What Is Vitamin D Functions Sources And Deficiency?

Vitamin D is a fat-soluble vitamin your body needs to absorb calcium and phosphate, the minerals that build and maintain strong bones. Your skin makes it when exposed to sunlight, and you also get it from certain foods and supplements. A true deficiency — measured by a specific blood test — is common worldwide, and it can weaken bones and cause other health problems over time.

What Is Vitamin D and How Does Your Body Make It?

Vitamin D is different from most vitamins. Your body can manufacture it on its own. When ultraviolet B (UVB) rays from sunlight hit your skin, they trigger a chemical reaction that converts a form of cholesterol into vitamin D3. From there, your liver and kidneys modify it into the active hormone your body actually uses.

That final activated form — calcitriol — is not really a vitamin at all. It behaves like a hormone. It travels through your bloodstream and tells your intestines to absorb calcium, your bones to release or store minerals, and your kidneys to hold onto calcium rather than dump it in urine.

Because sunlight drives production, several factors change how much your skin makes:

• Skin color: More melanin means less vitamin D produced per minute of sun exposure.
• Latitude and season: Above roughly 35 degrees latitude, UVB rays are too weak for skin production during winter months.
• Sunscreen and clothing: These block UVB rays, which reduces production.
• Age: Older skin makes vitamin D less efficiently.
• Time of day: Midday sun produces more vitamin D than early morning or late afternoon sun.

This is why deficiency can happen even in people who spend time outdoors.

What Does Vitamin D Do in the Body?

Vitamin D’s best-established job is managing calcium and phosphate. Without enough of it, your intestines absorb only a fraction of the calcium you eat. Your body then pulls calcium from your bones to keep blood levels normal. Over months and years, that borrowing weakens bone.

Beyond bone health, vitamin D receptors exist in many tissues — muscle, immune cells, blood vessels, and more. Researchers have studied whether vitamin D affects cancer, heart disease, autoimmune conditions, infections, and mood. The results are mixed. Some studies suggest associations, but large randomized trials testing whether vitamin D supplements actually prevent these conditions have generally not shown clear benefit in people who start out with adequate levels.

That gap matters. A receptor being present in a tissue does not prove that taking more vitamin D improves that tissue’s function.

What Foods Contain Vitamin D?

Very few foods naturally contain meaningful amounts of vitamin D. The main natural sources are fatty fish and fish liver oils. Some foods are fortified, meaning vitamin D is added during processing.

These figures come from USDA nutritional data and can vary by brand, preparation, and the specific fish or product. Fortified foods vary widely. In the US, milk is usually fortified, but cheese and yogurt often are not.

For most people, food alone does not supply enough vitamin D to meet recommended intake. That is not a flaw in the diet — it reflects the fact that humans evolved to make vitamin D from sunlight, not to eat it.

How Much Vitamin D Do You Need?

Recommended daily intake is usually expressed in International Units (IU). The Food and Nutrition Board of the National Academies sets the Recommended Dietary Allowance at 600 IU per day for adults up to age 70 and 800 IU per day for adults over 70. The Endocrine Society has suggested higher amounts for people at risk of deficiency, but that guidance reflects clinical practice more than universal agreement.

These recommendations assume minimal sun exposure. If you get regular sun, your actual need from food and supplements may be lower.

The upper limit — the amount considered safe for most healthy adults — is 4,000 IU per day from all sources combined. Very high doses taken long-term can cause vitamin D toxicity, which raises blood calcium and can damage the kidneys and heart. This is rare but real, and it almost always comes from supplements, not sun or food.

What Is Vitamin D Deficiency?

Vitamin D deficiency is defined by blood levels of 25-hydroxyvitamin D, the main form measured in testing. The most widely used threshold comes from the Institute of Medicine (now the National Academy of Medicine): a level below 12 ng/mL (30 nmol/L) is considered deficient. Levels between 12 and 20 ng/mL are sometimes called insufficient, though experts debate where the line should sit.

Some organizations, including the Endocrine Society, use a higher cutoff of 20 ng/mL for deficiency. This disagreement is real and not fully resolved. What is clear is that very low levels — below 12 ng/mL — are linked to bone disease.

What causes deficiency?

The most common causes are limited sun exposure, dark skin, older age, living at high latitude, and covering the skin for cultural or medical reasons. Certain medical conditions also interfere:

• Crohn’s disease, celiac disease, and other conditions that impair fat absorption
• Kidney or liver disease that blocks activation of vitamin D
• Obesity, because vitamin D gets sequestered in fat tissue
• Medications like certain anticonvulsants and steroids
• Exclusive breastfeeding without vitamin D supplementation for the infant

What are the symptoms?

Mild deficiency often causes no obvious symptoms. When it is severe or prolonged, it can cause bone pain, muscle weakness, and frequent falls in older adults. In children, severe deficiency causes rickets — soft, bowed bones and delayed growth. In adults, it causes osteomalacia, a condition of soft, painful bones.

Fatigue and vague aches are often attributed to low vitamin D, but the evidence linking mild deficiency to those symptoms is weak. Many people with low levels feel fine, and many people with fatigue have normal levels.

Who Should Get Tested?

Routine screening of all adults is not recommended by most major medical organizations. Testing is generally reserved for people with clear risk factors or symptoms suggesting bone disease.

Reasonable candidates for testing include people with osteoporosis or fractures from minor falls, those with malabsorption conditions, people with chronic kidney or liver disease, and anyone with symptoms of severe deficiency. If you are unsure whether testing applies to you, that is a conversation to have with your clinician.

How Is Deficiency Treated?

Treatment depends on how low the level is and why it dropped. Standard approaches include:

• Vitamin D2 (ergocalciferol) or D3 (cholecalciferol) supplements, taken orally
• Higher initial doses for severe deficiency, followed by a maintenance dose
• Calcium intake review, since vitamin D and calcium work together
• Addressing the underlying cause — for example, treating malabsorption or adjusting medications

Vitamin D3 is generally considered more effective than D2 at raising blood levels, though both work. Some clinicians recommend taking vitamin D with a meal containing fat, since it is fat-soluble, but the practical difference is modest.

Very large single doses given at long intervals have been studied, and some research suggests they may be less effective — or even carry slight risks — compared with smaller daily or weekly doses. This is an area where practice has shifted over time.

What About Sunlight and Supplements?

Sunlight remains the most efficient natural source of vitamin D for most people. Brief exposure of arms and legs — often cited as 10 to 30 minutes several times a week — can produce adequate amounts in lighter-skinned people at lower latitudes. The exact time depends on skin type, location, season, and time of day.

However, UV exposure also raises skin cancer risk. No major health organization recommends deliberately tanning or burning to make vitamin D. Getting some sun is reasonable; baking in it is not.

Supplements are the more predictable option, especially in winter or for people with limited sun exposure. They are inexpensive and widely available. The key is not to overdo it — more is not better, and the upper limit exists for a reason.