Vitamin D: Everything You Need to Know About Its Benefits, Sources, Deficiency, and Dosage

Vitamin D: Everything You Need to Know About Its Benefits, Sources, Deficiency, and Dosage

Vitamin D is one of the most important vitamins associated with bone and muscle health and the regulation of calcium and phosphorus in the body. Although it is called a “vitamin,” vitamin D has hormone-like properties because the body can produce it in the skin after exposure to ultraviolet B (UVB) radiation. It then undergoes several conversion processes inside the body until it reaches its active form. [Source: NIH Office of Dietary Supplements]

 

 

What Is Vitamin D?

Vitamin D is a fat-soluble vitamin that exists mainly in two dietary forms: Vitamin D2 (ergocalciferol) and Vitamin D3 (cholecalciferol). It can be obtained from food and dietary supplements, while the skin can produce vitamin D3 when exposed to ultraviolet B radiation.

After entering the body, vitamin D is not immediately in its final active form. It is first converted in the liver into 25-hydroxyvitamin D [25(OH)D], also known as calcidiol. This is the main indicator used to assess vitamin D stores in the body. It is then converted mainly in the kidneys into 1,25-dihydroxyvitamin D [1,25(OH)₂D], or calcitriol, which is the active hormonal form. [Sources: NIH and NCBI]

What Are the Functions of Vitamin D in the Body?

One of the most important functions of vitamin D is helping the intestines absorb calcium and phosphorus, two essential minerals needed to build and maintain strong bones. Vitamin D also contributes to normal muscle, nervous system, and immune system function. [Source: NIH Office of Dietary Supplements]

When vitamin D levels become severely or chronically low, calcium and phosphorus absorption may decrease. As a compensatory response, levels of parathyroid hormone (PTH) may increase, which can increase bone turnover and lead to impaired bone mineralization. In severe cases, vitamin D deficiency can cause rickets in children and osteomalacia in adults. [Source: NCBI]

How Does the Body Get Vitamin D?

There are three main sources:

1. Sun Exposure

The skin can produce vitamin D3 when exposed to ultraviolet B (UVB) radiation. The amount produced depends on several factors, including skin color, age, the amount of skin exposed to sunlight, time of day, season, geographic latitude, and protective clothing.

Therefore, there is no single amount of sun exposure that can be considered appropriate for everyone or for every country. Relying on sunlight alone may also be insufficient for some people. [Source: NCBI]

2. Food

Natural amounts of vitamin D are found in a relatively limited number of foods. Important sources include fatty fish such as salmon, tuna, and mackerel, as well as fish liver oil, egg yolks, and some foods fortified with vitamin D. [Source: NIH Office of Dietary Supplements]

3. Dietary Supplements

Vitamin D supplements are available mainly in the forms of D2 and D3. Supplements may be used when there is a nutritional or medical need. However, taking high doses without a medical reason or proper monitoring may lead to toxicity.

How Much Vitamin D Does a Person Need Each Day?

According to the National Institutes of Health (NIH), the recommended dietary allowance for most adults aged 19–70 years is approximately 600 IU per day (15 micrograms), while adults over 70 generally need about 800 IU per day (20 micrograms). [Source: NIH Office of Dietary Supplements]

Individual requirements may vary depending on age, health status, pregnancy, and other risk factors.

What Causes Vitamin D Deficiency?

Vitamin D deficiency can occur for several reasons and is not caused only by insufficient sunlight exposure. Important risk factors include:

Limited exposure to sunlight.

Low intake of foods containing vitamin D.

Fat-malabsorption problems or certain gastrointestinal disorders.

Some liver or kidney diseases.

Older age.

Higher skin pigmentation.

Obesity, which is associated with lower 25(OH)D levels and an increased risk of deficiency.

Certain medications that increase vitamin D metabolism or reduce its levels. [Sources: NCBI and NIH]

What Are the Symptoms of Vitamin D Deficiency?

Vitamin D deficiency may not cause obvious symptoms at first. Therefore, a person can have low vitamin D levels without experiencing any specific symptoms.

Severe or chronic deficiency may be associated with muscle weakness, bone or muscle pain, fatigue, and an increased risk of poor bone health and fractures. In children, severe deficiency can lead to rickets and skeletal deformities. [Source: NCBI]

How Is Vitamin D Measured?

The blood test 25-hydroxyvitamin D [25(OH)D] is used as the primary indicator of vitamin D status in the body.

In contrast, measuring 1,25-dihydroxyvitamin D [1,25(OH)₂D] is not the routine test used to evaluate vitamin D stores because it is affected by complex hormonal regulation and may remain normal even when vitamin D deficiency is present. [Sources: NIH and NCBI]

It is important to note that interpreting 25(OH)D levels is not always straightforward. Thresholds can differ between guidelines and laboratories, and the relationship between blood vitamin D levels and some non-bone-related health outcomes remains under scientific investigation.

Should Everyone Have a Vitamin D Test?

Not necessarily.

The Endocrine Society's 2024 guideline recommends against routine testing of 25(OH)D levels in generally healthy adults who do not have a medical reason that warrants testing. It also indicates that an optimal blood vitamin D level for preventing many diseases has not been firmly established through clinical trials.

This is important because an association between low vitamin D levels and a particular disease in observational studies does not necessarily mean that taking vitamin D supplements will prevent that disease.

Does Vitamin D Strengthen the Immune System?

Vitamin D plays a role in normal immune system function, and there is an association between vitamin D status and certain immune markers. However, this does not mean that high doses of vitamin D supplements can prevent or treat all infections.

Recent Endocrine Society guidelines indicate potential benefits for certain groups, such as children and adolescents for the prevention of rickets and possibly reducing some respiratory infections. However, they do not recommend routinely taking vitamin D doses above standard nutritional requirements for healthy adults under 75 years of age solely for disease prevention.

Vitamin D and Bone Health

The relationship between vitamin D and bone health is one of its best-established functions. Vitamin D helps the body absorb calcium and maintain calcium and phosphorus balance. Therefore, severe and chronic deficiency can lead to impaired bone mineralization.

Vitamin D becomes particularly important for people who are at risk of bone problems or fractures, depending on their medical condition and other risk factors.

Can Vitamin D Help With Weight Loss?

Observational studies have found an association between low vitamin D levels and obesity. People with obesity may also have lower levels of 25(OH)D.

However, this does not mean that taking vitamin D alone causes weight loss.

Current evidence suggests that the relationship between obesity and vitamin D is complex and may be affected by factors such as lifestyle, metabolism, and the distribution of vitamin D in the body. Therefore, vitamin D is not considered a treatment for weight loss.

What Happens When You Take Too Much Vitamin D?

Although vitamin D is important for health, excessive amounts can be harmful. High doses of supplements can cause increased calcium levels in the blood and urine. Possible symptoms include:

Nausea and vomiting.

Muscle weakness.

Excessive thirst.

Frequent urination.

Dehydration.

Kidney stones.

In severe cases of vitamin D toxicity, kidney damage, calcium deposits in tissues, and abnormal heart rhythms may occur, and the condition can become very serious. Vitamin D toxicity usually results from excessive supplement use rather than normal exposure to sunlight. [Source: NIH Office of Dietary Supplements]

For adults, the tolerable upper intake level is generally 4,000 IU per day from all sources. However, a doctor may prescribe higher doses for a specific period when treating confirmed vitamin D deficiency under medical supervision. [Source: NIH]

Vitamin D and Drug Interactions

Some medications can affect vitamin D levels or interact with vitamin D. Examples mentioned by NIH sources include certain anticonvulsants, steroids, and orlistat.

Additionally, combining vitamin D with certain thiazide diuretics may increase the risk of high calcium levels in some people. Therefore, patients should inform their doctor or pharmacist about all medications and dietary supplements they are taking. [Source: NIH Office of Dietary Supplements]

Conclusion

Vitamin D is an essential nutrient for bone and muscle health and calcium and phosphorus balance, and it also plays important roles in several body functions. People obtain vitamin D from sunlight, food, and dietary supplements. The 25(OH)D blood test is the primary indicator used to assess vitamin D status when testing is medically indicated.

However, taking high doses of vitamin D without a medical need or appropriate supervision is not necessarily better for health and may lead to toxicity and high calcium levels. Current evidence also does not support routinely using vitamin D supplements at doses above nutritional requirements for all healthy adults.

Therefore, the appropriate use and dosage of vitamin D depend on factors such as age, health status, diet, sun exposure, risk factors for deficiency, and, when appropriate, laboratory testing and medical evaluation. [Sources: NIH Office of Dietary Supplements and Endocrine Society]

Main Scientific Sources

National Institutes of Health (NIH) – Office of Dietary Supplements. Vitamin D Fact Sheet for Health Professionals.

Endocrine Society. Vitamin D for the Prevention of Disease: Clinical Practice Guideline, 2024.

NCBI Bookshelf. Vitamin D Deficiency – StatPearls.

NCBI Bookshelf. Screening for Vitamin D Deficiency in Adults – Evidence Review for the U.S. Preventive Services Task Force.

NCBI Endotext. Disorders in the Action of Vitamin D.