Causes and Risk Factors
Vitamin D deficiency develops when the skin makes too little, the diet supplies too little, or the body cannot absorb or process what it gets:- Limited sun exposure: Working indoors, covering the skin, and consistent sunscreen use reduce production.
- Older age: Aging skin produces vitamin D less efficiently, and the kidneys activate it less well.
- Darker skin: Melanin reduces the skin's vitamin D production from the same amount of sun.
- Malabsorption: Digestive conditions such as celiac disease and Crohn's disease, and prior gastric bypass surgery, impair absorption of this fat-soluble vitamin.
- Obesity: Vitamin D is sequestered in fat tissue, lowering blood levels.
- Limited dietary intake: Very few foods naturally contain vitamin D, so diets low in fish, eggs, and fortified dairy supply little.
- Kidney or liver disease and certain medications: Both organs help activate vitamin D, and some seizure and steroid medicines speed its breakdown.
Symptoms
Most people with vitamin D deficiency feel nothing at all, which is why it is usually discovered on a blood test. When deficiency is significant or prolonged, it can cause:- Bone pain and tenderness: Severe deficiency softens bone — rickets in children and osteomalacia in adults.
- Muscle weakness and aches: Particularly in the hips and thighs, sometimes contributing to falls in older adults.
- Fatigue: Nonspecific, but frequently reported.
- Bone thinning over time: Chronic deficiency contributes to osteopenia and osteoporosis and raises fracture risk.
Diagnosis
A simple blood test measuring 25-hydroxyvitamin D confirms the diagnosis. Levels below 20 ng/mL are generally considered deficient, while 20 to 30 ng/mL is often called insufficient. Routine screening of healthy adults without risk factors is not generally recommended; testing is targeted to people with risk factors, bone disease, unexplained muscle symptoms, or malabsorption, and calcium and parathyroid hormone levels are sometimes checked alongside.Treatment
Treatment is safe, inexpensive, and effective:- Supplementation: Over-the-counter vitamin D3 (cholecalciferol) taken daily is the mainstay; significant deficiency is often treated first with several weeks of higher prescription-strength dosing, then a maintenance dose.
- Dietary sources: Fatty fish, egg yolks, and fortified milk and cereals contribute modest amounts.
- Sensible sun exposure: Brief, regular time outdoors helps, balanced against skin cancer risk — an important consideration in Florida.
- Treating the underlying cause: Malabsorption and kidney disease require tailored management, and levels are rechecked after about three months to confirm correction.
Complications
Because vitamin D governs how much calcium the body can absorb, prolonged deficiency has consequences well beyond a low lab value:- Osteomalacia: Softening of adult bone that causes deep, aching bone pain, tenderness over the ribs and hips, and in advanced cases small stress fractures.
- Secondary hyperparathyroidism: When calcium absorption falls, the parathyroid glands work harder and draw calcium out of the skeleton, a pattern that overlaps with hyperparathyroidism and accelerates bone loss.
- Falls and fractures: Weakness in the hip and thigh muscles combined with thinner bone raises fracture risk, a particular concern for older adults living independently.
- Rickets in children: Severe deficiency during growth causes bowed legs, delayed milestones, and skeletal deformity — uncommon today, but still seen.
Correcting Vitamin D Without Overdoing It
Vitamin D deficiency is one of the easiest diagnoses to treat and one of the easiest to treat badly, because both underdosing and megadosing are common. The practical questions are which product, how much, when to recheck, and when a low level is a clue to something else.
Which product and how to take it
Vitamin D3 raises and holds the blood level more reliably than D2 and is the usual choice for daily maintenance; the prescription-strength weekly capsule used for repletion is often D2, which works well for that purpose. Either is absorbed better with a meal that contains some fat. For adults the recommended daily intake is 600 to 800 international units depending on age, with an upper limit of 4,000 for routine unsupervised use, and the maintenance dose after repletion usually falls within that range. Products at many times that amount are for supervised treatment of a documented deficiency.
When to recheck, and when to stop checking
The blood level is rechecked about three months after starting treatment, which is how long it takes to reach a steady state; testing sooner shows an incomplete result. Once the level is in range on a stable dose, repeated testing adds little unless the dose or absorption changes. Chasing a high level is not the goal; above the normal range there is no added bone benefit and a rising risk of high calcium. Our vitamin D in the Sunshine City article covers both ends of that range.
Calcium and bone medication go with it
Vitamin D only helps bone when there is calcium to absorb, and most adults do better meeting the calcium target from food rather than supplements. Anyone starting alendronate or another bone medication for osteoporosis or osteopenia has vitamin D corrected first, because those drugs work poorly and can drop calcium when the level is low. Our bone health after 50 guide sets out the rest of the plan.
Why more sun is not the answer here
The same ultraviolet exposure that makes vitamin D drives melanoma and other skin cancers, and Florida's UV index is high most of the year, so the trade is a poor one. Sunscreen and shade remain the rule, and a supplement corrects the deficiency more cheaply, predictably, and safely. Our Florida sun safety guide explains how to protect skin without giving up the outdoors.
When a low level is a clue to something else
A deficiency that does not correct with reasonable dosing suggests a reason it is not being absorbed: celiac disease, Crohn's disease, prior weight-loss surgery, or medications such as some seizure drugs and long-term steroids that speed its breakdown. A low vitamin D with a high calcium is checked for primary hyperparathyroidism before supplementing, which can push the calcium higher. Advanced chronic kidney disease needs the activated form, which is managed differently. Our endocrine team evaluates the cases that do not respond. The NIH Office of Dietary Supplements and MedlinePlus publish patient guidance on intake and testing.
If you have been taking vitamin D for months without a recheck, or a low level was never explained, schedule a visit.