Vitamin D Deficiency – Causes, Symptoms and Treatment

What Is Vitamin D Deficiency?

Vitamin D deficiency (medically known as hypovitaminosis D) is a condition in which the body lacks sufficient vitamin D to maintain its normal functions. Vitamin D is a fat-soluble vitamin that can be obtained through certain foods and is also produced in the skin when exposed to sunlight. It plays a vital role in bone health, immune function, muscle strength, and many other physiological processes. Vitamin D deficiency is considered one of the most widespread nutritional deficiencies worldwide.

Causes

Vitamin D deficiency can arise from a variety of causes:

• Insufficient sun exposure: Since the majority of vitamin D is synthesized in the skin through UV-B radiation, living at northern latitudes, spending most of the time indoors, or consistently using high-factor sunscreen can significantly reduce vitamin D production.
• Low dietary intake: Vitamin D is naturally present in only a few foods, such as fatty fish (salmon, herring), egg yolks, and fortified products. A limited diet may therefore contribute to a deficiency.
• Malabsorption: Conditions such as Crohn's disease, celiac disease, or pancreatic insufficiency can impair the absorption of vitamin D in the intestine.
• Kidney disease: The kidneys are responsible for converting vitamin D into its active form (calcitriol). Kidney disorders can disrupt this conversion.
• Liver disease: The liver is involved in the first step of vitamin D activation, so liver conditions can also lead to deficiency.
• Older age: The ability of the skin to produce vitamin D decreases with age.
• Darker skin tone: Higher melanin levels in the skin reduce the efficiency of vitamin D synthesis from sunlight.
• Obesity: Vitamin D can be sequestered in fatty tissue, reducing its bioavailability in the body.

Symptoms

Mild vitamin D deficiency often causes no obvious symptoms. However, a pronounced or long-lasting deficiency can lead to the following complaints:

• Fatigue, exhaustion, and general weakness
• Muscle aches and muscle weakness
• Bone pain, especially in the back, hips, and legs
• Increased susceptibility to infections due to a weakened immune system
• Low mood or depressive symptoms
• In children: rickets (softening and deformation of bones)
• In adults: osteomalacia (bone softening) and increased risk of osteoporosis (bone loss)

At-Risk Groups

Certain population groups are particularly prone to vitamin D deficiency:

• Older adults (aged 65 and above)
• Infants and toddlers (especially those exclusively breastfed without supplementation)
• People with darker skin living in low-sunlight regions
• Individuals who spend little time outdoors or cover most of their skin
• Pregnant and breastfeeding women
• People with chronic gastrointestinal or liver conditions

Diagnosis

Vitamin D deficiency is diagnosed through a blood test measuring the serum level of 25-hydroxyvitamin D (25-OH vitamin D), the primary marker of vitamin D status in the body. Values are expressed in nanomoles per liter (nmol/L) or nanograms per milliliter (ng/mL):

• Below 30 nmol/L (12 ng/mL): severe deficiency
• 30–50 nmol/L (12–20 ng/mL): deficiency
• 50–75 nmol/L (20–30 ng/mL): borderline insufficient
• Above 75 nmol/L (30 ng/mL): sufficient

Recommended Intake

The World Health Organization (WHO) and most national nutrition societies recommend a daily intake of 600–800 IU (15–20 µg) of vitamin D for adults and children when adequate sun exposure is not achieved. Individual requirements can vary depending on age, body weight, and level of sun exposure. For older adults, recommendations are often higher, up to 800–1000 IU per day.

Treatment and Supplementation

Treatment of vitamin D deficiency depends on the severity of the deficit and its underlying cause:

• Supplementation: The most common approach is taking vitamin D supplements (vitamin D3, also known as cholecalciferol) in the form of drops, capsules, or tablets. In cases of severe deficiency, higher initial doses may be prescribed, followed by a maintenance dose.
• Dietary adjustments: Increasing intake of vitamin D-rich foods such as fatty fish, egg yolks, and fortified dairy products can help improve levels.
• More sunlight: Regular, moderate sun exposure on the face, hands, and arms (without sunscreen) for approximately 15–30 minutes daily supports the body's own vitamin D production.
• Treating the underlying condition: If a medical condition such as malabsorption is the cause, it must be addressed concurrently.

Interactions and Toxicity

As a fat-soluble vitamin, vitamin D can accumulate in the body. Toxicity (hypervitaminosis D) is very rare with normal sun exposure and moderate supplementation, but can occur with very high doses taken over a prolonged period. Symptoms of overdose include nausea, weakness, elevated blood calcium levels (hypercalcemia), and in severe cases, kidney damage. High-dose supplementation should therefore always be medically supervised.

Vitamin D works closely together with calcium and vitamin K2. Vitamin K2 supports the proper incorporation of calcium into the bones and helps prevent calcium deposits in blood vessels. When supplementing with higher doses of vitamin D, co-supplementation with vitamin K2 is often recommended.

References

1. Holick MF. Vitamin D Deficiency. New England Journal of Medicine, 2007; 357(3):266-281.
2. World Health Organization (WHO): Vitamin D supplementation in infants, children and adolescents. WHO Guidelines (2022).
3. Pludowski P et al. Vitamin D supplementation guidelines. Journal of Steroid Biochemistry and Molecular Biology, 2018; 175:125-135.