What Are the Symptoms of Vitamin D Deficiency? Why Is It More Common in Winter?

In brief:

• The body makes most of its vitamin D when sunlight falls on bare skin. The season, the time of day, cloud, age and skin colour all affect how much is made (NIH).
• Severe deficiency causes rickets in children and osteomalacia, with bone pain and muscle weakness, in teenagers and adults (NIH).
• Adults need 15 mcg (600 IU) a day; after 71, 20 mcg (800 IU) (NIH).
• Routine vitamin D testing is not recommended for everyone; a doctor decides whether you need a test (MedlinePlus).
• Too much is harmful too. Toxicity almost always comes from supplements, not from the sun (NIH, NHS).

What does vitamin D do?

Vitamin D helps the body absorb calcium. Calcium is one of the main building blocks of bone. According to the NIH, muscles need vitamin D to move, nerves need it to carry messages between the brain and the body, and the immune system needs it to fight off bacteria and viruses.

Vitamin D comes from three sources (MedlinePlus):

• Sun: the body makes vitamin D when bare skin gets sunlight outdoors. Sunlight through a window does not produce vitamin D (NIH).
• Food: it occurs naturally in only a few foods: oily fish (salmon, sardines, mackerel, trout), fish liver oil, egg yolks, liver and a little in cheese (NIH, NHS). In some countries it is added to milk and breakfast cereals.
• Supplements: available as drops or tablets. Because it dissolves in fat, it is absorbed better when taken with a meal that contains some fat (NIH).

What are the symptoms of vitamin D deficiency?

MedlinePlus lists the symptoms that may be linked to vitamin D deficiency as follows:

• Bone pain
• Muscle weakness or muscle aches
• Soft or misshapen bones
• Weak bones and fractures

According to the NIH, severe deficiency causes rickets in children, in which bones soften and become misshapen, and osteomalacia, with bone pain and muscle weakness, in teenagers and adults.

Worth knowing: online you often see long lists linking vitamin D deficiency to tiredness, hair loss or low mood. Based on the review by the US National Academies, the NIH states that the evidence for effects of vitamin D beyond bone health is insufficient or conflicting. These symptoms can have other causes too.

Why is vitamin D deficiency more common in winter?

According to the NIH, the season, the time of day, the length of the day, cloud, air pollution, age and skin colour all affect how much vitamin D you make from the sun. In winter the days are shorter, sunlight is weaker and people spend less time outdoors.

The NHS says that in the UK you cannot make enough vitamin D from sunlight from October to early March. This period applies to the UK's latitude; Türkiye lies further south. Even so, the factors the NIH lists apply everywhere: less sun and less time outdoors in winter mean less vitamin D.

Vitamin D deficiency is also seen as a common problem in Türkiye. The Ministry of Health started a programme giving free vitamin D to babies in 2005 and a vitamin D support programme for pregnant women in 2011 (Özdemir et al., 2018). The same study reports that the deficiency persisted among pregnant women and their babies in Istanbul despite the programme.

Who is more likely to have vitamin D deficiency?

According to the NIH and MedlinePlus, the risk is higher in these groups:

• Babies fed only breast milk: breast milk alone does not provide enough vitamin D (NIH).
• Older people: the skin's ability to make vitamin D decreases with age (NIH).
• People who get little sun: those who rarely leave home, who cover their bodies and heads outdoors, and who use sunscreen (NIH, MedlinePlus).
• People with darker skin: the darker the skin, the less vitamin D it makes from the sun (NIH).
• People with conditions that affect fat absorption: Crohn's disease, coeliac disease, ulcerative colitis (NIH).
• People with obesity and those who have had weight-loss surgery (NIH)
• People with lactose intolerance or a milk allergy, and those on a vegan diet (NIH)
• People with kidney or liver disease: it becomes harder to turn vitamin D into a form the body can use (MedlinePlus).
• People taking certain medicines: some cholesterol medicines (statins), steroids and weight-loss medicines can affect vitamin D levels (MedlinePlus).

How is vitamin D deficiency diagnosed?

The level of 25-hydroxyvitamin D, or 25(OH)D for short, is measured in the blood. According to MedlinePlus, this is the most accurate way to see whether the body has enough vitamin D. Blood is taken from a vein in the arm; no special preparation is needed. Tell your doctor about any medicines and supplements you take.

According to MedlinePlus, routine vitamin D testing is not recommended for everyone. The test is usually ordered:

• if you have a bone disease or symptoms that may be linked to vitamin D deficiency;
• if you are in one of the risk groups above;
• if you take supplements and your level is being monitored.

We explain which tests to have and when in Tests Every Woman Should Have Once a Year.

The values given by the NIH (labs report results in nmol/L or ng/mL; 1 nmol/L = 0.4 ng/mL):

Cut-offs can differ slightly between laboratories and guidelines. Review your result with the range on your report and with your doctor.

How much vitamin D do you need a day?

The daily amounts recommended by the NIH:

1 microgram of vitamin D equals 40 IU (NHS). Recommendations can differ between countries: the NHS, for example, advises adults in the UK to take 10 mcg a day in the winter months. Talk to your doctor about how much you need, especially in pregnancy and if you have a medical condition.

Is too much vitamin D harmful?

Yes. According to the NIH, vitamin D toxicity raises the calcium level in the blood. In severe cases it can lead to kidney failure, calcification of soft tissues and heart rhythm problems. Toxicity almost always comes from taking too much of a supplement.

The NHS says adults (including pregnant and breastfeeding women) should not take more than 100 micrograms (4,000 IU) a day. You cannot get too much vitamin D from the sun (NHS). According to MedlinePlus, too much can cause nausea, vomiting, poor appetite, constipation, muscle weakness and weight loss.

When should you see a doctor?

• You have bone pain, muscle weakness or an unexplained fracture
• You are in one of the risk groups and have never been tested (for example, if you have a condition that affects fat absorption or have had stomach surgery)
• You are pregnant or planning a pregnancy; ask your doctor about the Turkish Ministry of Health's vitamin D support programme for pregnant women
• You have been taking high-dose vitamin D supplements for a long time and have noticed nausea, constipation or muscle weakness

Frequently asked

Does vitamin D deficiency cause hair loss?

The sources in this article do not list hair loss among the symptoms of vitamin D deficiency. According to the NIH, the evidence for effects of vitamin D beyond bone health is insufficient or conflicting. The NHS, on the other hand, lists hair loss among the less common symptoms of iron deficiency.

Can sunbathing fix vitamin D deficiency?

The sun is the main source of vitamin D, but it also carries a risk of skin cancer. According to MedlinePlus, to raise your level a doctor usually recommends supplements or foods containing vitamin D; this is safer than more sun exposure. Sunlight through a window does not produce vitamin D (NIH).

Do you need to fast for a vitamin D test?

According to MedlinePlus, no special preparation is needed for a vitamin D test. If it is done together with other tests, follow your doctor's or the laboratory's instructions.

D2 or D3?

Both raise vitamin D in the blood. According to the NIH, D3 may raise the level more and for longer. Decide with your doctor which one to use and how much. We explain the different forms in supplements in general in Does Every Food Supplement Have the Same Effect?.