Vitamin D Deficiency: When Low Numbers Actually Matter

Vitamin D is a fat-soluble hormone that the body makes from cholesterol in skin exposed to UVB sunlight and, to a much smaller extent, absorbs from food. Its measured form in blood is 25-hydroxyvitamin D — a storage form with a half-life of a few weeks that reflects your last month or two of intake and sun exposure.

The medical worry is not the number itself. It is what a truly low number does to calcium metabolism: parathyroid hormone rises, calcium leaches from bone, and over time this causes osteomalacia in adults or rickets in children. Everything else attributed to low vitamin D — fatigue, depression, cardiovascular risk, cancer — has a much weaker evidence base and is where clinicians and researchers still disagree.

Depending on where you live, between 20% and 50% of adults have a 25(OH)D under 20 ng/mL at least once in a given year. That is not a global epidemic. It is what the population distribution looks like. Modern life keeps most people indoors, and indoor UVB is essentially zero.

The population most likely to be persistently low: darker skin at higher latitudes, veiled clothing, older adults with reduced skin synthesis, people with obesity (fat sequesters vitamin D), inflammatory bowel disease or bariatric surgery patients (malabsorption), and those on long-term glucocorticoids or antiepileptics (accelerated catabolism).

Fatigue, low mood, muscle aches and hair loss are the four symptoms most commonly attributed to low vitamin D by internet articles. In practice, only muscle pain and proximal muscle weakness have a plausible mechanistic link at levels under 15 ng/mL, and even then randomised trials have shown mixed results.

A more reliable clinical picture: bone or muscle pain that has developed over months, difficulty rising from a squat or climbing stairs, waddling gait, tender ribs or sternum on pressure, and biochemistry showing low or low-normal calcium, low phosphate, elevated alkaline phosphatase and high PTH. That combination is osteomalacia. That is what "vitamin D deficiency" clinically looks like.

The standard evidence-based approach for confirmed deficiency in an otherwise healthy adult: cholecalciferol (vitamin D3) 50,000 IU weekly for 6–8 weeks as a loading course, followed by 800–2,000 IU daily maintenance. Recheck 25(OH)D at 3 months, not sooner — the serum takes time to stabilise.

Vitamin D2 (ergocalciferol) is a common prescription alternative, especially in the UK and NHS setting. It is roughly equally effective per international unit but has a slightly shorter serum half-life. Calcifediol (25-hydroxyvitamin D) works faster but is only used in specific specialist scenarios.

Chronic maintenance beyond three months should be personalised — most healthy adults do not need lifelong daily supplementation once repleted, unless a persistent risk factor remains (obesity, malabsorption, older age with fall risk, dark skin at high latitude).

1. What was my 25(OH)D level in ng/mL or nmol/L? A single unit-conversion changes the interpretation (multiply ng/mL by 2.5 to get nmol/L).

2. Were calcium, phosphate, PTH and alkaline phosphatase also checked? If not, ask why — those results usually decide treatment more than the vitamin D number alone.

3. If we start supplementation, what dose and for how long? What is the plan for rechecking, and when?

4. Am I in a group where trial evidence supports supplementation for outcomes beyond bone health? (In most cases, the honest answer is: no.)

5. Is my low level explained by an underlying condition — malabsorption, obesity, or a medication? Would treating the underlying cause fix it?