addition, other cofactors play a role:

  • Magnesium is required for the enzymatic activation of vitamin D3.

  • Zinc and vitamin A influence binding to receptors.

  • Healthy liver and kidney function are prerequisites for the complete metabolic pathway.

This complexity underlines that micronutrients act in networks – an idea that is increasingly the focus of modern nutritional science.

Conclusion – a harmonious interaction in the body

The pathway of vitamin D3 from the sun through the skin, liver, and kidneys to the cell impressively demonstrates the precise biochemical regulation at work in the human body. Vitamin D3 initiates the absorption and activation of calcium, while vitamin K2 controls the distribution of this mineral—an interplay of trigger and regulator .

This understanding opens up a scientifically sound view of the "sunshine vitamin": not as an isolated active ingredient, but as part of a complex physiological network. Research continues to decipher these connections – with the goal of understanding the subtle biochemical mechanisms that keep our health in balance.

The pathway of vitamin D3 at a glance

  1. UVB radiation hits skin

  2. Formation of cholecalciferol

  3. Conversion in the liver to calcidiol

  4. Activation in the kidney to calcitriol

  5. Cellular effect via VDR

  6. K2 activates transport proteins for calcium

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Two vitamins, one common goal Fat-soluble vitamins play a central role in human metabolism. They are not only involved in structural and regulatory processes, but also influence how the body absorbs, stores, and utilizes nutrients. Vitamin D3 and vitamin K2 , in particular, have been increasingly studied together in recent years—as a functional duo that plays a crucial role in calcium metabolism . Both vitamins act at different levels of the same physiological system. Vitamin D3 promotes calcium absorption, while vitamin K2 ensures that this calcium reaches the right places—especially the bone matrix. This interaction is increasingly being studied as a key mechanism for bone health and vascular stability . The scientific relevance lies in the realization that isolated considerations of individual vitamins may be insufficient. Studies indicate that a balanced balance of D3 and K2 is essential for the precise control of calcium levels—a process that is finely tuned by hormonal and enzymatic control circuits. The role of vitamin D3 in the body From precursor to active hormone Vitamin D3 (cholecalciferol) is not strictly speaking a classic vitamin, but rather a hormone precursor that is activated in several steps. Exposure to UVB radiation causes the skin to first synthesize 7-dehydrocholesterol , which is converted in the liver to 25-hydroxyvitamin D3 (calcidiol) . Finally, the kidneys produce 1,25-dihydroxyvitamin D3 (calcitriol) – the biologically active form. Calcitriol binds to vitamin D receptors (VDRs) in various tissues and regulates the expression of genes that control calcium and phosphate metabolism . A key effect is the increase of calcium absorption in the intestine , which contributes to the maintenance of stable serum calcium levels. Importance for bones, muscles and immune system At the cellular level, vitamin D3 influences the activity of osteoblasts (bone-building cells) and osteoclasts (bone-resorbing cells). This balance is essential for continuous bone remodeling . In addition to its role in the skeletal system, studies have also linked vitamin D3 to muscular function and immunological processes . Researchers are investigating, for example, how calcitriol affects cells of the innate and adaptive immune system. Despite these diverse approaches, research remains cautious, emphasizing mechanisms rather than therapeutic claims. Vitamin K2 – the underestimated regulator Difference between K1 and K2 Vitamin K exists in two main forms: K1 (phylloquinone) and K2 (menaquinone) . While K1 is found primarily in green leafy vegetables and is primarily involved in blood clotting, K2 has more specific effects on calcium metabolism . Vitamin K2 is predominantly produced by bacterial fermentation and is found in foods such as natto (fermented soybeans) , certain cheeses , and animal products . The different menaquinone variants (e.g., MK-4, MK-7) differ in their chain length and half-life—an aspect that is currently being intensively studied. Activation of GLA proteins The actual physiological function of vitamin K2 lies in the activation of so-called GLA proteins (γ-carboxyglutamate-containing proteins). These proteins bind calcium ions and regulate their storage in tissue. Particularly relevant are: Osteocalcin , which integrates calcium into the matrix in the bones Matrix Gla protein (MGP) , which inhibits calcium deposits in vascular walls Researchers suspect that vitamin K2 thus directs the distribution of calcium —away from soft tissues and toward bones and teeth. Observational studies have described a link between higher K2 intake and improved vascular elasticity , although causality has not yet been conclusively proven. The biochemical relationship between D3 and K2 The calcium control circuit in detail Vitamins D3 and K2 act at different but closely linked stages of the calcium cycle: Vitamin D3 increases calcium absorption from the intestine and increases blood levels. Vitamin K2 activates proteins that specifically incorporate this calcium into the bones. In the absence of K2, some of the calcium absorbed through D3 can remain in tissues where it is potentially undesirable. This maldistribution is associated with vascular calcification in experimental models. In the presence of K2, however, the calcium remains functionally bound—an example of biochemical synergy . Study situation on the combination D3 + K2 Several studies and meta-analyses examine the combined administration of vitamin D3 and vitamin K2. Results show that, in some studies, combined supplementation produces more favorable marker values ​​for bone metabolism and vascular health than the administration of either vitamin alone. For example, researchers reported that certain GLA-dependent proteins are only fully activated when sufficient K2 is available—even with optimal D3 levels. However, scientists also emphasize the limitations of previous studies: heterogeneous dosages, small sample sizes, and different study designs make it difficult to draw clear conclusions. Physiological balance instead of oversupply In modern research, the principle of balance is increasingly applied: more is not automatically better. Both D3 and K2 follow non-linear dose-response relationships . Individual differences in genetics, gut microbiome, or lipid metabolism can influence how efficiently vitamins are absorbed, activated, and utilized. Professional associations therefore recommend taking laboratory values ​​and individual metabolic status into account as part of medical care—especially when it comes to fat-soluble vitamins, which can be stored in the body. Diet and lifestyle also play an important role: sun exposure, liver function, intestinal health and dietary diversity influence the availability of both vitamins and thus the physiological balance of calcium metabolism . Conclusion – A scientifically strong duo Vitamins D3 and K2 form a complementary system within calcium metabolism. D3 promotes absorption and mobilization, while K2 controls targeted storage. Together, they support the finely tuned regulation of calcium in the body —a process that is equally relevant for bones, blood vessels, and cells. Research in this area is still in its early stages, but the findings point to a fascinating interplay that could help us better understand physiological mechanisms in the future. Instead of examining individual micronutrients in isolation, scientists are increasingly focusing on the interplay of biochemical networks—a principle that also opens up new perspectives in the case of vitamins D3 and K2. Briefly explained: How vitamin D3 and K2 work together Vitamin D3 → increases calcium absorption in the intestine Vitamin K2 → activates proteins that incorporate calcium into bones Result → Balance between absorption and storage

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Vitamin D deficiency in modern societies – what role D3 and K2 play in prevention

An underestimated health problem of our time Vitamin D deficiency is now considered one of the most common micronutrient deficiencies worldwide. Studies show that, particularly in regions of the Northern Hemisphere—including large parts of Europe— more than 40% of the population have suboptimal vitamin D levels . Low levels are also not uncommon in sunny countries, indicating profound changes in modern lifestyles. Vitamin D is often called the " sunshine vitamin " because the body can synthesize it through UVB radiation. However, urban life, office work, sunscreen, clothing, and increasing time spent indoors severely limit this natural production. At the same time, environmental factors such as air pollution, which absorbs UVB rays, contribute to a reduction in the body's own synthesis. Scientists therefore speak of a societal vitamin D deficiency —a development that is not only geographically induced, but primarily lifestyle-related. Research into the role of vitamins D3 and K2 opens up new perspectives on prevention and metabolic regulation without turning into medical recommendations. The physiological importance of vitamin D3 From sunlight to active hormone Vitamin D3 (cholecalciferol) is formed in the skin when UVB radiation hits the 7-dehydrocholesterol present in skin cells. This creates an inactive precursor that is converted to 25-hydroxyvitamin D (calcidiol) in the liver and finally activated in the kidneys to 1,25-dihydroxyvitamin D (calcitriol) . Calcitriol acts as a steroid-like hormone that exerts its effects in numerous tissues via specific vitamin D receptors (VDRs) . These receptors regulate the expression of genes involved in calcium uptake, cell division, and immune function . There is a direct relationship between sun exposure, season, and geographical location: the farther you live from the equator, the shorter the periods of effective UVB radiation per year. This explains why a pronounced seasonal fluctuation cycle in vitamin D levels is observed, particularly in northern countries. Functions in the human body Vitamin D3 influences several central systems: Calcium and phosphate metabolism: Promotes calcium absorption in the intestine and redeposition into the bone matrix. Muscles: Supports neuromuscular functions by regulating calcium channels in muscle cells. Immune system: Involvement in the activation of immune cells such as T lymphocytes and macrophages. Research describes vitamin D3 as a regulator of numerous cellular processes , not just as a bone vitamin. However, the evidence relates to molecular mechanisms, not to therapeutic effects in the narrower sense. Why modern lifestyle leads to deprivation Environmental and lifestyle factors Perhaps the most important factor is the lack of direct sunlight . 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Genetics: Variants in the vitamin D receptor gene or in metabolic enzymes alter individual needs. This diversity explains why a uniform “ideal value” is difficult to define scientifically and why preventive strategies should be considered individually. Vitamin K2 – the often overlooked player The difference between D3 and K2 Vitamin D3 increases calcium absorption—but without adequate regulation, this calcium cannot always be incorporated where it is needed. This is where vitamin K2 comes in. It activates proteins that bind calcium and transport it specifically into the bone matrix. The most important ones include: Osteocalcin – supports the incorporation of calcium into the bones. Matrix Gla protein (MGP) – inhibits calcium deposits in vessels. In the absence of K2, these proteins cannot fully perform their function, which can lead to dysregulation of calcium homeostasis . 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Vitamin K2 ensures that the absorbed calcium is incorporated into bones and teeth instead of being deposited in soft tissue or blood vessel walls. This interaction ensures a dynamic balance between calcium uptake and storage , which is essential for homeostasis. Findings from studies Combined studies of D3 and K2 show evidence of synergistic effects in several scientific studies: Improved markers of bone mineralization Reduced inactivity of GLA proteins with sufficient K2 availability Trend towards more favorable biomarkers of vascular health At the same time, there is consensus that further long-term and interventional studies are needed to understand the exact mechanisms and clinical relevance. Current research therefore focuses on the molecular interaction of both vitamins and their influence on gene expression and enzyme activity . Prevention through knowledge – not by chance Preventing vitamin D deficiency begins with awareness and education . 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The future of research will show how this knowledge can be translated even more precisely into preventive strategies – always with an eye on individual differences and scientifically proven mechanisms. 5 common causes of vitamin D deficiency Too little sun exposure Working and living indoors Geographical location and season Sun protection and clothing Insufficient activation by the liver or kidney

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