Vitamin D Deficiency — Symptoms, Causes & Treatment | MyMedicPlus

Vitamin D deficiency is the most prevalent nutritional deficiency globally, affecting approximately 1 billion people. Serum 25-hydroxyvitamin D [25(OH)D] is the primary measure of vitamin D status: deficiency is defined as below 50 nmol/L (below 20 ng/mL); insufficiency as 50-75 nmol/L; and sufficiency as above 75 nmol/L. Vitamin D functions as a prohormone — vitamin D3 (cholecalciferol) is synthesized in the skin from 7-dehydrocholesterol under UVB radiation (wavelength 290-315 nm), then hydroxylated in the liver to 25(OH)D (calcidiol — the main circulating storage form), then converted by the kidney to 1,25-dihydroxyvitamin D (calcitriol — the biologically active form). Calcitriol binds vitamin D receptors (VDR) expressed in over 200 cell types, regulating calcium absorption from the gut (by 30-40%), bone mineralisation, innate and adaptive immune function, insulin secretion, and myocyte function. Vitamin D is essential for calcium homeostasis, bone mineralisation, immune function, and neuromuscular health.

Insufficient sun exposure (most common cause globally): indoor lifestyle, high-latitude residence (above 35 degrees N or S), clothing covering the skin (cultural or religious practices), winter months (UVB radiation is insufficient at high latitudes between October and April). Reduced skin synthesis: dark skin pigmentation (melanin reduces UVB conversion — requires 3-6 times longer sun exposure for equivalent D3 synthesis), ageing skin (reduced 7-dehydrocholesterol — the elderly produce approximately 75% less vitamin D from the same UVB exposure compared to young adults). Obesity (fat sequestration of the fat-soluble vitamin reduces serum bioavailability). Malabsorption syndromes: Crohn's disease, coeliac disease, bariatric surgery (all impair fat-soluble vitamin absorption). Liver disease (impaired hepatic 25-hydroxylation) or chronic kidney disease (impaired renal 1-alpha-hydroxylation to active calcitriol). Dietary insufficiency (few foods naturally rich in D3: oily fish, egg yolks, UV-irradiated mushrooms, fortified plant milks and cereals). Medications: anticonvulsants (phenytoin, phenobarbitone — induce hepatic CYP enzymes increasing 25(OH)D catabolism); rifampicin; corticosteroids; cholestyramine.

Vitamin D deficiency is often entirely asymptomatic, particularly when mild to moderate — many patients are diagnosed incidentally on routine blood testing. Symptomatic deficiency (typically serum 25(OH)D below 25 nmol/L): bone pain and deep tenderness on pressure at the sternum, tibia, and lumbar spine — from inadequate osteoid mineralisation causing osteomalacia in adults; pressing on the sternum or shin reproduces the characteristic bone pain of osteomalacia; proximal muscle weakness (difficulty rising from a chair without using arms, climbing stairs, or lifting above the head — from impaired calcium-dependent muscle contraction and reduced vitamin D receptor-mediated gene expression in muscle); generalised muscle aches, fatigue, and malaise. Impaired gait, balance difficulties, and increased fall risk — the characteristic waddling gait of proximal myopathy and impaired proprioception increase falls in older adults; this is the most clinically consequential effect of symptomatic vitamin D deficiency in the elderly. Children presenting with rickets: bowing of weight-bearing long bones (genu varum — bow legs; or genu valgum — knock knees); craniotabes (softening of occipital and parietal skull bones — ping-pong ball sensation on pressure); delayed fontanelle closure beyond 18 months; rachitic rosary (firm, round, bony swellings at the costochondral junctions — palpable and visible along the chest wall); Harrison's sulcus (horizontal chest wall indentation from diaphragm pull on compliant softened ribs); delayed dentition and enamel defects; and growth delay with short stature. Severe hypocalcaemia from vitamin D deficiency: tetany (painful carpopedal spasm — positive Trousseau's sign on BP cuff inflation; Chvostek's sign — ipsilateral facial muscle twitching when tapping the facial nerve anterior to the ear), laryngospasm (inspiratory stridor), and hypocalcaemic seizures — most commonly in exclusively breastfed infants born to vitamin D-deficient mothers.

Serum 25-hydroxyvitamin D [25(OH)D]: the primary diagnostic test — reflects total vitamin D status from sun exposure and dietary intake; deficiency below 50 nmol/L (below 20 ng/mL). Serum PTH (parathyroid hormone): secondary hyperparathyroidism is an early and sensitive indicator of vitamin D insufficiency — PTH rises before 25(OH)D reaches the deficient threshold; useful for detecting functional insufficiency. Serum calcium and phosphate: hypocalcaemia and hypophosphataemia develop in clinically significant deficiency. Serum alkaline phosphatase (ALP): elevated in rickets and osteomalacia (from increased osteoblast activity trying to mineralise bone). X-ray: Looser's zones (pseudofractures — transverse radiolucent stress fracture lines in the pelvis, femur, tibia) in adults with osteomalacia; rachitic changes in children (metaphyseal cupping, fraying, and widening; bowing of weight-bearing long bones). DEXA scan: bone mineral density assessment (for osteoporosis risk stratification in adults with chronic deficiency). Note: serum calcitriol [1,25(OH)2D] is NOT an appropriate screening test — calcitriol may be normal or elevated due to secondary hyperparathyroidism even when 25(OH)D is severely low.

Severe deficiency (below 25 nmol/L): loading dose 300,000 IU vitamin D3 orally (as a single high-dose, or 50,000 IU weekly for 6-8 weeks — total 300,000 IU). Moderate deficiency (25-49 nmol/L): 800-2,000 IU D3 daily. Maintenance for all at-risk groups: 800-1,000 IU D3 daily. Calcium supplementation (1,000-1,200 mg/day elemental calcium — dietary or supplement) for osteomalacia, rickets, and those with inadequate dietary calcium. For rickets: 2,000-6,000 IU daily for 12 weeks then maintenance. Recheck 25(OH)D level 3 months after loading dose to confirm treatment adequacy. Vitamin D3 (cholecalciferol) is preferred over D2 (ergocalciferol) — D3 is more potent and longer-lasting in raising serum 25(OH)D levels. In chronic kidney disease (CKD stage 3-5): activated vitamin D analogues (alfacalcidol 0.25-1 mcg/day or calcitriol 0.25-0.5 mcg/day) are required as the kidney cannot adequately hydroxylate 25(OH)D to calcitriol — standard vitamin D3 supplementation is also given concurrently to replete stores. Magnesium: required for vitamin D metabolising enzymes (25-hydroxylase and 1-alpha-hydroxylase) — supplement magnesium concurrently in deficient patients to optimise vitamin D metabolism. Vitamin D toxicity (serum 25(OH)D above 375 nmol/L): causes hypercalcaemia, hypercalciuria, and nephrolithiasis — stop supplementation and ensure adequate hydration.

Osteomalacia in adults (inadequate osteoid mineralisation — diffuse bone pain, Looser's zones on X-ray which are pseudofractures or stress fracture lines through the cortex at the pelvis and femoral neck, and pathological fractures without significant trauma) and nutritional rickets in children (skeletal deformities, genu varum, genu valgum, rachitic rosary, dental defects, and linear growth failure). Osteoporosis and increased fragility fracture risk: chronic vitamin D deficiency causes secondary hyperparathyroidism — elevated PTH stimulates osteoclastic bone resorption, reducing bone mineral density and increasing hip, vertebral, and wrist fracture risk; combined calcium and vitamin D supplementation reduces hip fracture risk by 25-30% in elderly nursing home residents. Falls and sarcopenia in older adults: proximal myopathy and impaired neuromuscular coordination from vitamin D deficiency increase fall frequency and severity — vitamin D supplementation at 700-1000 IU daily reduces fall risk by approximately 20% in deficient elderly patients. Autoimmune and chronic inflammatory disease associations (epidemiological — causality not established): multiple sclerosis (higher incidence at high latitudes; low 25(OH)D increases relapse risk), type 1 diabetes mellitus, inflammatory bowel disease (Crohn's in particular), and rheumatoid arthritis are all more prevalent in populations with lower sun exposure. Increased susceptibility to respiratory infections: vitamin D is required for macrophage cathelicidin and defensin production — deficiency impairs innate mucosal immune defence; association with increased respiratory tract infection frequency and COVID-19 severity. Pregnancy complications: gestational diabetes (approximately 2x increased risk in severely deficient women), preeclampsia, small-for-gestational-age infants, and neonatal hypocalcaemia causing seizures in the first week of life.

Safe sun exposure: 10-30 minutes of midday summer sun (arms and legs uncovered, without sunscreen) at least 3 times per week is sufficient for adequate vitamin D synthesis in lighter-skinned individuals at temperate latitudes — varies significantly by latitude (above 35 degrees N or S in winter, UVB is insufficient regardless of sun exposure duration), season, skin phototype (darker skin requires 3-6 times longer exposure for equivalent synthesis), and age (elderly skin has reduced 7-dehydrocholesterol substrate). Dietary sources of vitamin D: oily fish (wild salmon 600-1000 IU per 100 g, mackerel 400 IU, canned tuna 150 IU, sardines 300 IU), egg yolks (40-50 IU each), UV-irradiated mushrooms (up to 400 IU per serving when placed gill-side up in midday sun), and fortified foods (plant milks, fortified orange juice, breakfast cereals, and margarine typically providing 80-100 IU per serving). Universal supplementation recommendations: pregnant and breastfeeding women (400-600 IU vitamin D3 daily — essential as breast milk is virtually vitamin D-free); exclusively or partially breastfed infants (400 IU D3 from birth until dietary intake is adequate); adults over 65 (800-1000 IU D3 daily — reduces fall and fracture risk by 20-25% in deficient patients); housebound individuals, nursing home residents, individuals with darker skin tones or those who cover their skin (1000 IU daily year-round). High-risk groups needing higher doses: obesity (BMI above 30 — fat sequestration of fat-soluble D3 requires 2-3x standard dose), malabsorption syndromes (Crohn's, coeliac disease, bariatric surgery), and chronic kidney disease (activated D analogues — alfacalcidol or calcitriol). Annual 25(OH)D monitoring for all high-risk groups.

See your GP for: persistent unexplained bone or muscle pain, muscle weakness (particularly proximal weakness — difficulty rising from a chair or climbing stairs), fatigue, or frequent infections in a person with risk factors for vitamin D deficiency (dark skin, minimal sun exposure, obesity, malabsorption, housebound). Testing is a simple blood test (25-hydroxyvitamin D). Seek urgent assessment for: vitamin D deficiency in an infant or young child with: bowing of legs (nutritional rickets), seizures from hypocalcaemia, or irritability and poor weight gain — these require treatment. See your GP urgently if vitamin D deficiency is associated with hypocalcaemia symptoms: muscle cramps, tingling in hands and feet, spasm, or seizures (tetany from severe hypocalcaemia). High-risk groups (pregnant or breastfeeding women, infants, elderly care home residents, dark-skinned individuals) should receive routine supplementation without the need for testing.