Vitamin D Deficiency

What vitamin D does

Vitamin D is a hormone-like vitamin. Its main job is helping the gut absorb calcium and phosphate, which bones need to mineralize. It also supports muscle function and has roles in the immune system.

You get it from:

• Sunlight: UVB rays make vitamin D in the skin. This is the main source for most people.
• Food: oily fish (salmon, trout, sardines), egg yolks, and fortified milk, plant milks, cereals and orange juice. Few foods naturally contain much.
• Supplements: vitamin D3 (cholecalciferol) or D2 (ergocalciferol).

The liver converts vitamin D to 25-hydroxyvitamin D, which is what blood tests measure. The kidneys then make the active form, calcitriol.

Vitamin D levels explained

Some laboratories and specialists use 30 ng/mL (75 nmol/L) as a target, which is why you may see different cutoffs. The 2024 Endocrine Society guideline does not recommend routine testing in healthy people without symptoms or risk factors.

Symptoms of vitamin D deficiency

Mild deficiency usually causes no symptoms. As levels fall further:

• Bone pain and tenderness, often in the lower back, pelvis, hips, legs and ribs.
• Muscle weakness, especially in the hips and thighs, causing difficulty climbing stairs or rising from a chair and a waddling walk.
• Muscle aches and cramps.
• Fatigue.
• Fractures from minor falls.
• Low mood is associated with low vitamin D, but supplements have not consistently been shown to improve depression.

In children

• Rickets: bowed legs or knock knees, thickened wrists and ankles, a bumpy ribcage, delayed walking and late tooth eruption.
• Poor growth.
• Seizures or spasms from low calcium in infants.

In adults

• Osteomalacia: soft bones with diffuse aches and weakness.
• Worsening of osteoporosis and fracture risk.

Causes and risk factors

Not enough sunlight

• Living far from the equator, especially in winter (in much of the US north of about 37 degrees latitude, winter sun is too weak to make vitamin D).
• Spending most time indoors, being housebound or in a care home.
• Covering most of the skin for cultural or religious reasons.
• Consistent use of high-SPF sunscreen.

Body factors

• Darker skin, which needs more sun to make the same amount.
• Older age: skin makes less vitamin D.
• Obesity: vitamin D is stored in fat and less is available in the blood.
• Pregnancy and breastfeeding.
• Exclusively breastfed infants, because breast milk is low in vitamin D.

Medical causes

• Malabsorption: celiac disease, Crohn's disease, cystic fibrosis, weight-loss surgery and liver disease.
• Chronic kidney disease, which reduces activation.
• Medicines: some anti-seizure drugs (phenytoin, carbamazepine, phenobarbital), long-term steroids, rifampin and some HIV medicines speed breakdown.

Diagnosis

A blood test for 25-hydroxyvitamin D confirms deficiency. Doctors may also check:

• Calcium, phosphate and alkaline phosphatase (raised in osteomalacia and rickets).
• Parathyroid hormone: low vitamin D often raises PTH (secondary hyperparathyroidism).
• Kidney and liver function.
• Celiac antibodies if malabsorption is suspected.
• X-rays or a bone density scan when bone disease is suspected.

How much vitamin D you need

Recommended daily intakes (US National Academies):

The tolerable upper intake level for adults is 4,000 IU (100 micrograms) a day from all sources, unless a doctor prescribes more. Breastfed and partly breastfed babies should get 400 IU of vitamin D drops daily, starting in the first few days of life.

Vitamin D deficiency treatment

• Vitamin D3 supplements: usually preferred over D2. For confirmed deficiency, a doctor may prescribe a higher dose for 6 to 12 weeks, then a maintenance dose. Follow your doctor's plan rather than self-treating with very high doses.
• Calcium: adequate calcium from diet (about 1,000 to 1,200 mg a day for adults) or supplements if intake is low.
• Repeat testing: after about 3 months for severe deficiency or malabsorption.
• Treating the cause: such as celiac disease.
• Special forms: calcitriol or similar active vitamin D for kidney failure or certain rare disorders.

Take vitamin D with a meal containing some fat for better absorption.

Too much vitamin D

Vitamin D toxicity is almost always from supplements, not sun or food. It raises calcium levels (hypercalcemia), causing nausea, vomiting, thirst, frequent urination, confusion, kidney stones and kidney damage. Avoid taking several products that each contain vitamin D without checking the total.

Prevention

• Get modest sun exposure on arms and face in spring and summer, without burning. Needs vary with skin type and location.
• Eat oily fish and fortified foods.
• Take a daily supplement of 600 to 800 IU if you are at risk, especially in winter.
• Give vitamin D drops to breastfed babies.

Frequently asked questions

What are the signs of low vitamin D?

Mild deficiency often has no signs. More severe deficiency causes bone pain (especially in the back, hips and legs), muscle weakness and aches, fatigue, and fractures after minor falls. In children, it can cause bowed legs, swollen wrists, delayed walking and poor growth (rickets).

What causes vitamin D deficiency?

The main causes are limited sun exposure, darker skin, older age, obesity, and a diet low in vitamin D. Medical causes include malabsorption (celiac disease, Crohn's disease, weight-loss surgery), kidney and liver disease, and medicines such as some anti-seizure drugs and long-term steroids.

What is a normal vitamin D level?

Most experts consider a 25-hydroxyvitamin D level of 20 ng/mL (50 nmol/L) or higher adequate for bone health. Below 12 ng/mL (30 nmol/L) is deficient. Some labs use 30 ng/mL (75 nmol/L) as the lower limit of normal. Levels above 50 ng/mL offer no proven extra benefit.

How much vitamin D should I take daily?

Adults aged 19 to 70 need 600 IU (15 micrograms) a day, and people over 70 need 800 IU (20 micrograms). Infants need 400 IU. If you are deficient, your doctor may prescribe a higher dose for a few weeks. Do not exceed 4,000 IU a day without medical advice.

How long does it take to correct vitamin D deficiency?

With treatment, blood levels usually rise within 6 to 12 weeks. Symptoms such as bone pain and muscle weakness can take several months to improve, and bone healing in osteomalacia may take 6 months or longer. A repeat blood test after about 3 months confirms progress.

Can low vitamin D cause tiredness?

Fatigue is commonly reported with low vitamin D, especially when deficiency is severe. However, tiredness has many causes, and supplements help mainly when a real deficiency is present. If you feel persistently tired, a doctor can check vitamin D along with other causes such as anemia and thyroid problems.