Vitamin D Deficiency: Signs, Causes and What to Do

Vitamin D has a reputation as the sunshine vitamin, and it earns it: your skin makes most of it when exposed to sunlight. But many people in the US spend most daylight hours indoors, and only a handful of foods contain much of it naturally.

This article explains what vitamin D does, the signs that you may be low, who is most at risk, how deficiency is confirmed, and what you can do about it. It can help you decide whether to ask for a test, but only a blood test and your doctor can tell you whether you are deficient.

What does vitamin D do in your body?

Vitamin D helps your body absorb calcium and phosphorus from food, which you need to build and keep strong bones. According to the NIH Office of Dietary Supplements (ODS), it also plays roles in muscle function, the immune system, and reducing inflammation.

Without enough vitamin D, your body cannot absorb calcium well, even if you eat plenty of it. Over time, it pulls calcium from your bones, which weakens them.

Vitamin D comes from three places: sunlight on your skin, food, and supplements. It is one piece of the bigger picture covered in our nutrition and wellness guide.

What are the signs of vitamin D deficiency?

Many people with low vitamin D have no symptoms at all, especially early on. When signs do appear, they are often vague and easy to blame on something else.

Sign	What it may feel like	Why it happens
Bone pain	A dull ache in the lower back, hips, legs, or ribs	Poor calcium absorption softens bone
Muscle weakness	Trouble climbing stairs or getting up from a chair	Vitamin D supports muscle function
Muscle aches or cramps	General aching, sometimes spasms	Low calcium and vitamin D affect muscles
Fatigue	Persistent tiredness	Linked with low levels, though many things cause it
Frequent fractures	Bones breaking from minor falls	Weakened bones over time
Rickets (children)	Bowed legs, delayed growth, bone pain	Bones do not harden properly

Because fatigue and aches have so many possible causes, they are not enough on their own to say you are deficient. If you are worn out much of the time, it is worth looking at the wider list in why you feel tired all the time.

What happens with severe, long-term deficiency?

Severe deficiency over months or years can cause rickets in children and osteomalacia in adults. Osteomalacia is a softening of the bones that causes bone pain and muscle weakness. Long-term low vitamin D also contributes to osteoporosis, a condition in which bones become thin and fragile, raising the risk of fractures as you age.

Who is most at risk of low vitamin D?

Your risk is higher if your skin makes less vitamin D, you absorb less from food, or you need more than usual. ODS lists several groups at higher risk.

• People who get little sun. Those who are homebound, work indoors, cover most of their skin, or live in northern states, especially in winter.
• People with darker skin. Higher melanin, the pigment that colors skin, reduces how much vitamin D the skin makes from sunlight.
• Older adults. Skin makes vitamin D less efficiently with age, and older adults often spend more time indoors.
• Breastfed infants. Breast milk is low in vitamin D, so pediatric guidance recommends a supplement for breastfed babies.
• People with obesity. Body fat holds onto vitamin D, so more may be needed to reach the same blood level.
• People with conditions that limit fat absorption. Such as Crohn's disease, celiac disease, cystic fibrosis, or after gastric bypass surgery.
• People taking certain medicines. Some anti-seizure drugs, steroids, and weight-loss drugs can lower vitamin D levels.

How is vitamin D deficiency diagnosed?

Vitamin D deficiency is diagnosed with a blood test that measures 25-hydroxyvitamin D, the main form of vitamin D circulating in your blood. Symptoms alone cannot confirm it.

Results are reported in nanograms per milliliter (ng/mL) or nanomoles per liter (nmol/L). According to ODS, levels below 12 ng/mL (30 nmol/L) are linked with deficiency, and levels of 20 ng/mL (50 nmol/L) or higher are considered adequate for bone health in most people. Experts do not fully agree on the ideal level, so your doctor will interpret your result alongside your health and risk factors.

Blood level (25-hydroxyvitamin D)	General interpretation (ODS)
Below 12 ng/mL (30 nmol/L)	Associated with deficiency
12 to 20 ng/mL (30 to 50 nmol/L)	Potentially inadequate for some people
20 ng/mL (50 nmol/L) and above	Generally adequate for bone health
Above 50 ng/mL (125 nmol/L)	Possibly linked with adverse effects

Routine screening of people without symptoms or risk factors is not generally recommended. Ask your doctor about testing if you are in a higher-risk group or have symptoms such as bone pain or muscle weakness.

How much vitamin D do you need?

Most adults need 600 IU (15 micrograms) of vitamin D a day, and adults over 70 need 800 IU (20 micrograms), according to ODS. Infants up to 12 months need 400 IU (10 micrograms).

The tolerable upper intake level, the most you can take daily without likely harm, is 4,000 IU (100 micrograms) for adults and teens. Your doctor may prescribe a higher dose for a short time to correct a confirmed deficiency, but do not take high doses on your own.

Food labels list vitamin D as a percent Daily Value, which is set at 20 micrograms (800 IU) for adults and children 4 years and older.

Which foods contain vitamin D?

Few foods are naturally rich in vitamin D, so fortified foods supply much of what Americans get from their diet. Fatty fish is the best natural source.

• Fatty fish: salmon, trout, sardines, mackerel, and canned light tuna
• Cod liver oil
• Fortified milk: most US cow's milk is fortified with vitamin D
• Fortified plant milks: many soy, almond, and oat milks (check the label)
• Fortified breakfast cereals and some orange juice
• Egg yolks, in small amounts
• Mushrooms exposed to UV light, labeled as such

A varied diet built from whole foods gives you the best chance of covering vitamin D along with other nutrients; see our guide to eating a whole foods diet.

Many people who are low in one nutrient are low in others too, and iron is a common companion; our list of everyday foods rich in iron can help there.

How much sun do you need for vitamin D?

There is no single safe amount of sun that works for everyone. How much vitamin D your skin makes depends on the season, time of day, where you live, cloud cover, skin color, and how much skin is exposed.

Sunscreen and glass windows block much of the UVB light your skin needs. However, the American Academy of Dermatology advises against getting vitamin D from sun exposure or tanning beds because ultraviolet light raises skin cancer risk. Food and supplements are the safer route, and you should keep using sun protection.

How is vitamin D deficiency treated?

Deficiency is usually treated with vitamin D supplements, sometimes at a higher dose for several weeks, followed by a maintenance dose. Your doctor will choose the dose based on your blood level, age, and health conditions.

Supplements come as vitamin D2 (ergocalciferol) or vitamin D3 (cholecalciferol). Both raise blood levels, though D3 may raise them a little more and keep them up longer. Taking vitamin D with a meal that contains some fat may help absorption.

If your bones or muscles have been affected, your doctor may also check calcium, recommend weight-bearing exercise, or test bone density. Strength work helps protect both, as explained in strength training after 40.

Can you take too much vitamin D?

Yes. Too much vitamin D, almost always from high-dose supplements rather than food or sun, can raise blood calcium. ODS notes this can cause nausea, vomiting, muscle weakness, confusion, pain, loss of appetite, dehydration, excessive urination and thirst, and kidney stones. Very high levels can cause kidney failure and irregular heartbeat. Stay at or below 4,000 IU a day unless your doctor directs otherwise.

When should you see a doctor?

See your doctor if you have ongoing bone pain, muscle weakness, or fractures from minor falls, or if you are in a high-risk group and have never been tested. Your primary care provider can order a blood test and look for other causes of your symptoms.

Talk to your doctor before starting supplements if you have kidney disease, kidney stones, sarcoidosis, or take medicines such as thiazide diuretics or steroids, which can interact with vitamin D. A registered dietitian (RD or RDN) can help you plan food sources.

Call 911 or go to the emergency room if you have signs of a broken bone such as a deformed limb or inability to bear weight, or, if you have been taking high-dose vitamin D, severe confusion, a very irregular heartbeat, fainting, or persistent vomiting.

What is the bottom line?

Vitamin D deficiency is common, often silent, and easy to check with a blood test. Bone pain, muscle weakness, and fatigue can be signs, but they have many causes, so testing matters more than guessing.

Most adults need 600 IU a day, or 800 IU over age 70, from fatty fish, fortified foods, and supplements if needed. If you are at higher risk, ask your doctor whether a test makes sense for you.