Vitamin D: Why There Is No Universal Effective Dose

Molecular model with a glowing accent on chemical bonds, illustrating the precision of polygenic risk calculation and DNA analysis in the Apixmed Prism lab.

Two people both discover they have a vitamin D deficiency and start taking the same dose of supplements — say, 1000 IU daily. After a few months, one of them gets tested and sees their 25(OH)D level has risen to normal. The other gets almost the same result as before. Is it a difference in diet? Not necessarily. Lifestyle? Similar. So why does the same dose of vitamin D work differently?

The answer isn't always obvious. How the body obtains, transports, and uses vitamin D is influenced by several layers of factors — from sun exposure to individual genetic characteristics.

Why Vitamin D Levels Can Differ Even with a Similar Lifestyle

Vitamin D enters the body in two ways: it is synthesized in the skin under the influence of UV light, and it comes from food or supplements. In practice, blood vitamin D concentration is influenced simultaneously by several factors (Hyppönen et al., Nutrients, 2022):

Amount and quality of sun exposure — latitude, season, time of day, skin type.
Body weight — adipose tissue stores vitamin D, reducing its availability in the blood.
Age — as skin ages, it synthesizes vitamin D less efficiently.
Gastrointestinal status — certain conditions reduce absorption of fat-soluble vitamins.
Diet and supplement-taking habits.

All of these factors are well known. But even accounting for each of them, variability in response to the same dose remains. And this is where genetics enters the picture.

For more on the features of vitamin absorption — see the article "Why Vitamin Deficiency Isn't Always Solved by Supplements".

How the Body Obtains, Converts, and Uses Vitamin D — and Where Individual Differences Arise

The path vitamin D takes in the body is not a single step, but a series of consecutive transformations. And at each one, there is room for individual variation.

First, vitamin D (in the form of D3 or D2) enters via the skin or food. In the liver, it is converted into 25-hydroxyvitamin D — this is the value measured in a blood test. Then, in the kidneys, a second activation step occurs: 25(OH)D is converted into 1,25-dihydroxyvitamin D — the active form, which directly affects cells (Saponaro et al., Int. J. Mol. Sci., 2020).

Before the active form of vitamin D reaches tissues, it is transported by a carrier protein — vitamin D-binding protein (VDBP), encoded by the GC gene. This protein determines how much vitamin D circulates in the blood in bound and free forms. Different variants of the GC gene influence VDBP concentration and, correspondingly, blood 25(OH)D levels.

Finally, the active form of vitamin D interacts with the vitamin D receptor (VDR) — a protein present in many of the body's tissues. This interaction triggers the corresponding cellular response. Variants of the VDR gene can influence the characteristics of this interaction.

Vitamin D thus passes through four key points:

intake and synthesis
conversion in the liver
transport by VDBP
interaction with the VDR receptor in tissues

At each of these stages, genetic characteristics can alter the final result.

3D model of skin tissue, liver, kidneys, and a cell connected by a glowing light path, illustrating how genetic predisposition and lifestyle affect various body systems.

Which Genetic Features May Influence Vitamin D Levels and Action

Large-scale GWAS studies have identified several loci consistently associated with blood 25(OH)D levels. Among them are variants of the genes GC (group-specific component), CYP2R1 (cytochrome P450 family 2 subfamily R member 1), and CYP24A1 (cytochrome P450 family 24 subfamily A member 1). For the context of transport and receptor response, the most thoroughly studied are GC and VDR (vitamin D receptor) (Hyppönen et al., Nutrients, 2022).

GC and Vitamin D Transport

The GC gene encodes the vitamin D-binding protein (VDBP), which carries 25(OH)D and 1,25(OH)₂D in the blood. Studies note that certain variants of this gene are associated with reduced serum VDBP concentration and lower 25(OH)D levels. This means: even with adequate vitamin D intake and normal sun exposure, the measured blood level may be lower if the transport protein is produced in smaller amounts or with different functional characteristics. So the 25(OH)D blood level reflects the current state but does not fully explain its causes.

VDR and Tissue Response

The vitamin D receptor, encoded by the VDR gene, is present in cells of bone, muscle, the immune system, and many other tissues. When the active form of vitamin D binds to VDR, it triggers a chain of cellular reactions.

A 2022 meta-analysis described associations between VDR gene polymorphisms and variability in vitamin D-related measures. Carriers of certain variants showed differences in the rise of 25(OH)D levels at the same dosage compared to other groups (Usategui-Martín et al., Nutrients, 2022).

This doesn't mean a particular VDR variant automatically leads to any specific condition. Genetic variants can influence the characteristics of tissue response to vitamin D, and the blood 25(OH)D level doesn't always reflect all individual characteristics of its use by the body.

The Role of Genetics Among Other Factors

It's important not to overestimate the role of genetics in isolation from other factors. Sun exposure, diet, physical activity, body weight, and gastrointestinal status all continue to act regardless of which gene variants are present in the genome.

Genetics doesn't cancel out the influence of lifestyle. It explains why, with the same behavior and the same external conditions, the result can differ.

Therefore, an approach to maintaining vitamin D levels always consists of several parts:

Regular sun exposure appropriate to the season and latitude.
A balanced diet with adequate fat-soluble vitamin content.
If needed — supplements at a dosage agreed upon with a doctor.
Monitoring 25(OH)D levels via blood test.

Genetic information complements this list — it helps better understand why a standard approach may need individual adjustment.

Genetics and Blood Tests: How They Complement Each Other

A blood test for 25(OH)D answers one question: what is the current vitamin D level. This is important and necessary information — without it, it's hard to assess the current state and track dynamics.

But a blood test doesn't explain why that level is what it is. Why it rises poorly despite regular supplement intake. Why in some people it's stable, while in others it fluctuates seasonally even with the same lifestyle.

Genetic information is a different level of data. It shows individual characteristics that remain stable regardless of season or current habits. GC gene variants provide context regarding vitamin D transport in the blood. VDR gene variants — regarding the characteristics of the tissue response to it.

Together, the blood test and genetic data provide a fuller picture. The test shows where you stand now. Genetics helps understand why exactly there — and what to consider in the long term.

The Genetic Report and Blood Test Results: How They Work Together

The question "why isn't 1000 IU of vitamin D enough for me?" doesn't arise from nowhere. Each person's body has its own route for vitamin D — from synthesis in the skin to interaction with receptors in tissues. External factors set the conditions. Genetic characteristics determine how the body responds under those conditions.

A blood test remains the basic monitoring tool: it shows the current 25(OH)D level. Genetic information is a complement that explains the individual characteristics of maintaining that level. This makes it possible to assess not only the current vitamin D level, but also to interpret this indicator in the context of the body's individual characteristics.

Check your individual vitamin D absorption characteristics with the Apixmed Prism DNA test.

Genetic test results are not a diagnosis and do not replace a physician's consultation. The Apixmed Prism report provides genetic context that complements examination results and supports decision-making together with a doctor.

References

Hyppönen, E., Vimaleswaran, K. S., Zhou, A. (2022). Genetic Determinants of 25-Hydroxyvitamin D Concentrations and Their Relevance to Public Health. Nutrients, 14(20), 4408. https://doi.org/10.3390/nu14204408
Usategui-Martín, R., De Luis-Román, D.-A., Fernández-Gómez, J. M., Ruiz-Mambrilla, M., Pérez-Castrillón, J.-L. (2022). Vitamin D Receptor (VDR) Gene Polymorphisms Modify the Response to Vitamin D Supplementation: A Systematic Review and Meta-Analysis. Nutrients, 14(2), 360. https://doi.org/10.3390/nu14020360
Asghari, G., Yuzbashian, E., Najd-Hassan-Bonab, L., Mirmiran, P., Daneshpour, M. S., Azizi, F. (2023). Association of rs2282679 polymorphism in vitamin D binding protein gene (GC) with the risk of vitamin D deficiency in an Iranian population: season-specific vitamin D status. BMC Endocr. Disord., 23(1), 220. https://doi.org/10.1186/s12902-023-01463-7