by Laurent Glatz – for Athletic Carnivore
Vitamin D is called a vitamin, but it acts like a hormone. It influences immunity, bone density, muscle function, insulin sensitivity, testosterone production, and inflammation regulation. Yet, between the ingested form, the circulating form, and the active form, several critical steps determine its true effectiveness.
Vitamin D is not just a nutritional intake. It is a metabolic chain.
D2 or D3: A Fundamental Difference
There are two main forms.
Vitamin D2, or ergocalciferol, primarily comes from plant or fungal sources.
Vitamin D3, or cholecalciferol, is produced by the skin under UVB exposure and is found in animal products, especially fatty fish, eggs, and certain animal fats.
D3 is biologically more effective. It raises blood 25(OH)D levels more steadily and sustainably than D2. At equivalent doses, D3 produces a greater increase and has a longer half-life.
The blood marker used to assess status is 25-hydroxyvitamin D. Values generally considered sufficient are above 30 ng/mL, with many data suggesting optimal function between 40 and 60 ng/mL.
But the number only tells part of the story.
Absorption: Dependent on Lipids
Vitamin D is fat-soluble. Its intestinal absorption depends on the presence of dietary fats and proper bile function.
It is incorporated into lipid micelles, absorbed in the small intestine, then transported via chylomicrons.
With a fat-rich meal, absorption can exceed 70 to 80%. On an empty stomach or in cases of fat malabsorption, it drops significantly.
This means supplementation taken without fats can be partially ineffective.
Storage: A False Sense of Security
Once absorbed, vitamin D is mainly stored in adipose tissue and secondarily in muscle.
This is where obesity becomes a determining factor.
In individuals with high fat mass, a larger proportion of vitamin D is sequestered in adipose tissue. The result: circulating blood concentrations are lower for the same intake.
Several studies show that at equivalent doses, obese subjects have lower plasma levels and often require higher intakes to reach the same concentrations.
This is not an absorption deficiency. It is a volumetric dilution and tissue storage phenomenon.
Activation: The Central Role of Magnesium
Vitamin D ingested or synthesized by the skin is biologically inactive.
It must undergo two transformations.
The first occurs in the liver, where it is converted into 25-hydroxyvitamin D.
The second mainly takes place in the kidney, where it becomes 1,25-dihydroxyvitamin D, the active hormonal form.
Both steps depend on magnesium.
Magnesium acts as an enzymatic cofactor in the successive hydroxylations. A magnesium deficiency can therefore limit vitamin D activation, even if intake or sun exposure is sufficient.
In other words, a correct blood level of 25(OH)D does not guarantee optimal activation if magnesium status is deficient.
In Western populations, magnesium intake is often below recommendations, usually around 200 to 300 mg per day, while physiological needs range between 350 and 420 mg depending on sex and body mass.
Hormonal and Metabolic Interactions
Vitamin D modulates the expression of over 1,000 genes. It influences insulin sensitivity, inflammatory cytokine production, and muscle function.
Chronic deficiency is associated with decreased muscle strength, increased fracture risk, impaired immunity, and possible hormonal disruption.
But excess is not harmless.
Prolonged massive intakes can cause hypercalcemia, tissue calcifications, and mineral imbalances, especially if vitamin K2 intake is insufficient.
The Functional Reality
Taking vitamin D is not enough.
Storing it guarantees nothing.
Measuring it does not tell the whole story.
Its effectiveness depends on:
- lipid intake during absorption,
- magnesium status for activation,
- fat mass for distribution,
- overall mineral balance, notably calcium and vitamin K2.
In a diet rich in fatty animal products and with adequate sun exposure, dietary and cutaneous D3 can cover a significant portion of needs.
But in contexts of sedentary lifestyle, low sun exposure, obesity, and magnesium deficiency, the system can jam at multiple levels.
The question is therefore not only: are you taking enough vitamin D?
But rather: can your body absorb, release, and activate it effectively?
Because as often in physiology, ingestion is only the first step of a far more demanding process.
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