Vitamin D deficiency is one of the most common nutritional problems in the world, and one of the most commonly missed. Over a billion people are estimated to be deficient or insufficient - and a significant proportion of them have no idea.
The symptoms are vague. Fatigue, low mood, muscle weakness, frequent colds. They overlap with stress, poor sleep, and a dozen other things. Most people don't get tested unless a doctor specifically thinks to check. Here's how to recognise it and what to do if you have it.
What Vitamin D Actually Does
Vitamin D isn't really a vitamin. It's a hormone - one your body produces when UV light hits your skin, and which controls over 200 biological processes.
The most well-known roles are bone health (it regulates calcium absorption) and immune function. But vitamin D also plays a part in muscle function, mood regulation, inflammation control, and insulin sensitivity. When levels drop, those systems don't work as well as they should.
Symptoms of Vitamin D Deficiency
These symptoms overlap with a lot of other conditions, which is exactly why deficiency gets missed. If you're experiencing several of them together, a blood test is worth requesting.
Fatigue That Sleep Doesn't Fix
This is the most reported symptom, and the most dismissed. People are told they're tired because of stress, or not sleeping well, or working too hard - and vitamin D is never mentioned.
A 2015 study in the North American Journal of Medical Sciences found a direct correlation between vitamin D levels and reported fatigue severity in otherwise healthy adults. When vitamin D was restored to normal levels, fatigue scores dropped significantly.
Bone or Muscle Pain
Vitamin D is essential for calcium absorption. Without enough of it, bones lose density and become achy - particularly in the back, legs, and ribs.
Muscle weakness and pain are also common, especially in the upper arms and thighs. Some people describe a deep aching feeling in their bones that's hard to localise. In severe cases this progresses to osteomalacia (softening of the bones) in adults, or rickets in children.
Low Mood or Depression
Multiple studies have found associations between low vitamin D and depression, particularly seasonal depression (SAD - seasonal affective disorder). The link makes biological sense: vitamin D receptors are found in areas of the brain associated with mood regulation, and the hormone influences serotonin production.
A 2020 meta-analysis in the Journal of Affective Disorders found that vitamin D supplementation had a modest but consistent positive effect on depressive symptoms across 53 randomised trials.
Frequent Illness or Slow Recovery
Vitamin D plays a direct role in the immune response. It activates T-cells - the white blood cells that fight infection - and regulates inflammatory responses. When levels are low, that activation is slower.
People with low vitamin D consistently show higher rates of respiratory infections in population studies. The UK NHS specifically cites vitamin D supplementation as reducing the risk of acute respiratory infections.
Hair Loss
Not as well-known as the others, but vitamin D deficiency has been linked to alopecia areata - an autoimmune condition that causes patchy hair loss - and to general hair thinning.
Vitamin D receptors in hair follicles appear to play a role in the hair growth cycle. When those receptors aren't being activated, the follicle can go into a prolonged resting phase.
Impaired Wound Healing
If cuts or injuries are taking longer than usual to heal, vitamin D may be a factor. It's involved in the production of compounds that control inflammation and form new skin tissue. Several studies in surgical patients have found that those with adequate vitamin D heal significantly faster post-operation.
Who's Most at Risk?
Some people are much more likely to be deficient than others.
People with limited sun exposure are the obvious group - anyone who works indoors most of the day, lives in a northern climate (above roughly 37 degrees latitude), or wears full-length clothing outdoors consistently.
People with darker skin have more melanin, which reduces the skin's ability to synthesise vitamin D from sunlight. Darker-skinned individuals living in low-sunlight countries are at particularly high risk.
Older adults synthesise vitamin D from sunlight less efficiently as they age. The kidneys also become less effective at converting vitamin D to its active form.
Overweight individuals - vitamin D is fat-soluble and can become sequestered in fat tissue, making it less available to the rest of the body even at seemingly adequate intake levels.
People who avoid dietary fat - vitamin D is only absorbed alongside fat. Very low-fat diets can impair absorption from both food and supplements.
How to Get Your Vitamin D Levels Tested
Ask your GP for a 25-hydroxyvitamin D blood test. This is the standard test for vitamin D status.
The results come back in nmol/L (UK/Europe) or ng/mL (US). Below 30 nmol/L (12 ng/mL) is considered deficient. Below 50 nmol/L (20 ng/mL) is insufficient. Most researchers working in this area consider 75-125 nmol/L (30-50 ng/mL) to be optimal.
How to Raise Your Vitamin D Levels
Sunlight
The most natural route. Bare skin (arms and face at minimum) exposed to direct midday sun for 15-30 minutes produces meaningful vitamin D. But this only works when the UV index is 3 or above - which means it's essentially useless in the UK and Canada from October to March.
Food
Very few foods contain useful amounts of vitamin D. Fatty fish (salmon, sardines, mackerel) and egg yolks are the main dietary sources. Fortified foods (some milks, cereals, and orange juices) contribute a little. But getting enough from food alone without supplements is genuinely difficult.
Supplements
For most people in northern latitudes, supplementation is the practical solution - especially through winter. The NHS recommends 10mcg (400 IU) daily for adults. Many researchers and clinicians consider this too low, particularly for people who are already deficient, and suggest 1,000-2,000 IU daily is a safer target for most adults.
The form matters. Vitamin D3 (cholecalciferol) is more effective than D2 at raising and maintaining blood levels. Take it with a meal containing fat for best absorption.
If you've tested deficient, your GP may prescribe a higher loading dose (typically 20,000-50,000 IU weekly for a set period) to restore levels quickly before moving to a maintenance dose.