Vitamin D deficiency: symptoms, causes, and solutions
Vitamin D, often called the "sunlight vitamin", is incredibly important for your health. And not just for its contribution to strong bones. Its role is much broader than that. What does this essential vitamin do for you? And how can you make sure you get your daily dose of vitamin D?
Why is vitamin D so important?
Vitamin D occupies a unique position in the world of vitamins. What's remarkable is that your body can produce it itself under the influence of sunlight.
By the way, we call it a "vitamin", but vitamin D is actually a fat-soluble prohormone . It's a precursor to hormones that influence all sorts of important bodily functions.
So much for the theory – the important thing, of course, is what vitamin D does for you. And that's quite a lot.
Vitamin D for strong bones
The best-known function of vitamin D? It promotes the absorption of calcium and phosphate from the intestines.¹ These minerals are essential for building and maintaining strong bones and teeth.²
If you don't get enough vitamin D, your body can't absorb calcium efficiently. This can lead to bone problems. In children who are still growing, it can even lead to deformities.
In adults, a long-term deficiency can lead to osteomalacia (bone softening). The risk of osteoporosis (bone loss) also increases significantly.
The role of vitamin D for your immune system
But vitamin D's role extends far beyond bone health. It plays a crucial role in the immune system³ and helps regulate cell division.⁴ Vitamin D also contributes to healthy muscle function.⁵
Recent research shows a possible link between vitamin D deficiency and a variety of conditions, including diabetes, autoimmune diseases, and depression.⁶
Vitamin D deficiency: a global epidemic
A vitamin D deficiency is usually not noticed right away. This is precisely the risk, as the deficiency develops gradually. The signs are subtle at first, so you often don't notice them until (too) late. Sometimes the symptoms are attributed to other causes. Therefore, it's important to be aware so you can recognize the signs of a vitamin D deficiency.
Around the world, vitamin D deficiency is a serious issue. Approximately 15% of study participants have a serum 25-hydroxyvitamin D level of less than 30 nmol/L, 47.9% less than 50 nmol/L, and 76.6% less than 75 nmol/L. 6 In Europe, approximately 40% of the population is deficient, with severe deficiencies in 13%.⁷ Specific studies for Belgium and the Netherlands point in the same direction.⁸
Symptoms of a vitamin D deficiency
Fatigue and listlessness
A typical example of the vague, hard-to-recognize symptoms of a vitamin D deficiency is persistent fatigue. You might feel constantly exhausted and listless, even when you're getting enough sleep. Such a lack of energy can significantly hinder your daily life, both personally and professionally.
Muscle weakness and muscle pain
Vitamin D is important for muscle function.⁵ A deficiency can lead to muscle weakness, pain, and sometimes even cramps. This can affect all muscle groups, but is often most noticeable in the legs and pelvis. People with a deficiency may therefore have difficulty climbing stairs, rising from a chair, or standing for extended periods.
Bone pain and risk of bone fractures
As we've mentioned, vitamin D is crucial for bone health. A deficiency can lead to bone pain, particularly in your lower back, pelvis, and legs. You risk weakening your bones, making you more likely to fracture, even from a minor fall. In older adults, brittle bones due to osteoporosis are a known—and feared—problem.
Weaker immunity
Your immune system also needs a substantial amount of vitamin D. A deficiency can lead to reduced immunity. This can, in turn, lead to infections more quickly, such as colds, flu, or respiratory infections. One study showed that individuals with severe vitamin D deficiency were 33% more likely to be hospitalized for a respiratory infection, compared to those with adequate levels.⁹
Mood swings and depressive feelings
There's growing evidence that vitamin D plays a role in brain function and mood. A deficiency can be linked to feelings of low mood, irritability, anxiety, and symptoms similar to depression.¹⁰ This effect is particularly noticeable during the dark winter months.
Other symptoms
There may also be other, less obvious signs of a vitamin D deficiency. These include hair loss, slow wound healing, or even dental problems. These symptoms are often overlooked.
Causes of Vitamin D Deficiency
Vitamin D deficiency usually arises from a combination of factors related to sunlight exposure, diet, and individual risk factors. Being aware of these causes is the first step toward effective prevention and treatment.
Lack of exposure to sunlight
The primary way our bodies produce vitamin D is through skin exposure to UVB radiation from sunlight. However, this natural production is hampered, in part, by the lack of intense sunlight, especially in winter. Moreover, we spend even less time outdoors during this time.
Even in summer, natural vitamin D production remains subpar for many people. This is especially true in our region. When the sun does shine on a warm summer day, we block it out with sunscreen, parasols, and clothing.
Diet with too little vitamin D
Besides sunlight as the main source of vitamin D, diet also contributes. However, this contribution is limited. Only a few foods naturally contain a significant amount of vitamin D. Oily fish like salmon, mackerel, and herring are good examples. This vitamin is also found in egg yolks. However, most people don't eat enough of these products. For vegans and vegetarians, it can be even more difficult to get enough vitamin D from their diet.
Risk groups and medical factors
Certain groups of people are at higher risk for vitamin D deficiency. The elderly are one such at-risk group. Their skin produces vitamin D less efficiently, and they generally spend less time outdoors. People with darker skin also often have a vitamin D deficiency. They have more pigment in their skin, which reduces UVB absorption.
Obese individuals may also require higher doses of vitamin D because fat tissue can store vitamin D. Medical conditions that affect fat absorption, such as Crohn's disease, celiac disease, or cystic fibrosis, can reduce vitamin D absorption in the intestines.¹¹ Finally, kidney or liver problems can hinder the conversion of vitamin D to its active form.¹²
Diagnosis of a vitamin D deficiency
Wondering if you have a vitamin D deficiency? Since the symptoms can be very vague, a reliable medical test is essential. The most accurate way to determine a person's vitamin D status is with a blood test that measures the level of 25-hydroxyvitamin D (also called 25(OH)D). Visit your doctor for a blood test.
However, even if your vitamin D levels are still within the official reference range, it may be beneficial to take more of the vitamin. The "normal" vitamin D levels you see on a blood test result are statistical reference values . This means they are based on measurements in the average population , not on what is biologically ideal.
In countries like Belgium and the Netherlands, where there's little sunshine, a large portion of the population consistently has a low daily temperature. If "low" becomes the norm, the reference value will shift accordingly.
Types of vitamin D
Most people talk about "vitamin D" by default, but did you know there are actually several forms of vitamin D? They each have their own unique properties and sources. The most common types are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol).
Vitamin D2
Vitamin D2 is primarily found in plant sources and fungi, such as mushrooms exposed to ultraviolet light. Some dietary supplements contain vitamin D2. However, it is generally considered less potent than D3. This is primarily due to the way the body metabolizes and utilizes these forms of vitamin D.
Vitamin D3
Vitamin D3 is the most common form of vitamin D. It's also the form naturally produced when exposed to sunlight. D3 is also found in animal products such as oily fish and egg yolks. Because of its more effective action, this form is also preferred in dietary supplements.
Do you need vitamin D supplements?
When : Vitamin D supplements are an effective way to supplement deficiencies, especially during the darker months or if you don't get enough from sunlight and food.
Which : Vitamin D3 is generally better absorbed by the body and retained longer than vitamin D2.
How much : The required dose varies widely, but 20 mcg is a good maintenance dose. Don't just combine different supplements with vitamin D, as too much is also harmful, although you're unlikely to build up an overdose.¹³
Multivitamins with vitamin D
Many people choose a daily multivitamin that also contains vitamin D. This can be a convenient way to ensure you get enough not only vitamin D, but also other essential vitamins and minerals.
When choosing a multivitamin supplement, make sure that the amount of vitamin D matches the recommended daily amount you need, depending on your specific situation.
Why Smart Vitamins from Essentials?
Stress, hormonal fluctuations, and a busy lifestyle can lead to deficiencies, even with a healthy diet. Smart Vitamins for Her from Insentials replenishes these where needed. Each capsule contains a 20 mcg dose of vitamin D3. But that's not all. These multivitamins combine vitamin D3 with several other essential vitamins, minerals, and omega-3s : DHA from fish oil , iron , vitamin E , Coenzyme Q10 , folic acid , vitamin K2 , vitamin B8 or biotin , and vitamin B12 .
All ingredients have been carefully selected based on scientific evidence . They are dosed effectively to provide targeted support for your body. No fillers or unnecessary ingredients, just the nutrients you need, in bioavailable forms.
A true all-in-one solution for your daily nutritional needs.
¹ Vitamin D contributes to the normal absorption/utilisation of calcium and phosphorus
² Vitamin D contributes to the maintenance of normal bones. Vitamin D contributes to the maintenance of normal teeth.
³ Vitamin D contributes to the normal functioning of the immune system.
⁴ Vitamin D plays a role in the cell division process.
⁵ Vitamin D contributes to normal muscle function.
⁶ Kaur J, Khare S, Sizar O, Givler A. Vitamin D Deficiency. 2025 Feb 15. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2025 Jan–. PMID: 30335299. https://pubmed.ncbi.nlm.nih.gov/30335299/
⁷ Cashman KD, Dowling KG, Škrabáková Z, Gonzalez-Gross M, Valtueña J, De Henauw S, Moreno L, Damsgaard CT, Michaelsen KF, Mølgaard C, Jorde R, Grimnes G, Moschonis G, Mavrogianni C, Manios Y, Thamm M, Mensink GB, Rabenberg M, Busch MA, Cox L, Meadows S, Goldberg G, Prentice A, Dekker JM, Nijpels G, Pilz S, Swart KM, van Schoor NM, Lips P, Eiriksdottir G, Gudnason V, Cotch MF, Koskinen S, Lamberg-Allardt C, Durazo-Arvizu RA, Sempos CT, Kiely M. Vitamin D deficiency in Europe: pandemic? Am J Clin Nutr. 2016 Apr;103(4):1033-44. doi: 10.3945/ajcn.115.120873. Epub 2016 Feb 10. PMID: 26864360; PMCID: PMC5527850. https://pmc.ncbi.nlm.nih.gov/articles/PMC5527850/
⁸ Verbakel, MR, Verkaik-Kloosterman, J., Dinnissen, CS et al. Vitamin D status of adults in the North of the Netherlands: cross-sectional results from the Lifelines cohort study. Eur J Clin Nutr 79, 1114–1120 (2025). https://doi.org/10.1038/s41430-025-01667-1
⁹ Bournot AR, Hart KH, Johnsen S, et al. Association between serum 25-hydroxyvitamin D status and respiratory tract infections requiring hospital admission: unmatched case-control analysis of ethnic groups from the United Kingdom Biobank cohort. The American Journal of Clinical Nutrition, 2025. DOI: 10.1016/j.ajcnut.2025.101179.
¹⁰ Wang, L., Su, S., Liu, Y. Meta-analysis of the effect of vitamin D on depression. Frontiers in Psychiatry, 2025 Jul 31;16:1622796. https://pubmed.ncbi.nlm.nih.gov/40821024/
¹¹ Margulies SL, Kurian D, Elliott MS, Han Z. Vitamin D deficiency in patients with intestinal malabsorption syndromes--think in and outside the gut. J Dig Dis. 2015 Nov;16(11):617-33. doi: 10.1111/1751-2980.12283. PMID: 26316334. https://pubmed.ncbi.nlm.nih.gov/26316334/
¹² Christakos S, Dhawan P, Verstuyf A, Verlinden L, Carmeliet G. Vitamin D: Metabolism, Molecular Mechanism of Action, and Pleiotropic Effects. Physiol Rev. 2016 Jan;96(1):365-408. doi: 10.1152/physrev.00014.2015. PMID: 26681795; PMCID: PMC4839493.
¹³ Zemp J, Erol C, Kaiser E, Aubert CE, Rodondi N, Moutzouri E. A systematic review of evidence-based clinical guidelines for vitamin D screening and supplementation over the last decade. Arch Public Health. 2025 Aug 29;83(1):221. doi:10.1186/s13690-025-01709-x. PMID: 40883815; PMCID: PMC12395781. https://pmc.ncbi.nlm.nih.gov/articles/PMC12395781/