We usually call vitamin D a “vitamin,” but technically, it is a steroid hormone precursor that is produced in the skin from cholesterol when exposed to sunlight. There are vitamin D receptors (VDR) in almost every cell of the body: immune cells, muscles, brain, adipose tissue, pancreas...

Because of this, vitamin D is:

• Not only for bones
• But also critical for immunity, inflammation, muscle strength, mood and sleep.

And exactly for that reason, even small mistakes can lead to the situation:
“I am taking vitamin D, but nothing is changing.”

In this article, we will especially focus on three main points:

1. Wrong dose
2. Wrong form
3. Wrong timing

And in the end, we will come to this question:

“Why does vitamin D not work on its own, and why do my symptoms continue even when my blood test looks high?”

Our goal is to make you look at the whole system, not just one number.

1. Wrong dose: The “everyone takes 2000 IU” advice is still everywhere, but how accurate is it?

a) Why should the dose definitely be individual?

The classic approach looks like this:

“Let everyone take 1000–2000 IU per day and the problem is solved.”

Science, however, paints a more complex picture.

Both older and newer studies say this:

• In people taking the same dose of vitamin D, the achieved blood level of 25(OH)D can vary widely.
• Overweight and obese individuals may need about 1.5 to 3 times higher doses than normal weight individuals to reach the same target blood level.

The main reasons:

• Vitamin D is a fat-soluble molecule. As fat mass increases, it is more widely “distributed,” and the level measured in blood can stay lower.
• In obesity, chronic inflammation and genetic differences related to VDR can also change the response to vitamin D.

So the correct sentence is:

Two people swallowing the same tablet may experience that the dose is “just right” for one, but too low or too high for the other.

Additionally:

• If inflammation is high (markers like CRP, IL-6, TNF-alpha), vitamin D receptor signalling can be suppressed.
• Polymorphisms in the VDR gene can change the intracellular response even at the same 25(OH)D level.

That is why our starting question should always sound like this:

“What is this person’s weight, body fat percentage, kidney and liver status, gut absorption, medications and genetic background?”

Safe upper limit and the problem of “too much”

There is another extreme:

Using higher than necessary doses of vitamin D for months or years “just to be safe and not deficient.”

International authorities (IOM, NIH and many guidelines) keep the upper safe daily limit for healthy adults at around 4000 IU per day.
In some clinical situations (for example, severe deficiency or malabsorption), temporarily higher doses can be used under medical supervision, but this is entirely a medical decision.

Real-life examples:

In case series and reviews, people who used 10,000 IU and above for months to years due to mislabeled products or dosing errors have been reported with:

• Hypercalcemia (too much calcium in the blood)
• Impaired kidney function
• Nephrocalcinosis (calcium deposition in the kidney)

The risk of toxicity is especially higher in these groups:

• Stage 4–5 chronic kidney disease
• Primary hyperparathyroidism
• Granulomatous diseases such as sarcoidosis or tuberculosis
• Recurrent kidney stone history or previous hypercalcemia

Because hypercalcemia can occur at lower doses in these patients, guidelines recommend that vitamin D replacement in these groups should definitely be done under specialist supervision.

Simple translation:

It is not “the more, the better.”

We need an approach that asks:

“For this person, for this duration, is this dose reasonable?”

b) Loading dose: When does it make sense, and when is it risky?

What we call a “loading dose” means giving a relatively high total dose over a certain period in order to correct a deficiency quickly.

The common point in many national guidelines and a recent review is:

• A total of about 300,000 IU of D3
• Divided over 5–10 weeks (for example, 50,000–60,000 IU per week)
• Then switching to a lower maintenance dose (for example, 1000–2000 IU per day)

This approach is generally used in situations like:

• Marked symptomatic deficiency (signs of osteomalacia, muscle weakness, bone pain)
• When strong osteoporosis treatments (zoledronic acid, denosumab, teriparatide, etc.) are about to be started, and a deficiency needs to be corrected quickly

The 2024 analysis by Tóth and colleagues reported that a total loading dose of 300,000 IU, divided over several weeks, was both effective and safe, and that with a subsequent maintenance dose of 2000 IU per day, most patients reached 25(OH)D levels above 30 ng/mL.

What is the most common mistake?

Turning the loading regimen into:

“A dose that is continued for months or years as if it were maintenance”

and not noticing it.

For whom is a loading dose more dangerous?

Even standard loading schemes can be risky in the following groups and should definitely be arranged by specialists such as endocrinologists, nephrologists or rheumatologists:

• Stage 4 or higher chronic kidney disease, end-stage renal failure
• Primary hyperparathyroidism (calcium is already high)
• Sarcoidosis, tuberculosis and other granulomatous diseases
• Those with a history of hypercalcemia or recurrent kidney stones

Guidelines are recommended in these groups:

• The loading plan should be made by a specialist, and
• Serum calcium should definitely be checked after loading.

Because it behaves like a hormone, the difference between “bolus” and “daily” matters

Physiology in nature looks like this:

• The skin produces small amounts of vitamin D every day when exposed to sunlight.
• So the body is used to a hormone system that works in a “continuous, low dose” pattern.

Some regimens tested in clinical trials are far from this pattern:

• In one trial, where older women were given a single yearly dose of 500,000 IU cholecalciferol, more falls and fractures were seen compared to placebo.
• In trials like ViDA, where high bolus doses such as 60,000–100,000 IU monthly were given, the risk of falls and fractures did not clearly decrease; in some analyses, an increase in fall risk was even reported.

The picture is similar for respiratory infections:

• In studies using daily or weekly low to moderate doses (equivalent of 400–1200 IU per day), early meta-analyses showed a mild protective signal,
• As newer and larger trials were added, it became clear that the effect is much smaller than initially thought, and in the general population, there might be no meaningful overall difference.

In short:

• If there is any benefit, it seems to appear more with physiological, daily or regular weekly dosing,
• Not with very large and infrequent bolus doses.

Simple strategy: Think of it as a hormone and respect its rhythm

A more sensible way to look at it:

• Do not think of vitamin D as a “once-a-year depot injection,”
• Think of it as a hormone that is supposed to work in small amounts every day.

So a practical strategy looks like this:

• If the deficiency is severe, do a time-limited loading under medical supervision
• Then definitely switch to a lower and regular maintenance dose
• If there are high-risk conditions (kidney, parathyroid, granulomatous diseases), get a specialist opinion first
• Avoid unnecessary repeated mega bolus doses

This way, you correct the deficiency and at the same time use a hormone-like molecule in a way that is more aligned with the body’s rhythm.

c) Too high a dose also has a cost

Using higher than necessary doses of vitamin D for a long time can:

• Raise blood calcium excessively (hypercalcemia)
• Increase the risk of kidney stones and kidney damage
• Promote calcium deposition in soft tissues and blood vessels

So the logic you want to follow is:

“Do not set the dose aggressively, set it intelligently. Decide the target range together with your doctor, and monitor the person.”

2. Wrong form: D2, D3, and what should a vegan do?

a) D2 and D3 are not the same thing

There are two main forms of vitamin D:

• D2 (ergocalciferol): plant or fungus-derived
• D3 (cholecalciferol): skin synthesis and animal sources

Classic meta-analyses showed that D3 raises serum 25(OH)D levels more effectively than D2.

A newer and more comprehensive meta-analysis published in 2025 took it a step further:

• In groups supplemented with D2,
• The D3 form 25(OH)D3 in the blood actually decreased,
• And the total 25(OH)D levels remained lower compared to D3.

In other words:

• If you give the same IU dose, you are more likely to reach a higher and more stable 25(OH)D level with D3.

Therefore:

• For the general population, many guidelines and experts prefer the D3 form.
• For vegans, lichen-derived plant-based D3 products are now more common, so they are no longer forced to rely only on D2.

b) Vitamin D should not be thought of as “just a capsule”

Form is not only D2 or D3, but also:

1. Liquid drops
2. Softgels
3. Tablets or hard capsules
4. Sprays, orodispersible (mouth dissolving) forms, etc.

Since vitamin D is a fat-soluble molecule, carriers that contain fat can provide an advantage in terms of absorption.

In the study by Dawson Hughes and colleagues, D3 taken with a fat-containing meal increased serum 25(OH)D significantly more than when taken with a fat-free meal.

New small studies report that fat-based softgels or emulsions may provide higher bioavailability compared with some dry tablets or sprays.

Practical conclusion:

• When choosing a form, do not look only at “how many IU?”
• Fat-based drops or softgel forms may have an advantage in bioavailability, especially when taken with fat.
• But most importantly, choose a form that you can use regularly and consistently.

3. Wrong timing: Swallowing a capsule on an empty stomach and expecting a big effect

a) Taking it with fat increases absorption

In a randomised controlled trial, D3 supplementation:

• Taken with a fat-containing meal
• Raised serum 25(OH)D levels are significantly higher when taken with a fat-free meal.

Pocket-friendly practical tip:

Instead of swallowing vitamin D right after your morning coffee on an empty stomach, it makes more sense to add it to a meal that contains healthy fat, for example:

• A salad with olive oil
• Avocado
• Eggs
• Nuts like walnuts, almonds, hazelnuts

b) Morning or evening?

The half-life of vitamin D is long, so the exact hour, like “8:00 vs 11:00”, is not critical.

But the sleep side of the story matters.

A systematic review and meta-analysis published in 2022 showed that vitamin D supplementation can significantly improve sleep quality scores, although the results were not fully consistent across all dimensions of sleep disturbance.

Possible mechanisms:

• The presence of vitamin D receptors in brain regions that regulate sleep
• Effects on melatonin regulation and inflammation

Practical suggestion:

• If your sleep is already very sensitive and you notice discomfort when you take it at night, shifting vitamin D to a main meal during the day is a safer strategy.
• There is no single answer to whether it “wakes you up or calms you down”; your own body awareness is crucial here.

4. Why does vitamin D not work on its own?

Here, we move a bit towards a “functional medicine” perspective.

Vitamin D is not a solo singer performing alone on the stage. It is more like an orchestra conductor. The instruments it conducts include:

• Magnesium
• Vitamin K2
• Calcium–phosphorus balance
• Vitamin D receptor (VDR) and intracellular response
• Gut absorption, liver and kidney metabolism

a) Without magnesium, vitamin D does not work properly

In a randomised controlled trial, magnesium supplementation significantly altered vitamin D metabolites and played an important role in optimising vitamin D status.

Because:

• Many enzymes involved in vitamin D metabolism are magnesium-dependent.
• If magnesium levels are low, the vitamin D you give cannot be fully utilised.

Simple steps:

Increase magnesium-rich foods in your daily diet:

• Green leafy vegetables (spinach, chard, etc.)
• Nuts (almonds, hazelnuts, cashews)
• Whole grains
• Legumes

If needed, magnesium supplementation can be considered. This mineral not only interacts with vitamin D, but also participates in more than 300 biochemical reactions in the body and plays a key role in the nervous system, muscle function and energy metabolism.

b) Vitamin K2 and the question “Where will the calcium go?”

Vitamin D increases calcium absorption. But will this calcium go to bone or to blood vessels?

Here, vitamin K2 comes in.

A recent meta-analysis showed that vitamin K supplementation can slow the progression of coronary artery calcification (CAC) and improve calcification markers.

Vitamin K2:

• Helps activate certain proteins (such as MGP) that direct calcium into bone.
• In K2 deficiency, a so-called “calcium paradox” can appear:
  • Loss of minerals in bone
  • Increased calcification in blood vessels

Practical steps:

Vitamin K2 sources:

• Natto (fermented soy)
• Some fermented cheeses
• Egg yolk
• Certain meat products

Many well-formulated D3 supplements, especially at moderate to higher doses, are combined with K2.

If you plan to use vitamin D at moderate to high doses for a long time, it is a good idea to discuss your K2 status and vascular health with your doctor or dietitian.

c) Receptor and inflammation layer

Some studies have shown that genetic variations in the vitamin D receptor (VDR) may be associated with obesity, inflammation, insulin resistance and gut barrier function.

The message here:

• It is not just “how much D you take,”
• But also how the cell perceives it, the receptor response and the inflammatory environment that matter.

So:

• Chronic stress
• Poor sleep
• A diet high in sugar and ultra-processed foods
• A lifestyle full of chronic inflammation

cannot be “fixed” solely by vitamin D supplements.

5. Is your vitamin D test high, but your symptoms still continue?

One of the most common sentences in practice:

“My vitamin D test is now normal, even high, but I still feel tired, have muscle pain and low mood.”

There are several possible explanations:

1. The source of your symptoms was not only your vitamin D level.

• Iron deficiency
• B12 or folate deficiencies
• Thyroid dysfunction
• Chronic stress and sleep disorders
• Insulin resistance and blood sugar instability
• Chronic low-grade inflammation
  may all play a role.

Large and recent trials (like VITAL with tens of thousands of participants) show that 2000 IU per day of D3 in the general population:

• Does not significantly reduce cancer or major cardiovascular events,
• Does not dramatically change fracture and fall risk on its own,
• Yet it can still be important for bone health when there is a deficiency.

In other words, vitamin D is one piece of the overall health puzzle, not the single explanatory variable.

2. Intracellular response may be weak.

• Genetic variations in VDR
• Gut permeability and microbiota imbalances
• Chronic inflammation and oxidative stress

can change cellular response even at the same 25(OH)D value.

3. Co-factor deficiencies can lock the picture.
Especially:

• Magnesium
• Vitamin K2
• Low protein intake and broader micronutrient gaps

can affect both muscle function and vitamin D metabolism.

4. Sleep and circadian rhythm disturbances may still be ongoing.

• Vitamin D supplementation can improve sleep quality, but the effect is moderate and not enough to solve all sleep problems on its own.

Bottom line:

Instead of looking only at the 25(OH)D value and saying “all good,” you need to read your vitamin D level together with:

• Other vitamins and mineral status
• Sleep
• Stress
• Gut health
• Metabolic profile

6. A practical action plan: using vitamin D smartly

Now, let us translate the science into daily life.

1) Get tested: know where you are

• If possible, check your baseline 25(OH)D level.
• At first assessment, it is helpful to also look at:
  • Calcium
  • Magnesium
  • Kidney function (creatinine, etc.)
  • PTH, if necessary

2) Individualise the dose

When deciding the dose, consider:

• Your weight and body fat percentage
• Chronic conditions (kidney, liver, parathyroid, etc.)
• Medications you are using
• Diet and sunlight exposure

General guidelines:

For healthy adults, total intake from diet plus supplements is usually targeted around 600–800 IU per day.

For most adults, 4000 IU per day is accepted as an upper safe limit; this should not be exceeded long-term unless your physician clearly recommends otherwise.

If you have a deficiency, the safest approach is to plan with your doctor and or dietitian:

• Target range
• Whether you need a loading phase
• Maintenance dose

3) Choose the right form

• In general, D3 is preferred.
• If you are vegan, choose products containing plant-based D3 from lichen.
• For absorption, fat-containing drops or softgels are practical and have good bioavailability.

4) Take it at the right time and in the right way

• Do not take it on an empty stomach. Take it with a main meal that includes healthy fat.
• If your sleep is very sensitive and you notice discomfort when taking it at night, shifting it to daytime meals can be more comfortable.
• This small adjustment can significantly improve the rise in your serum 25(OH)D levels.

5) Do not forget the cofactors

Add magnesium and vitamin K-rich foods to your daily diet:

• Magnesium: green leafy vegetables, nuts, whole grains, legumes
• K1: green leafy vegetables
• K2: natto, fermented cheeses, egg yolk, certain meats

If you are going to use vitamin D at moderate to high doses for a long time, discussing K2 and vascular health with your clinician is a good idea.

6) Think symptom-focused, do not put all the burden on one capsule

If your vitamin D level has returned to normal, but you still feel:

• Tired
• Aches in muscles and joints
• Mentally foggy

Then the problem is most likely multifactorial.

Here, the functional perspective comes in:

• Your sleep quality
• Stress level
• Gut health (bloating, sensitivity, bowel habits)
• Blood sugar regulation (reactive hypoglycemia, insulin resistance)
• Other micronutrient status (iron, B12, folate, omega-3 etc.)

All need to be evaluated together. Expecting a single capsule to fix all this is not realistic.

Conclusion

In this article, we used up-to-date scientific data, especially on:

Vitamin D dose response and the effect of body weight, daily versus bolus (high single dose) regimens, D2–D3 differences and vegan options, the relationship between magnesium and K2, effects on sleep quality, immunity and musculoskeletal health.

But one important reminder:

• This article is not for making medical diagnoses.
• It does not replace a personal treatment or dosing plan.

For your own vitamin D dose, duration and combinations, you should always consult:

• Your doctor
• And if possible, your dietitian.

In summary: vitamin D is a powerful ally when used intelligently, but it is not a miracle on its own. The goal is not only to “normalise” a lab test, but to improve the entire system together: sleep, stress, nutrition, movement, gut health and micronutrient balance.

References:

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Frequently Asked Questions (FAQ) About Vitamin D

What is vitamin D, and why is it important?

Vitamin D is a fat-soluble vitamin and hormone precursor that plays a crucial role in calcium and phosphorus absorption, bone health, immune system function, and muscle strength. It is produced in the skin through sun exposure and can also be obtained from dietary supplements and certain foods naturally containing vitamin D.

What causes vitamin D deficiency?

Vitamin D deficiency can result from inadequate sun exposure, limited dietary intake, malabsorption disorders, darker skin pigmentation, which reduces vitamin D synthesis, obesity, and certain medical conditions like gastric bypass surgery. Low vitamin D levels can lead to health problems such as nutritional rickets in children and osteomalacia in adults.

How much vitamin D do I need daily?

RDAs are set assuming little sun exposure; individual needs may be higher or lower depending on sun, body weight, and health status. Some individuals may require higher doses to achieve adequate vitamin D levels, especially those with vitamin D insufficiency.

What are the best sources of vitamin D?

Vitamin D can be produced by the skin when exposed to sunlight (UVB rays). Dietary sources include fatty fish, cod liver oil, egg yolks, and fortified foods. Dietary supplements, especially vitamin D3, are effective for maintaining optimal vitamin D status.

How is vitamin D status measured?

Serum vitamin D status is assessed by measuring 25-hydroxyvitamin D [25(OH)D] levels in the blood. Levels below 30 nmol/L (12 ng/mL) indicate vitamin D deficiency, while levels between 30 and 50 nmol/L may suggest insufficiency. Adequate vitamin D levels are generally considered to be above 50 nmol/L (20 ng/mL).

Can vitamin D supplementation improve immune health?

Vitamin D plays an important role in immune system regulation. Adequate vitamin D levels may reduce the risk of autoimmune diseases and support immune health, although more research is needed to confirm specific benefits of supplementation beyond bone health.

Are there risks associated with vitamin D toxicity?

Yes. Excessive intake of vitamin D supplements can lead to vitamin D toxicity, causing hypercalcemia and related symptoms such as nausea, weakness, and kidney damage. The tolerable upper intake level for most adults is 4,000 IU per day, and supplementation above this should be supervised by a healthcare professional.

Does vitamin D supplementation prevent chronic diseases?

While vitamin D is essential for bone health, evidence from randomised controlled trials is inconclusive regarding its role in preventing cardiovascular disease, cancer, or type 2 diabetes. Maintaining adequate vitamin D levels is important, but supplementation alone is not a guaranteed preventive measure.

Who is at increased risk of vitamin D insufficiency?

Groups at higher risk include older adults, people with darker skin, individuals with limited sun exposure, those who have undergone gastric bypass surgery, people with fat malabsorption disorders, and breastfed infants without supplementation.

How can I improve vitamin D absorption?

Vitamin D absorption is enhanced when taken with dietary fat. Choosing vitamin D supplements in fat-based formulations and taking them with meals containing healthy fats can improve absorption and serum vitamin D levels.