Reviewed by:
Dr. Patricia Núñez de Aysa — Longevity expert physician, specialist in rehabilitation and non-surgical traumatology.
Dr. Joel Cuesta Gascón — Physician expert in sports medicine, rehabilitation, and nutrition.
Last updated: May 2026.

In 30 seconds

• Yes, in Spain, vitamin D supplementation is necessary in many cases. Despite being a Mediterranean country, between 33-40% of adults are deficient, and up to 65% have insufficient levels.
• Vitamin D is not just "for bones": it is practically a hormone with effects on the immune system, muscle, fertility, blood glucose, and brain health.
• The form with the most evidence is vitamin D3 (cholecalciferol), taken daily (not monthly), between 1,000 and 4,000 IU depending on the context.
• The combination D3 + K2 + magnesium is synergistic: D3 absorbs calcium, K2 directs it to the bones, and magnesium activates D3. This combination works best for bones, muscles, and the cardiovascular system.

We live in the country of sunshine... but we're deficient

One of the great paradoxes of public health in Spain: we have sun all year round, yet most adults have insufficient levels of vitamin D. The data:

• 33-40% of Spanish adults have a deficiency (serum 25-hydroxyvitamin D levels below 20 ng/mL).
• Up to 65% of adults have levels below 30 ng/mL, considered insufficient.
• The deficiency is even greater in people over 65, pregnant women, and residents in urban areas with high pollution.

The data is surprising but has an explanation: living in a sunny country does not guarantee sufficient vitamin D. It is one of the most undervalued nutrients in current health.

What is vitamin D (and why it's almost a hormone)

Vitamin D belongs to the group of fat-soluble vitamins: it is stored in fatty tissues, dissolves in fat, and is better absorbed when taken with food that contains some fat.

The body obtains it through two pathways:

• Skin synthesis by exposing the skin to UVB radiation from the sun.
• Dietary intake: fatty fish (salmon, sardines), eggs, liver, fortified dairy products.

But its action in the body is much broader than that of a classic vitamin. In fact, today it is practically considered a hormone, due to the number of tissues where it has receptors and the variety of functions it regulates:

• Bone health: maintains calcium and phosphorus levels in plasma and promotes bone mineralization.
• Immune system: modulates innate and adaptive immune responses.
• Muscle function: regulates muscle contraction and strength.
• Fertility: influences both male and female reproductive health.
• Glycemic control: deficiencies are associated with poorer insulin sensitivity.
• Brain health: low levels have been linked to an increased risk of dementia and depression.

If there's so much sun in Spain, why is there such a deficiency?

This is the most frequent question. And it has several answers:

1. UVB radiation is not the same all year round

In winter, in Spanish latitudes, the UVB radiation that reaches the skin is insufficient to activate the cutaneous synthesis of vitamin D. From October to March, almost none is produced.

2. Our lives are mostly indoors

Office work, covered transport, indoor leisure. UVB radiation does not pass through glass: being next to a sunny window doesn't count. In addition, we dress covered for a good part of the year.

3. Sun protection (fair and necessary)

Sunscreens significantly reduce cutaneous vitamin D synthesis. It is logical to protect oneself, but it also means that effective UVB exposure is reduced.

4. The typical Spanish diet provides little vitamin D

Few foods are rich in vitamin D, and those that have the most (fatty fish, liver, eggs) are not always consumed in sufficient quantities.

5. Groups at higher risk of deficiency

• Older people (lower cutaneous synthesis with age).
• Pregnant and breastfeeding women (higher demand).
• People with dark skin in northern or temperate latitudes.
• People with obesity (vitamin D is sequestered in adipose tissue).
• Patients with intestinal malabsorption (celiac disease, Crohn's, bariatric surgery).
• People on chronic treatment with omeprazole or proton pump inhibitors (they alter absorption).

How to choose a good vitamin D supplement

There are several key criteria for success:

1. Always choose vitamin D3 (cholecalciferol)

There are two main forms:

• Vitamin D3 (cholecalciferol): the one synthesized by the skin and present in animal-derived foods.
• Vitamin D2 (ergocalciferol): of plant or fungal origin.

The evidence is clear: D3 is more effective and more stable than D2 in raising and maintaining serum 25(OH)D levels. Its potency and duration of action are superior, making it the recommended option for correcting deficiencies.

2. Frequency: daily, not monthly

There is a widespread misunderstanding about this in Spain. A good part of the available pharmacological supplements are administered weekly, biweekly, or monthly in very high doses. This is not the most physiological approach.

Vitamin D works best with a daily and constant intake. The idea of "a mega dose per month that is slowly released" does not align with how the body regulates this vitamin. Immune, muscular, and metabolic functions benefit from stable serum levels, not sporadic peaks.

3. Dose: between 1,000 and 4,000 IU/day

The safe and effective dose for healthy adults is between 25-100 µg/day (1,000-4,000 IU). This is above the Recommended Nutrient Intakes (RNIs), which are set at 800 IU/day.

Why higher than recommended? Because:

• RNIs were calculated primarily to prevent rickets, not to achieve optimal levels.
• The percentage of insufficiency in the current population indicates that recommended doses are insufficient.
• Some people with decreased intestinal absorption need higher doses.

If you regularly consume fatty fish and eggs, a dose of 1,000-2,000 IU/day may be sufficient. If your diet is poor in vitamin D or you have risk factors, doses can go up to 4,000 IU/day. In any case, it is advisable to check with a blood test.

4. Take it with food

Being fat-soluble, its absorption improves when taken with food that contains some fat. The time of day is irrelevant: what matters is consistency and long-term use.

The D3 + K2 + magnesium combination: why it makes more sense

One of the most evidence-backed recommendations today is to combine vitamin D3 with vitamin K2 (menaquinone) and magnesium. Each plays a specific and complementary role:

Why vitamin K2

Vitamin D3 increases intestinal calcium absorption. Vitamin K2 directs that calcium to the correct place: to the bone, not to the arteries. It does this by activating proteins such as osteocalcin (which incorporates calcium into the bone) and matrix Gla-protein (which prevents calcium deposition in vascular tissues).

Without sufficient K2, calcium absorbed by the action of D3 can end up depositing where it shouldn't, which has been linked to vascular calcification. With K2, calcium goes to the bone and improves bone mineral density.

Current evidence indicates that the D3 + K2 combination improves bone density and optimizes calcium metabolism with superior effects to D3 alone.

Why magnesium

Magnesium acts as a cofactor in the enzymatic activation of vitamin D. Without sufficient magnesium, the vitamin D you ingest is not adequately converted into its active form. In other words, taking vitamin D without adequate magnesium can limit its real effect.

In addition, magnesium:

• Is a cofactor in the synthesis of vitamin K-dependent proteins.
• Improves muscle function and reduces inflammation.
• Is involved in over 300 bodily reactions.

Combined supplementation of D3 + K2 + magnesium synergistically supports:

• Bone health and mineral density.
• Muscle and neuromuscular function.
• Cardiovascular health (preventing arterial calcification).
• Optimal serum 25(OH)D levels.
• Immune function.

This is exactly the logic behind our Vit D3 + K2 + Magnesium formula: 2,000 IU of vitamin D3, 50 µg of vitamin K2 MK-7, and 400 mg of magnesium bisglycinate, in a single daily capsule. Formulated by doctors.

Quick table: doses according to context

As always, it is advisable to get a blood test to confirm your starting point (serum 25-hydroxyvitamin D) and adjust the dose to your actual context.

When to measure your vitamin D levels?

It is advisable to measure them:

• If you have persistent fatigue without a clear cause.
• If you have diffuse bone or muscle pain.
• If you have had fractures with little traumatic cause.
• If you have mood swings (depression, irritability).
• If you have recurrent infections.
• If you are diagnosed with osteopenia, osteoporosis, or are at risk.
• If you have thyroid pathology, malabsorption, or take interfering medication.
• In perimenopause and menopause (due to associated bone risk).

The desirable minimum value is 30 ng/mL, although many guidelines recommend aiming for 40-60 ng/mL for an optimal health profile.

Contraindications and precautions

Vitamin D3 supplementation at recommended doses is safe. However, caution is advised in:

• Hypercalcemia or calcium metabolism disorders.
• Primary hyperparathyroidism.
• Sarcoidosis and some granulomatous diseases.
• Patients with severe kidney failure.
• Treatment with high-dose thiazides (caution due to risk of hypercalcemia).

If you take chronic medication or have an underlying medical condition, consult your doctor beforehand.

Conclusion

The short answer to the title is: yes, in Spain, most adults need to supplement vitamin D. Not as a fad, but due to epidemiology.

But doing it right means understanding several things:

• Choose vitamin D3, not D2.
• Take it daily, not monthly.
• Adjust the dose between 1,000-4,000 IU depending on the context.
• Combine it with vitamin K2 and magnesium for optimal synergy.
• Consume it with some fat for better absorption.
• Confirm with blood tests before and during follow-up.

If you want an option that combines all of this in a single capsule, discover Vit D3 + K2 + Magnesium, formulated by doctors with 2,000 IU of D3, 50 µg of K2 MK-7, and 400 mg of magnesium bisglycinate.

If you have questions about your specific case, schedule a free medical consultation or write to us on WhatsApp: our doctors will guide you at no cost.

Frequently asked questions about vitamin D

How much vitamin D per day is recommended for adults?

Between 1,000 and 4,000 IU/day depending on the context (diet, sun exposure, risk factors). Current evidence places the optimal dose above classic RNIs (800 IU), especially in countries with a high prevalence of insufficiency like Spain.

Is vitamin D3 or vitamin D2 better?

Vitamin D3 (cholecalciferol). It is more effective and stable than D2 in raising and maintaining serum 25(OH)D levels.

Can I take vitamin D every day without risk?

Yes, at the recommended doses (1,000-4,000 IU/day), it is safe for continuous use in healthy adults. Very high doses require medical supervision.

Why is vitamin K2 recommended with D3?

Because K2 directs the calcium absorbed by D3 to the bone and prevents it from being deposited in the arteries. The combination improves bone and cardiovascular health compared to D3 alone.

And why add magnesium?

Because magnesium enzymatically activates vitamin D. Without sufficient magnesium, the D3 you take is not adequately converted into its active form.

Does vitamin D cause weight gain?

No. It provides no calories and has not been associated with weight gain. In people with obesity, higher doses may be needed because the vitamin is "sequestered" in adipose tissue.

Can I take vitamin D during pregnancy or breastfeeding?

Yes, it is recommended during these stages (demand increases), but the dose and format should be reviewed with your doctor or midwife.

Does vitamin D improve immunity?

Evidence indicates that adequate vitamin D levels favorably modulate the immune response. Deficiency is associated with increased susceptibility to respiratory infections.

Can I combine vitamin D with other supplements?

Yes, it is compatible with most: magnesium, omega-3, calcium, B complexes. The clearest synergy is with K2 and magnesium. If you take anticoagulants (warfarin, acenocoumarol), consult first due to vitamin K interaction.

Bibliographical references

• González-Molero I, Morcillo S, Valdés S, et al. Vitamin D deficiency in Spain: a population-based cohort study. Eur J Clin Nutr. 2011;65(3):321-328.
• García-Dorta A, Medina-Vega L, Villacampa-Jiménez JJ, et al. Baseline Levels of Vitamin D in a Healthy Population from a Region with High Solar Irradiation. Nutrients. 2021;13(5):1647.
• Díaz-Rizzolo DA, Kostov B, Gomis R, Sisó-Almirall A. Paradoxical suboptimal vitamin D levels in a Mediterranean area: a population-based study. Sci Rep. 2022;12(1):19645.
• Sosa Henríquez M, Gómez de Tejada Romero MJ. Cholecalciferol or Calcifediol in the Management of Vitamin D Deficiency. Nutrients. 2020;12(6):1617.
• Van Tienhoven XA, Ruiz de Chávez Gascón J, Cano-Herrera G, et al. Vitamin D in Reproductive Health Disorders. Int J Mol Sci. 2025;26(5):2256.
• Chen Y, Zhi X. Roles of Vitamin D in Reproductive Systems and Assisted Reproductive Technology. Endocrinology. 2020;161(4):bqaa023.
• Navale SS, Mulugeta A, Zhou A, et al. Vitamin D and brain health: an observational and Mendelian randomization study. Am J Clin Nutr. 2022;116(2):531-540.
• Littlejohns TJ, Henley WE, Lang IA, et al. Vitamin D and the risk of dementia and Alzheimer disease. Neurology. 2014;83(10):920-928.
• Cesareo R, Attanasio R, Caputo M, et al. AME/AACE Italian Position Statement: Clinical Management of Vitamin D Deficiency in Adults. Nutrients. 2018;10(5):546.
• Vieth R. Vitamin D supplementation: cholecalciferol, calcifediol, and calcitriol. Eur J Clin Nutr. 2020;74(11):1493-1497.
• Aaseth JO, Finnes TE, Askim M, Alexander J. The Importance of Vitamin K and the Combination of Vitamins K and D for Calcium Metabolism and Bone Health. Nutrients. 2024;16(15):2420.
• Van Ballegooijen AJ, Pilz S, Tomaschitz A, Grübler MR, Verheyen N. The Synergistic Interplay between Vitamins D and K for Bone and Cardiovascular Health. Int J Endocrinol. 2017;2017:7454376.
• Bleizgys A. Zinc, Magnesium and Vitamin K Supplementation in Vitamin D Deficiency. Nutrients. 2024;16(6):834.
• Dai Q, Zhu X, Manson JE, et al. Magnesium status and supplementation influence vitamin D status and metabolism: results from a randomized trial. Am J Clin Nutr. 2018;108(6):1249-1258.
• Capozzi A, Scambia G, Lello S. Calcium, vitamin D, vitamin K2, and magnesium supplementation and skeletal health. Maturitas. 2020;140:55-63.

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