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What is vitamin D deficiency?
A vitamin D deficiency is defined as a situation where blood levels fall below the threshold required for the body to function properly. This level is measured by testing for 25-hydroxyvitamin D, the storage form produced by the liver.
Medical terminology distinguishes between two conditions.Insufficiency refers to a sub-optimal level, which is common and often asymptomatic. Deficiency corresponds to a more pronounced shortfall, which may have consequences for bone and muscle health.
This distinction is important, as insufficiency affects a very large proportion of the adult population in France, whilst a confirmed deficiency remains rarer. In both cases, only a blood test can provide a definitive diagnosis: symptoms alone are never sufficient to make a diagnosis.
Symptoms of vitamin D deficiency
The difficulty with vitamin D deficiency lies in the non-specific nature of its symptoms. None of the signs are unique to this condition, and each one can be explained by around ten other causes. This explains why it so often goes unnoticed for months.
Fatigue and low energy
This is the most common reason for seeking medical advice. It is a persistent form of fatigue, present upon waking, which does not subside after a full night’s sleep. It sets in gradually, which makes it all the more difficult to identify: people eventually come to regard it as normal.
This fatigue is often put down to work, stress or the time of year. However, the fact that it frequently appears in late autumn, just as the reserves built up over the summer begin to run out, is worth noting.
Bone and muscle pain
Vitamin D helps maintain normal muscle function, and muscle tissue has its own receptors for this vitamin. Insufficient levels frequently result in:
- generalised muscle weakness, particularly in the thighs and shoulders
- muscle pain with no apparent cause, unrelated to physical exertion
- dull bone pain, often localised in the lower back, pelvis or legs
- prolonged recovery after training in active individuals
In athletes, these symptoms are almost always attributed to the training load. This is a possibility that should not be dismissed too quickly if the symptoms persist despite a reduction in training volume.
Recurrent infections
Vitamin D contributes to the normal functioning of the immune system. Increased susceptibility to winter infections, a succession of colds or a prolonged recovery period are among the reasons for investigating vitamin D status.
Here again, the seasonal correlation is striking: the period when vitamin D synthesis via the skin is non-existent in our latitudes is also the time when the risk of infection is at its highest.
Other, less well-known signs
Several symptoms are less commonly linked to vitamin D, even though they regularly feature in clinical descriptions: hair loss, delayed wound healing, cramps, a feeling of mental fog and joint pain.
In infants and young children, severe deficiency can lead to rickets, characterised by impaired mineralisation of the developing skeleton. In adults, the equivalent condition isosteomalacia. These conditions require medical management only.
The causes of vitamin D deficiency
Vitamin D deficiency does not occur by chance: it results from three underlying factors that are particularly prevalent in Northern and Western European countries.
Lack of exposure to sunlight
This is the main cause. The skin can only produce vitamin D3 when exposed to UVB rays, and in our latitudes these are too weak from October to March to allow for significant synthesis. For nearly half the year, therefore, vitamin D production in the skin is virtually nil, regardless of how much time is spent outdoors.
Added to this are modern lifestyles: working indoors, travelling by car, the routine use of sun protection and limited skin exposure for much of the year.
Structurally low dietary intake
Very few foods contain significant amounts of vitamin D. Oily fish and cod liver oil are the only truly rich sources, followed at a considerable distance by egg yolk and certain mushrooms.
As a result, meeting the nutritional reference value of 15 µg per day through diet alone requires regular consumption of oily fish, which most people do not eat. Details of the sources and their vitamin D content are set out in our guide to foods rich in vitamin D.
Individual factors
Even with identical exposure and diet, not everyone produces the same amount of vitamin D. Several factors come into play:
- skin pigmentation: darker skin requires a longer exposure time to synthesise the same amount
- age: the skin’s ability to synthesise vitamin D gradually decreases over the years
- excess weight: as vitamin D is fat-soluble, it is distributed throughout body fat, which reduces its availability in the blood
- intestinal absorption disorders, which limit the absorption of dietary intake
- certain medications, which accelerate its metabolism
Who is most at risk of deficiency
Certain groups have multiple risk factors and warrant particular attention:
- older people, due to reduced vitamin D synthesis in the skin and often reduced exposure to sunlight
- infants and young children, for whom supplementation is usually provided as part of paediatric care
- pregnant or breastfeeding women, whose requirements increase
- people with dark skin living in regions with little sunshine
- people with very little exposure to the sun, whether due to their lifestyle, occupation or for medical reasons
- people with chronic liver, kidney or bowel disease
During the menopause, this issue is also particularly acute, as the drop in hormone levels is accompanied by a weakening of bone mass. Our article on supplements for the menopause addresses all these needs.
How do you know if you’re deficient?
The only reliable way is a blood test for 25-hydroxyvitamin D, prescribed by a doctor. This test measures the ‘storage’ form ofthe vitamin, which truly reflects your status over the past few weeks.
There are two points worth noting before going ahead with this. Firstly, this test is not automatically reimbursed: coverage is restricted to certain specific clinical situations. Secondly, interpreting the results is the doctor’s responsibility: the thresholds vary depending on the reference standards and must be assessed in the context of your personal circumstances, your treatments and your medical history.
Taking supplements blindly on the basis of vague symptoms is not a good approach. Conversely, waiting for a blood test result to ensure your intake meets the nutritional reference values doesn’t make much sense either, particularly in winter.
How to correct a vitamin D deficiency
Correction relies on three complementary approaches; none is sufficient on its own in most situations.
- Sun exposure: around 15 to 20 minutes a day, exposing the face and forearms, outside the hours of peak sunlight, during the warmer months. This is the most effective method, but it is ineffective from October to March.
- Diet: regular consumption of oily fish, two to three times a week, significantly improves intake but does not fully meet requirements.
- Supplementation: this steps in when the first two approaches are not sufficient, particularly during periods of low sunlight.
Regarding dosage selection, our guide on vitamin D3 and choosing the right dosage sets out the guidelines for each individual’s profile. As for when to take it, our article on vitamin D: morning or evening distinguishes between common misconceptions and what really matters.
A confirmed deficiency, diagnosed by a blood test, requires medical management and should not be treated with self-medication. The loading doses prescribed in such cases are entirely different from the doses provided by a dietary supplement.
Frequently Asked Questions
What are the symptoms of a vitamin D deficiency?
A vitamin D deficiency mainly manifests as persistent tiredness, muscle weakness, bone pain or generalised aches, and increased susceptibility to infections. These may be accompanied by hair loss, cramps and delayed wound healing.
These symptoms have one thing in common: they are non-specific. Each can be explained by numerous other causes, making it difficult to identify a deficiency without a blood test. If several of these symptoms occur together, particularly towards the end of winter, it is advisable to speak to your doctor.
How long does it take to raise your vitamin D levels?
Vitamin D levels are gradually restored over several weeks. Taking the supplement for just a few days does not produce a measurable change in blood levels, regardless of the dose used.
The actual time taken depends on your starting level, the dosage used and, above all, how regularly you take it. Only a follow-up blood test, arranged with your doctor, allows you to objectively monitor this progress.
Is a vitamin D deficiency serious?
A moderate deficiency is common and can be easily corrected, but a severe and prolonged deficiency is not to be taken lightly: it affects bone mineralisation and muscle function.
In children, a severe deficiency can lead to rickets in the growing skeleton; in adults, to osteomalacia. These severe forms have become rare in France, precisely thanks to screening and supplementation, but they serve as a reminder of why the issue deserves to be taken seriously.
Can a deficiency be corrected through diet alone?
It is difficult, and rarely sufficient in cases of established deficiency. There are few foods rich in vitamin D, and meeting the nutritional reference value through diet alone requires a consumption of oily fish that few people can maintain over time.
Diet remains a useful tool for maintaining vitamin D levels and preventing the condition from worsening, but it is best used to complement sun exposure and, if necessary, supplementation.
Which vitamins should be taken in combination to tackle persistent fatigue?
Vitamin D is not the only vitamin linked to fatigue. B vitamins play a role in energy metabolism and the normal functioning of the nervous system, whilst magnesium helps reduce fatigue and is involved in converting vitamin D into its active form.
Our selection of B-complex vitamin supplements and our guide to the best vitamin and mineral supplements enable you to put together a well-rounded regimen. However, if fatigue persists over a long period, you should seek medical advice, as there are many possible causes and vitamin D is just one of them.
Vitamin D deficiency is one of the most common nutritional deficiencies, and its symptoms are subtle enough to be overlooked for months. If several of these signs apply to you, speak to your doctor: a blood test will provide a clear answer where symptoms merely offer a hint. To understand the full role of this vitamin, our guide ‘Vitamin D: benefits, deficiency and when to take it’ covers the subject in detail.
