Vitamin D Deficiency: Symptoms, Causes, and Best Food Sources

The Sunshine Vitamin Problem

Vitamin D is unique among nutrients because your body can manufacture it when your skin is exposed to ultraviolet B (UVB) radiation from sunlight. In a process that mirrors industrial vitamin synthesis, UVB rays convert a cholesterol compound called 7-dehydrocholesterol in your skin into previtamin D3, which is then transformed into vitamin D3 through body heat. Your liver converts D3 into 25-hydroxyvitamin D (the form measured in blood tests), and your kidneys activate it into calcitriol, the hormone form that influences hundreds of biological processes.

In theory, regular sun exposure should prevent deficiency in most people. In practice, an estimated 1 billion people worldwide have insufficient vitamin D levels, making it one of the most widespread nutritional deficiencies in human history. The reasons are numerous and compounding. Modern life keeps most people indoors during peak UVB hours (roughly 10am-3pm). People living above approximately 37 degrees latitude, which includes most of Europe, Canada, Russia, northern China, and the northern United States, receive insufficient UVB radiation from October through March to produce meaningful vitamin D. Sunscreen with SPF 30, while important for skin cancer prevention, blocks 97% of UVB radiation. Darker skin contains more melanin, which acts as a natural sunscreen and requires 3-5 times more sun exposure to produce equivalent vitamin D. Elderly people synthesize vitamin D less efficiently even with adequate sun exposure due to thinner skin and reduced 7-dehydrocholesterol levels. And clothing covering most of the body further limits UVB exposure.

The result is widespread deficiency across every continent, climate, age group, and demographic, even in sunny countries where you would not expect it.

What Vitamin D Does in Your Body

Vitamin D's most established role is in calcium homeostasis and bone health. It dramatically enhances calcium absorption in the gut: without sufficient vitamin D, your body absorbs only 10-15% of dietary calcium, compared to 30-40% with adequate levels. Severe deficiency causes rickets in children (soft, deformed bones) and osteomalacia in adults (bone pain and weakness). Less severe deficiency contributes to osteoporosis and increased fracture risk, particularly in elderly populations.

But the discovery of vitamin D receptors in virtually every tissue type in the human body, including the brain, heart, immune cells, pancreas, muscles, and intestines, has revealed that this nutrient's influence extends far beyond bones. Vitamin D modulates immune function, with multiple studies linking deficiency to increased susceptibility to respiratory infections, autoimmune conditions, and prolonged illness duration. It influences muscle function, and deficiency is independently associated with muscle weakness, impaired balance, and increased fall risk, particularly dangerous for older adults. Research has linked low vitamin D to higher rates of depression, with the relationship being strongest during winter months when both sunlight and vitamin D levels decline simultaneously. Emerging evidence suggests roles in cardiovascular health, cancer prevention, and metabolic function, though causal relationships are still being established through ongoing research.

Recognizing the Symptoms

Vitamin D deficiency symptoms are frustratingly vague and overlap with dozens of other conditions, which is why so many cases go undiagnosed for years. Persistent fatigue and a general sense of low energy are the most common complaints. Muscle weakness and aches, particularly in the lower back, hips, and legs, develop gradually. Bone pain, especially in the shins, can occur. Frequent infections or illness suggest impaired immune function. Low mood, particularly seasonal depression during winter months, has a well-documented association with vitamin D status. Impaired wound healing, hair loss, and difficulty concentrating round out the typical symptom picture.

The only definitive way to assess your vitamin D status is a blood test measuring 25-hydroxyvitamin D. Levels below 30 nmol/L (12 ng/mL) indicate deficiency, 30-50 nmol/L indicates insufficiency, and 50-125 nmol/L is considered adequate by most guidelines, though some researchers argue that 75-100 nmol/L represents the optimal range.

Best Food Sources of Vitamin D

Very few foods naturally contain significant vitamin D, which is a major reason deficiency is so prevalent even in well-fed populations.

Fatty fish are the best natural sources by a considerable margin. Salmon leads at approximately 11 micrograms (440 IU) per 100g, with wild-caught varieties containing 3-4 times more than farmed fish due to their natural diet of plankton and smaller fish. Mackerel provides about 7.3 micrograms, sardines about 4.8 micrograms, herring about 4.2 micrograms, and canned tuna about 1.7 micrograms per 100g. Eating fatty fish 2-3 times per week provides a substantial foundation for vitamin D intake.

Egg yolks provide about 2.1 micrograms per 100g of whole egg, with a single yolk containing roughly 1 microgram. Eggs from chickens raised outdoors ("pasture-raised") or fed vitamin D-enriched feed can contain 3-6 times more vitamin D than conventional eggs. For most people, eggs represent the most accessible and affordable daily vitamin D source.

Cod liver oil is the most concentrated food source at about 34 micrograms (1,360 IU) per tablespoon, but its strong fishy flavour limits its appeal. Modern capsule forms eliminate the taste issue and provide a reliable daily supplement in food form.

Mushrooms occupy a unique position as the only significant plant source. Like human skin, mushroom tissue can synthesize vitamin D when exposed to ultraviolet light. UV-treated mushrooms can provide 10+ micrograms per 100g. Look for labels that specify "UV-treated," "high vitamin D," or "sun-dried" varieties. Regular mushrooms grown in darkness contain negligible vitamin D.

Fortified foods are a significant source in many countries. Milk, some orange juices, certain breakfast cereals, and some yogurt brands are commonly fortified. Fortification levels vary by brand and country, so checking nutrition labels is important.

How Much Do You Need?

The recommended daily allowance is 15 micrograms (600 IU) for adults up to age 70 and 20 micrograms (800 IU) for those over 70. However, many researchers now consider these figures too conservative, and organizations including the Endocrine Society suggest that 25-50 micrograms (1,000-2,000 IU) daily may be more appropriate for achieving optimal blood levels, particularly for people with limited sun exposure.

The upper tolerable intake is 100 micrograms (4,000 IU) per day, though toxicity is rare and virtually always results from excessive supplementation rather than food or sunlight. You cannot overdose on vitamin D from sun exposure alone, as your skin has built-in regulatory mechanisms.

If you think you may be deficient, a blood test is the only reliable way to know. If supplementation is warranted, vitamin D3 (cholecalciferol) is more effective at raising blood levels than D2 (ergocalciferol). Take supplements with a meal containing fat, since vitamin D is fat-soluble and absorption improves significantly when consumed with dietary fat.

Explore all vitamin D rich foods in our database, or use the food search to check the vitamin D content of any specific food.

All nutritional values sourced from the USDA FoodData Central database.