Vitamin D, beyond the sun and bones

Why is vitamin D deficiency so common, even in sunny countries? What does the current scientific evidence say? When should it be measured? Why is it not always a good idea to take supplements "just in case"? Specialists in Endocrinology and Nutrition, Gynecology and Obstetrics, and Psychiatry at the Clínica Universidad de Navarra this core topic health.

 

A vitamin that acts as a hormone

Vitamin D is fat-soluble (it dissolves in fats and oils), but its active form—calcitriol—behaves like a true hormone. After being synthesized in the skin thanks to solar radiation or incorporated through per diem expenses supplements, it is activated in the liver and kidneys and acts on numerous tissues. 

"Vitamin D not only regulates calcium and phosphorus absorption; it also plays a role in muscle function, the immune system, metabolism, and inflammation regulation," explains Dr. Ana Hernández Moreno, manager area the Clínica Universidad de Navarra Pamplona and a specialist in endocrinology and nutrition. 

This "multifunctional" nature explains why its deficiency can go unnoticed or manifest itself with nonspecific symptoms such as fatigue, muscle weakness, or diffuse bone pain. 

Why is vitamin D deficient even in sunny countries?

The paradox is clear: living in a country with many hours of sunshine does not guarantee adequate levels of vitamin D. The explanation lies in lifestyle habits and biological factors. 

We spend most of the day indoors, we use sunscreen—necessary to prevent skin cancer—and we tend to expose ourselves to the sun at times when radiation is insufficient to stimulate skin synthesis. Furthermore, for several months of the year, UVB radiation is insufficient even in Mediterranean latitudes. 

Other risk factors include advanced age, dark skin, obesity, chronic diseases, or a per diem expenses in foods rich in vitamin D. "Even though we live in a sunny country, the combination of all these factors means that deficiency remains common," emphasizes Dr. Hernández. 

Bone health: the best-known role

The link between vitamin D and bone health is the best established scientifically. Its function is essential for calcium to be absorbed properly and incorporated into the bone. In the words of Dr. Álvaro Ruiz Zambrana, a specialist ingynecology and obstetricsat the Clínica Universidad de Navarra, "calcium is the building blocks of bone and vitamin D is the cement: if one of the two is missing, the bone does not form properly." 

This balance is especially important during stages such as menopause, when the decline in estrogen accelerates bone loss and increases the risk of osteoporosis and fractures. However, throughout a woman's life, there are various times of particular vulnerability when vitamin D plays a core topic role, such as during pregnancy or breastfeeding. 

During pregnancy, nutritional needs increase. The fetus obtains calcium and vitamin D from the mother, which can lead to deficiencies if these needs are not adequately met. "Vitamin D is essential for the baby's development and also for maintaining the mother's bone health," explains Dr. Ruiz Zambrana. In addition, its relationship with glucose metabolism and blood pressure control has been observed, linking it to the risk ofgestational diabetesorpreeclampsia. 

Duringbreastfeeding, vitamin D is also shared with the newborn through breast milk, so maintaining adequate levels remains important, especially during prolonged breastfeeding. 

 
 

Beyond the bone: metabolism, defenses, and muscle

research shown that vitamin D plays a role in muscle and neuromuscular function, so a deficiency is associated with an increased risk of falls, especially in older people, and a general feeling of weakness. 

It also acts as an immune system modulator, helping to regulate the inflammatory response. Although it is not a substitute for vaccines or treatments, adequate levels of this vitamin have been linked to reduced vulnerability to respiratory infections. 

Food and supplements: when to take them and when not to

per diem expenses alone rarely cover vitamin D requirements. Foods that naturally contain vitamin D are scarce: fatty fish, egg yolks, and liver. Therefore, in certain cases, supplementation is recommended. 

"It's not about taking vitamin D 'just in case,'" warns Dr. Hernández. In healthy people without deficiencies or risk factors, no benefits have been demonstrated. Furthermore, excessive and prolonged consumption can cause adverse effects. 

Vitamin D is not a panacea, but neither is it a minor detail. Understanding its real role, based on evidence, allows us to take advantage of its benefits where they are needed and avoid unnecessary excesses. This balance is part of a comprehensive approach to health. 

Where do we get vitamin D?

• 80–90%: skin synthesis through exhibition  
• 10–20%: food and supplements 

The most commonly used ranges for 25-hydroxyvitamin D (25-OH-D) in routine internship are: 

• Déficit: < 20 ng/mL.
• Insufficiency: 20–30 ng/mL.
• Sufficiency/Optimal level for most people: ≥ 30 ng/mL, with many experts placing the goal range goal 30–50 ng/mL. 

Who should have their vitamin D levels measured?

Clinical guidelines agree that routine vitamin D testing is not necessary in healthy individuals without risk factors. However, it is indicated in certain groups:  

• Older people (especially those living in nursing homes).
• Patients with osteoporosis or previous fractures.  
• Pregnant women or women with polycystic ovary syndrome without supplementation. 
• People with little exhibition (work , very indoor lifestyle). 
• People with dark skin or high phototypes. 
• People with obesity or diseases that cause intestinal malabsorption (celiac disease, inflammatory bowel disease, bariatric surgery, or gastric resection). 
• People with liver disease or chronic kidney failure. 
• Infants without adequate supplementation. 
• People who take certain drugs that affect vitamin D metabolism (anticonvulsants, glucocorticoids, some antiretrovirals).