Vitamin D Deficiency in Adults: Symptoms and Solutions

Vitamin D deficiency is one of the most widespread nutritional deficits in France: according to ANSES, around 80% of French adults have insufficient levels at least part of the year. Yet the symptoms are often attributed to stress or transient fatigue, delaying diagnosis.

What Is the Role of Vitamin D in the Body?

Vitamin D is a fat-soluble vitamin synthesized primarily by the skin under the effect of solar ultraviolet B (UVB) radiation. Diet provides only a minority fraction of daily requirements.

Its best-known role concerns bone health: vitamin D promotes intestinal absorption of calcium and phosphorus, two minerals essential for skeletal and dental mineralization. But its action extends well beyond this.

Vitamin D acts as a hormone by binding to receptors present in more than 30 different tissues. It modulates immune system function, regulates inflammation, supports muscle function and contributes to nervous system balance. Its presence determines the expression of dozens of genes, which explains the diversity of clinical manifestations observed in deficiency.

What Are the Causes of Vitamin D Deficiency in Adults?

Vitamin D deficiency does not result solely from poor diet. Three main factors explain its high prevalence.

Insufficient Sun Exposure

In France, between October and March, the angle of solar rays is too low to trigger cutaneous vitamin D synthesis. Even in good weather, modern lifestyles (remote work, indoor activities, sunscreen use) greatly limit this production.

A daily exposure of 15 to 30 minutes in full sun, between 11am and 3pm, of arms and face without sunscreen, is theoretically sufficient in summer to cover daily needs for fair skin. In practice, few adults achieve this regularly.

A Diet Poor in Vitamin D

Foods rich in vitamin D are few and often absent from French dietary habits: oily fish (smoked salmon, herring, mackerel, sardines, canned tuna), calf liver, egg yolks. Plant sources remain negligible. A vegan diet is particularly associated with deficiency risk.

Individual Risk Factors

Certain profiles present increased vulnerability:

• People with darker skin: melanin significantly reduces cutaneous synthesis
• People who are overweight: vitamin D is stored in adipose tissue, reducing its serum availability
• Older adults: cutaneous synthesis naturally decreases with age
• People with intestinal malabsorption (Crohn’s disease, bariatric surgery)
• People with kidney or liver insufficiency impairing vitamin D activation

What Are the Symptoms of Vitamin D Deficiency in Adults?

The symptoms of vitamin D deficiency are varied, progressive and non-specific. They develop over several weeks or months before being clearly identified.

Persistent Fatigue and Low Energy

Chronic fatigue is the most frequently reported symptom in adults with deficiency. It manifests as exhaustion disproportionate to effort, difficulty recovering and persistent lack of energy even after a full night’s sleep.

This link is explained by vitamin D’s role in mitochondrial function and cellular energy production. Several controlled trials show significant improvement in fatigue following supplementation in deficient subjects.

Bone and Muscle Pain

Diffuse pain in the back, legs, hips and ribs is a characteristic symptom of vitamin D deficiency. It results from impaired bone mineralization, a condition termed osteomalacia in adults.

Muscle pain, cramps and feelings of muscle weakness frequently accompany these bone manifestations. Vitamin D is involved in muscle contraction and skeletal muscle protein synthesis.

Bone Fragility and Fracture Risk

Prolonged deficiency accelerates bone demineralization and is a recognized risk factor for osteoporosis. This risk is particularly high in women after menopause.

Meta-analyses show that a blood vitamin D level above 50 nmol/L significantly reduces hip fracture incidence in at-risk populations. Dietary supplements combining vitamin D3 and calcium are widely prescribed for prevention in the elderly.

Weakened Immunity and Recurrent Infections

Vitamin D modulates immune response by activating T cells and regulating cytokine production. Deficiency is associated with increased susceptibility to winter respiratory infections: colds, flu, bronchitis.

To strengthen the immune system naturally, maintaining adequate vitamin D levels is a fundamental lever, alongside a balanced diet and regular physical activity.

Mood Disturbances and Seasonal Low Mood

Vitamin D receptors are present in many brain areas, including those involved in mood regulation. Deficiency is associated with increased risk of seasonal depression and anxiety states.

The correlation is particularly marked in winter, when cutaneous synthesis is nil. Combining vitamin D deficiency correction with relaxation techniques or essential oils for stress and anxiety fits within a holistic well-being approach.

Who Is Most at Risk of Vitamin D Deficiency?

Several profiles accumulate risk factors:

• Adults over 65: in France, more than 80% have deficiency. Cutaneous synthesis decreases with age and outdoor activities diminish.
• Pregnant and breastfeeding women: requirements increase during pregnancy. Maternal deficiency affects infant bone development. HAS recommends systematic supplementation.
• People with darker skin: melanin reduces cutaneous synthesis 6 to 8 fold. In a country with limited sunshine, this factor represents increased deficiency risk.
• Obese individuals: storage of vitamin D in adipose tissue reduces its availability in the bloodstream.

How to Diagnose Vitamin D Deficiency?

Diagnosis relies on a blood test measuring 25-hydroxyvitamin D (25-OH-D), prescribed by a doctor. Standard interpretation thresholds are:

• Below 25 nmol/L: severe deficiency, osteomalacia risk
• Between 25 and 50 nmol/L: insufficiency
• Between 50 and 125 nmol/L: adequate status
• Above 125 nmol/L: excessive intake, toxicity risk

High-dose supplementation without prior blood testing is not recommended. Vitamin D excess can cause hypercalcemia and kidney disorders. Only a healthcare professional can adapt dosage to test results.

How to Increase Vitamin D Levels Naturally?

Getting Appropriate Sun Exposure

Between April and September, a daily exposure of 15 to 30 minutes (arms and face, without sunscreen), between 11am and 3pm, is generally sufficient to cover daily needs for fair skin. This duration should be extended for medium or darker skin tones.

Outside this period, or when lifestyle limits sun exposure, supplemental intake is necessary.

Adapting Diet to Vitamin D-Rich Foods

The most concentrated vitamin D foods are oily fish (smoked salmon, herring, mackerel, sardines, canned tuna), calf liver and egg yolks. Consuming these foods two to three times per week contributes to regular dietary intake, without correcting confirmed deficiency.

Using Vitamin D3 Dietary Supplements

For confirmed deficiency or preventive supplementation in winter, dietary supplements of vitamin D3 (cholecalciferol) are recommended over vitamin D2 (ergocalciferol), which has lower bioavailability. Usual dosage ranges from 800 to 2,000 IU per day depending on age and deficiency level.

Single high-dose ampoule presentations are discouraged: they generate transient blood peaks and irregular tissue exposure.

For women approaching menopause, vitamin D3 is a pillar of osteoporosis prevention. See the comparison of best menopause supplement brands.

Frequently Asked Questions