Vitamin D Deficiency: Symptoms, Causes and What to Do

## What is vitamin D deficiency

Vitamin D is not technically a vitamin — it behaves more like a hormone. Your body produces it when your skin is exposed to sunlight, and it plays a central role in bone strength, immune function, muscle performance, and mood regulation. When levels fall too low, these systems quietly begin to underperform — often without a single obvious symptom at first.

Unlike most vitamins, dietary sources of vitamin D are limited. Very few foods contain meaningful amounts. This makes sunlight the primary source for most people, which is why deficiency has become so widespread in modern, indoor-heavy lifestyles.

## Common symptoms

Vitamin D deficiency rarely announces itself dramatically. More often, it presents as a cluster of vague, easy-to-dismiss complaints that gradually worsen over months.

Persistent fatigue is one of the most common signs — the kind that does not improve with rest and affects daily function. Bone pain or a deep aching in the legs, back, or hips is another hallmark, caused by insufficient mineralisation of bone tissue. Muscle weakness, particularly in the thighs and upper arms, can make climbing stairs or lifting feel harder than they should.

Low mood, difficulty concentrating, and a general sense of flatness are associated with low vitamin D — the connection to mood appears to involve receptors in the brain. Frequent colds, respiratory infections, or slow recovery from illness may reflect impaired immune function. Slow wound healing has also been reported in people with confirmed deficiency.

## Causes

Limited sun exposure is the leading cause globally. People who work indoors, live in northern or southern climates with long winters, cover their skin, or spend most of their time away from direct outdoor light are at particular risk.

Diet contributes, but rarely enough to compensate. Fatty fish such as salmon and mackerel, egg yolks, and fortified dairy or cereals contain some vitamin D — but typical dietary intake covers only a fraction of what the body needs.

Absorption problems play a significant role in some people. Conditions such as Crohn's disease, coeliac disease, and other gastrointestinal disorders reduce the gut's ability to absorb fat-soluble vitamins including vitamin D. Obesity can also affect how the body stores and activates it. Kidney and liver disorders impair the conversion of vitamin D into its active form.

## Who is at risk

Anyone can become deficient, but some groups face a much higher risk. Older adults produce less vitamin D in the skin even when exposed to sunlight, and tend to spend less time outdoors. People with darker skin tones have more melanin, which reduces the skin's synthesis rate — this is not abnormal, it simply means more sun exposure time is needed.

Pregnant and breastfeeding women have higher requirements, as do exclusively breastfed infants who receive no supplementation. People with thyroid conditions, autoimmune disorders, or chronic kidney or liver disease may also have impaired vitamin D metabolism. Those already experiencing fatigue from iron-deficiency anaemia are sometimes found to have concurrent low vitamin D.

## When to test

If you recognise several of the symptoms described here — particularly fatigue, bone pain, muscle weakness, or low mood — it is worth speaking with a doctor about testing. The standard test is a 25-hydroxyvitamin D (25-OH D) blood test, which reflects your body's stored vitamin D level.

Testing is especially appropriate if you have risk factors such as limited sun exposure, a malabsorption condition, a family history of osteoporosis, or if you are planning a pregnancy. Repeated illness without a clear cause is also a reasonable prompt to check vitamin D alongside other markers.

## Understanding your result

Vitamin D levels are typically reported in nanomoles per litre (nmol/L) or nanograms per millilitre (ng/mL). Broadly, results fall into three zones: deficient, insufficient, and sufficient — though exact cut-offs vary slightly by laboratory and guideline. A deficient result means levels are low enough to affect body function. An insufficient result means levels are below the optimal range but not severely low. Your doctor will interpret your result in the context of your symptoms, health history, and any other conditions you have.

## What to do next

If you suspect a deficiency, the most important step is to get tested rather than guess. Vitamin D levels vary widely between individuals, and the appropriate response depends on where your result falls.

Practical steps that apply broadly include increasing time outdoors during midday hours when UV intensity is highest, including more vitamin D-containing foods in your diet, and considering whether lifestyle factors — such as working entirely indoors or living at high latitude — are limiting your exposure year-round.

Vitamin D does not act in isolation. Fatigue, for example, can also stem from iron deficiency, thyroid dysfunction, or autoimmune conditions — all of which can coexist with low vitamin D. A basic blood panel that checks vitamin D alongside iron, thyroid hormones, and inflammatory markers often gives a more complete and actionable picture than testing a single value alone.

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