Vitamin D deficiency is one of the most common nutritional deficiencies in the developed world, affecting an estimated 1 billion people globally. Despite being called a vitamin, it functions more like a hormone — receptors for vitamin D exist in virtually every tissue in the body, and its effects extend far beyond bone health. Here is what you need to know.
Why Deficiency Is So Common
The primary source of vitamin D is synthesis in the skin triggered by UVB radiation from sunlight. Modern lifestyles work against this: most people spend the majority of their time indoors, sunscreen blocks UVB synthesis, and people living above 35 degrees latitude cannot synthesize meaningful vitamin D from sunlight for several months of the year. Darker skin requires more sun exposure to produce the same amount of vitamin D as lighter skin. Obesity reduces bioavailability because vitamin D is fat-soluble and gets sequestered in adipose tissue.
What Vitamin D Does
- Bone health: Regulates calcium and phosphorus absorption; deficiency causes rickets in children and osteomalacia and osteoporosis in adults
- Immune function: Modulates both innate and adaptive immune responses; deficiency is associated with increased susceptibility to respiratory infections
- Muscle function: Deficiency causes muscle weakness and increases fall risk in older adults
- Mood and mental health: Vitamin D receptors are present throughout the brain; deficiency is associated with depression, though causality is debated
- Cardiovascular health: Observational studies link deficiency to increased cardiovascular risk; intervention trials have shown mixed results
Recognizing Deficiency
Mild to moderate deficiency is often asymptomatic, which is why it goes undetected. When symptoms do occur, they include:
- Fatigue and low energy disproportionate to activity level
- Bone pain, particularly in the back, hips, and legs
- Muscle weakness and aches
- Frequent infections, particularly respiratory
- Depression or low mood, especially in winter months
- Slow wound healing
The only reliable way to assess vitamin D status is a blood test measuring 25-hydroxyvitamin D. Levels below 20 ng/mL are considered deficient; 20-30 ng/mL is insufficient; above 30 ng/mL is generally considered adequate. Many experts recommend 40-60 ng/mL as optimal.
Food Sources
Very few foods naturally contain significant vitamin D. The best dietary sources:
- Fatty fish (salmon, mackerel, sardines): 400-600 IU per serving
- Cod liver oil: 1,360 IU per tablespoon
- Egg yolks: 40 IU per yolk
- Fortified foods (milk, orange juice, cereals): 100-150 IU per serving
- UV-exposed mushrooms: variable, up to 400 IU per serving
Diet alone is rarely sufficient to maintain adequate vitamin D levels without sun exposure or supplementation.
Supplementation
Vitamin D3 (cholecalciferol) is more effective than D2 at raising blood levels and is the preferred supplemental form. Standard supplementation doses:
- Maintenance (adequate baseline): 1,000-2,000 IU daily
- Correction of deficiency: 4,000-5,000 IU daily for 8-12 weeks, then retest
- Upper tolerable limit: 4,000 IU daily for long-term use without medical supervision
Vitamin D is fat-soluble — take it with a meal containing fat for best absorption. Vitamin K2 (MK-7 form) is often recommended alongside vitamin D supplementation to direct calcium to bones rather than soft tissues, though the evidence for this combination is still developing. Magnesium is required for vitamin D metabolism; deficiency in magnesium can impair the response to supplementation.