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Vitamin D Deficiency

Vitamin D deficiency, also known as hypovitaminosis D, is a medical condition where vitamin D levels in the body are below normal.

This can result from inadequate exposure to sunlight, poor dietary intake, or conditions that limit vitamin D absorption or conversion to active metabolites. The condition can impair bone mineralisation, leading to diseases such as rickets in children and osteomalacia and osteoporosis in adults.

The normal process of Vitamin D absorption
The normal process of Vitamin D absorption

Signs and Symptoms

Child with rickets
Child with rickets

Vitamin D deficiency is often asymptomatic and may only be detected through blood tests. However, it can cause significant health issues:

  • Rickets: A childhood disease marked by impeded growth and deformity of long bones.
  • Osteomalacia: A bone-thinning disorder in adults characterised by muscle weakness and bone fragility.
  • Osteoporosis: Reduced bone mineral density and increased bone fragility.
  • Increased Risk of Fractures: Due to weakened bones.
  • Myopathy: Muscle aches, weakness, and twitching due to reduced blood calcium levels.
  • Periodontitis: Inflammatory bone loss leading to potential tooth loss.
  • Respiratory Infections and COVID-19: Increased risk and severity.
  • Schizophrenia: Associated with lower levels of vitamin D.

Risk Factors

Individuals most at risk include those with limited sun exposure due to climate, clothing, or lifestyle habits. Other risk factors include:

  • Age: Elderly individuals due to decreased sunlight exposure and dietary intake.
  • Obesity: Excess fat tissue can sequester vitamin D.
  • Darker Skin: Higher melanin levels reduce vitamin D synthesis from sunlight.
  • Malabsorption Conditions: Such as untreated celiac disease or inflammatory bowel disease.
  • Breastfeeding: Infants exclusively breastfed may require supplementation, especially if they have limited sun exposure.


Mapping of several bone diseases onto levels of vitamin D (calcidiol) in the blood
Mapping of several bone diseases onto levels of vitamin D (calcidiol) in the blood

Vitamin D deficiency is diagnosed by measuring serum concentrations of 25-hydroxyvitamin D (25(OH)D). Levels are classified as:

  • Severe Deficiency: <12 ng/mL (30 nmol/L)
  • Deficiency: <20 ng/mL (50 nmol/L)
  • Insufficient: 20–29 ng/mL (50–75 nmol/L)
  • Normal: 30–50 ng/mL (75–125 nmol/L)


Vitamin D2 supplements
Vitamin D2 supplements

Treatment involves supplementation and increased sunlight exposure. Dosages vary based on the severity of deficiency and patient-specific factors:

  • Initial Phase: High-dosage treatment to rapidly elevate serum levels, given daily, weekly, or as a single high dose.
  • Maintenance Phase: Continuous lower doses to maintain adequate levels.

Special considerations include higher dosages for patients with malabsorption disorders or those having undergone bariatric surgery.


Adequate sunlight exposure and dietary intake are very important. Foods such as oily fish, fortified milk, and mushrooms can help maintain vitamin D levels. Supplements, such as cholecalciferol (D3) or ergocalciferol (D2), are also commonly used.


Vitamin D is synthesised in the skin under UVB exposure and undergoes conversion in the liver and kidneys to its active form. Deficiency can result from decreased production, dietary intake, absorption, or conversion. This leads to impaired calcium absorption, increasing bone resorption and risk of fractures.


The prevalence of vitamin D deficiency varies significantly based on geographic location and population demographics. For instance, severe deficiency (<30 nmol/L) affects 13% of Europeans, 5.9% of Americans, and 7.4% of Canadians.


The link between diet and rickets was identified in the early 20th century, leading to the discovery of vitamin D. Fortification of milk with vitamin D in the 1930s significantly reduced the prevalence of rickets in the United States.


Emerging studies suggest associations between vitamin D deficiency and various conditions, including certain cancers, cardiovascular disease, and impaired immune function. While supplementation has shown benefits in reducing mortality in critically ill patients, more research is needed to establish definitive therapeutic guidelines.

Normal bone vs. osteoporosis
Normal bone vs. osteoporosis

Self-assessment MCQs (single best answer)

Which condition in children is caused by vitamin D deficiency?

Which of the following is NOT a symptom of vitamin D deficiency?

What is the normal range for serum 25-hydroxyvitamin D levels?

Which of the following populations is at higher risk for vitamin D deficiency?

What is the main method for diagnosing vitamin D deficiency?

Which of the following foods is a good source of vitamin D?

Which of the following is a treatment for vitamin D deficiency?

Vitamin D undergoes conversion to its active form in which organs?

What condition is characterised by bone-thinning and muscle weakness in adults due to vitamin D deficiency?

During which decade was milk fortification with vitamin D introduced in the United States?


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