Vitamin D deficiency: The symptoms you ignore and the effects on the body

Vitamin D. Why all the fuss about vitamin D?

"Are you kidding me? You sit there listening to what they say about vitamin D deficiency and you believe them? Is it possible to talk about vitamin D deficiency in our country?" Anyone, living in Greece, would ask this question, and we wouldn't blame them, as common sense tells us that vitamin D, since it requires almost exclusively sunlight to be synthesized in the skin, should normally be deficient in a very small percentage of the Greek population. However, scientific reasoning takes on a completely different dimension and explains that there are many different reasons why our beloved sun, unfortunately, most of the time, does not seem to be enough, as recent epidemiological data show that around 70% of the Greek population is deficient in vitamin D.

Hey, wait, don't rush. What is vitamin D?

Vitamin D (or vitamin D3). What is it and where do we get it from?

Vitamin D is a fat-soluble vitamin, that is stored in the body, specifically in the liver and fatty tissue, with the help of fat from our diet. What did you think? That fat is only stored and serves no purpose? In addition to vitamin D, it helps in the absorption of vitamins A, E, K. Apart from the few dietary sources rich in vitamin D, the largest amount needed by our body is synthesized in our skin, through UVB solar radiation and available cholesterol substrates, under normal conditions. The World Health Organization recommends brief exposure to the sun (15-20 minutes) early in the morning or late in the afternoon, when solar radiation does not pose the risks we are all aware of.

The few dietary sources of vitamin D include fatty fish, mushrooms, liver, egg yolks, and fortified products.

However, despite the sunshine that characterizes a Mediterranean country like Greece, around 7 in 10 Greeks are deficient or lacking in vitamin D, while nearly 1 billion people worldwide appear to be unable to reach optimal vitamin D levels. The World Health Organization has already sounded the alarm about the recognized public health issue of vitamin D deficiency, which reached pandemic levels, and one might even say "hidden" proportions, because the majority of symptoms are silent in nature, especially when we are not in a deficiency state but in insufficiency.

And this is one of the most important reasons why vitamin D deficiency and insufficiency need our attention...!

Lack of vitamin D & sun. How can its paradoxical nature be explained?

The reasons why the percentage is so high for a country with as much sunshine as ours are multifactorial, including the country's latitude, dark skin color, working mainly indoors, frequent – and necessary – use of sunscreen, few dietary sources of vitamin D, obesity and the simultaneous presence of various pathological conditions, which may contribute to the malabsorption not only of vitamin D but also of other essential micronutrients.

This article aims to provide you with reliable information and encourage you not to ignore your 25(OH)D₃ levels in your tests, as levels < 30ng/mL indicate mild or moderate insufficiency, while levels < 20 ng/mL indicate a severe deficiency. Both of these conditions require timely treatment, as vitamin D is a cornerstone of our health, with adequate levels working in our favor across a wide range of health parameters.

Vitamin D deficiency. Why is it so important and where should we focus our attention?

The fact that vitamin D receptors have been identified and located throughout almost the entire body is of utmost importance, as it makes us realize that vitamin D probably does something very well, with the result that almost all systems need it... Receptors are protein molecules in the tissues of the human body that recognize vitamin D and activate its action. Moving on to the "main course," the key roles of the "queen of vitamins" are as follows:

Immune system. Vitamin D appears to play a central role in regulating the immune response. According to a systematic review, conducted in 2025, low vitamin D levels have been associated with an increased risk of upper respiratory tract infections and increased production of pro-inflammatory cytokines. Vitamin D supplementation was associated with increased activity of macrophages, cells that work on the body's first line of defense, while also contributing to a reduction in overall inflammation, by enhancing the production of anti-inflammatory cytokines. In addition, a 2022 meta-analysis by the scientific group of Pereira and colleagues noted that vitamin D deficiency was associated with increased symptom severity and increased hospitalization time. due to Covid-19.
Maintaining muscle and bone health. Perhaps the role we all know is that vitamin D plays a leading role in bone health and calcium and phosphorus homeostasis. We discussed musculoskeletal health in a previous article and analyzed the action of vitamins D₃ and K₂ against the onset of osteopenia and osteoporosis. Indeed, vitamin D deficiency has been associated with reduced bone density and an increased risk of osteomalacia in adults, as well as rickets in children, reducing bone strength and increasing the risk of fractures. Nevertheless, vitamin D supplementation in deficient individuals has been shown to reduce the risk of fracture, particularly in the hip region, in the elderly, by 20%.
Mental health. The "sunshine vitamin" displays a huge impact on cognitive function and mood. Specifically, it appears to act on areas of the brain that constantly work together to help us think clearly and feel good, such as the hypothalamus, hippocampus, and prefrontal lobe. Recent data confirm that low vitamin D levels with an increased risk of depression, Alzheimer's disease, and schizophrenia, emphasizing the need for adequate levels to protect our mental health.
Cardio-metabolic health. Vitamin D effects extend to parameters, related to normal cardiovascular health and healthy metabolic function. Low vitamin D levels are associated with an increased risk of non-alcoholic fatty liver disease and type II diabetes mellitus. The onset of these chronic diseases involves mechanisms that are affected by low vitamin D levels, such as reduced insulin sensitivity, high blood pressure, and reduced endothelial function.

Finally, vitamin D deficiency has been associated with an increased risk of various types of cancer, such as breast and prostate cancer, and appears to be involved in reproductive health, particularly in women with Polycystic Ovarian Syndrome (PCOS), where supplementation has been shown to improve ovulation and insulin resistance, thereby combating infertility.

Vitamin D3 supplementation. When and for how long?

If vitamin D deficiency or insufficiency is diagnosed, it is vital to correct it as soon as possible, as its long-term effects can have a direct impact on a number of functions in the human body, as mentioned above. Vitamin D supplementation should be taken consistently for a period of 3-4 months, following a meal rich in fats (i.e., containing olive oil, nuts, or full-fat/light dairy products).

Extra tip: If the chosen vitamin D supplementation does not help correct your blood serum levels, ask yourself the following questions:

Am I taking the right form of vitamin D?
Am I taking the right dosage?
Am I taking the supplement consistently and correctly?

If you check all of the above, there may be a mild deficiency or lack of magnesium, which is an extremely important collaborator of vitamin D.

Conclusion

Vitamin D is a topic of much discussion regarding health and well-being, as it participates in numerous processes in the human body that are critical to its proper functioning. The percentage of people with insufficient vitamin D levels is now a recognized public health issue, requiring reliable information and personalized guidance. However, the good news is that it is something we can intervene and correct with proper monitoring, so that we can turn the scales of wellness... in our favor.

Scientific References

Hebbar, R., Gangwar, V., Gundeti, M., Tyagi, A., & Srivastava, S. (2025). Systematic Review: The Effects of Vitamin D3 Deficiency on Health. CME Journal Geriatric Medicine, 17, 26-31.

Mikola, T., Marx, W., Lane, M. M., Hockey, M., Loughman, A., Rajapolvi, S., ... & Ruusunen, A. (2023). The effect of vitamin D supplementation on depressive symptoms in adults: A systematic review and meta‐analysis of randomized controlled trials. Critical reviews in food science and nutrition, 63(33), 11784-11801.

Pereira‐Santos, M., Costa, P. D. F., Assis, A. D., Santos, C. D. S., & Santos, D. D. (2015). Obesity and vitamin D deficiency: a systematic review and meta‐analysis. Obesity reviews, 16(4), 341-349.

Pereira, M., Dantas Damascena, A., Galvão Azevedo, L. M., de Almeida Oliveira, T., & da Mota Santana, J. (2022). Vitamin D deficiency aggravates COVID-19: systematic review and meta-analysis. Critical reviews in food science and nutrition, 62(5), 1308-1316.

Rosanoff, A., Dai, Q., & Shapses, S. A. (2016). Essential nutrient interactions: does low or suboptimal magnesium status interact with vitamin D and/or calcium status?. Advances in nutrition, 7(1), 25-43.

Talib, M., Rachdi, M., Papazova, A., & Nicolis, H. (2024). The Role of Dietary Patterns and Nutritional Supplements in the Management of Mental Disorders in Children and Adolescents: An Umbrella Review of Meta-Analyses: Le rôle des habitudes alimentaires et des suppléments nutritionnels dans la prise en charge des troubles mentaux chez les enfants et les adolescents: une méta-revue de méta-analyses. The Canadian Journal of Psychiatry, 69(8), 567-589.