May 4, 2023
Vitamins and Minerals

Vitamin D: A Comprehensive Guide

Major Team

What Is Vitamin D?

Vitamin D (also known as “calciferol”) is a fat-soluble vitamin and a hormone the body produces. Vitamin D is primarily made in the skin when exposed to the sun and aids in regulating calcium and phosphorus levels in the body. Several body organs and tissues have vitamin D receptors, implying crucial roles beyond bone health.

Vitamin D comes in two forms: D2 (ergocalciferol) and D3 (cholecalciferol). The only difference between them is in their side-chain structures. And even though the presence of fat in the stomach boosts absorption, it can happen without dietary fat too. Neither aging nor obesity affects this process.[1]

What Does It Do?

The presence of vitamin D is necessary for the absorption of calcium, which is the primary component of bone. Vitamin D also handles many other cellular functions and has anti-inflammatory, antioxidant, and neuroprotective properties that support immune health, muscle function, and brain cell activity.

How much vitamin D your skin produces varies according to several factors, including latitude, season, time of the day, and skin pigmentation. Vitamin D production may decline or cease entirely during the winter based on your lifestyle and geographic location. While sunscreen is essential for preventing skin cancer, it can also reduce vitamin D production.[2]

What Vitamin D Offers You

Fighting Off Disease

Boosting vitamin D levels can reduce the risk of multiple sclerosis (MS). According to a 2018 review of population-based studies, low vitamin D levels are connected to an increased risk of MS.[3]

Low vitamin D levels have also been associated with an augmented risk of heart diseases like hypertension, heart failure, and stroke. While it is unclear whether vitamin D deficiency contributes to heart disease or indicates poor health in chronic condition cases, increasing vitamin D levels may still be beneficial.[4]

Moreover, vitamin D supports the immune system. Individuals with inadequate vitamin D levels may be at higher risk of infections and autoimmune diseases, such as rheumatoid arthritis, type 1 diabetes, and inflammatory bowel disease.[5]

Bone Health and Muscle Strength

Vitamin D is paramount for healthy bones and can help prevent falls and fractures in older adults. Several studies have found that low vitamin D levels are associated with a higher risk of fractures in seniors.

A meta-analysis of 12 randomized controlled trials showed that higher intakes of vitamin D supplements, around 500 to 800 IU/day, reduced hip and non-spine fractures by about 20%.[6] Another study found that taking vitamin D with calcium had a small protective effect against all fracture types in older men and postmenopausal women aged 65 or more.[7]

Vitamin D may also help increase muscle strength, which contributes to preventing falls, a common problem that can lead to disability and death in older people. Taking 700 to 1,000 IU of vitamin D daily can lower the risk of falls by 19%, according to a combined analysis of multiple studies.[8]

Decreased Risk of Premature Death

According to a report in the Archives of Internal Medicine, taking vitamin D supplements may potentially lower overall mortality rates. A combined analysis of various studies found that consuming moderate amounts of vitamin D supplements resulted in a significant 7% reduction in mortality from any cause.[9]

Additionally, a recent meta-analysis suggested that the decrease in mortality rates could be attributed primarily to a reduction in cancer mortality.[10] Nonetheless, further research is needed to make sweeping claims about the relationship between vitamin D and mortality. 

Lower Cancer Mortality

While vitamin D may not play a role in reducing the incidence of cancer, research, including randomized trials, suggests that maintaining higher vitamin D levels may improve survival in those who develop cancer. In the VITamin D and OmegA-3 TriaL (VITAL), individuals who were assigned to take vitamin D showed a lower death rate from cancer, and this benefit appeared to increase with time since starting on vitamin D.[10]

Sources of Vitamin D

Vitamin D3 is not naturally abundant in many foods. Its highest amounts are in fatty fish and fish liver oils, while other sources with lower content include egg yolks, cheese, and beef liver.

Some mushrooms contain vitamin D2, and some commercially available mushrooms go through ultraviolet light exposure to have their vitamin D2 level increased. In summary, vitamin D sources 

In summary, vitamin D sources comprise:

  • Cod liver oil; 
  • Salmon, swordfish, tuna, and sardines;
  • Vitamin D-fortified orange juice;
  • Vitamin D-fortified dairy and plant milk;
  • Egg yolk;
  • Beef liver;
  • Fortified cereals.[11]

How Much Vitamin D Should You Take?

The current recommended daily allowance for Vitamin D is 400 to 800 IU/day, although this may be too low for many adults. For moderate supplementation, a dose of 1,000 to 2,000 IU of vitamin D3 is sufficient to cover the needs of most of the population. However, higher daily doses may vary from 20 to 80 IU per kilogram of body weight.

The Upper Tolerable Intake Level in the United States and Canada is 4,000 IU/day. The real Upper Tolerable Intake Limit may be as high as 10,000 IU/day, although there is limited evidence on health outcomes with dosages close to this number.

It is advisable to supplement with vitamin D3 (cholecalciferol) rather than D2 (ergocalciferol), as D3 is more efficient in increasing blood levels.[12]


Vitamin D deficiency can occur for many reasons, such as inadequate dietary intake, poor absorption, or a metabolic need for higher amounts. If someone is not consuming enough vitamin D through their diet and receiving enough sun exposure, they may be at risk for deficiency. Certain groups of people are at increased risk of deficiency, including those who cannot tolerate or do not consume milk, eggs, and fish, as well as individuals following a vegan diet. Additionally, people with inflammatory bowel disease or other conditions that affect fat digestion may be at risk, as vitamin D is a fat-soluble vitamin that relies on the gut's ability to absorb dietary fat.

Obesity may also increase the risk of vitamin D deficiency, as the vitamin accumulates in excess fat tissues and is not readily available for use by the body when needed. Conversely, vitamin D blood levels rise when obese people lose weight. A deficiency may also happen to individuals who have had gastric bypass surgery, which often involves the removal of the upper portion of the small intestine responsible for vitamin D absorption.

Prolonged vitamin D deficiency can lead to conditions like rickets (soft bones and skeletal deformities in infants and children caused by failure of bone tissue to harden) and osteomalacia (weak and softened bones in adults reversible with supplementation). It is important to monitor vitamin D intake and speak with a healthcare provider if there are concerns about deficiency.

Are There Any Side Effects?

Vitamin D toxicity is rare and usually occurs from taking high supplement doses. It is unlikely to result from food sources or sun exposure because the body regulates the amount of vitamin D produced from these sources.

To prevent toxicity, it is advised not to take more than 4,000 IU/day of vitamin D supplements without consulting with a doctor. Symptoms include weight loss, anorexia, irregular heartbeat, and hardening of blood vessels and tissues due to high levels of calcium in the blood, which can potentially damage the heart and kidneys.[13] If you experience any of these symptoms, it is crucial to seek medical attention immediately.


1. Office of Dietary Supplements. (2022, August 12). Vitamin D - Fact Sheet for Health Professionals. National Institutes of Health. Retrieved February 23, 2023, from

2. Mayo Clinic Staff. (2021, February 9). Vitamin D. Mayo Clinic. Retrieved February 23, 2023, from

3. Sintzel, M. B., Rametta, M., & Reder, A. T. (2018). Vitamin D and Multiple Sclerosis: A Comprehensive Review. Neurology and therapy, 7(1), 59–85.

4. Saponaro, F., Marcocci, C., & Zucchi, R. (2019). Vitamin D status and cardiovascular outcome. Journal of endocrinological investigation, 42(11), 1285–1290.

5. Martens, P. J., Gysemans, C., Verstuyf, A., & Mathieu, A. C. (2020). Vitamin D's Effect on Immune Function. Nutrients, 12(5), 1248.

6. Bischoff-Ferrari HA, Willett WC, Wong JB, et al. Prevention of Nonvertebral Fractures With Oral Vitamin D and Dose Dependency: A Meta-analysis of Randomized Controlled Trials. Arch Intern Med. 2009;169(6):551–561. doi:10.1001/archinternmed.2008.600

7. Avenell A, Mak JC, O’Connell D. Vitamin D and vitamin D analogues for preventing fractures in post-menopausal women and older men. Cochrane Database of Systematic Reviews. 2014(4).

8. Bischoff-Ferrari H A, Dawson-Hughes B, Staehelin H B, Orav J E, Stuck A E, Theiler R et al. Fall prevention with supplemental and active forms of vitamin D: a meta-analysis of randomised controlled trials BMJ 2009; 339 :b3692 doi:10.1136/bmj.b3692

9. Autier P, Gandini S. Vitamin D Supplementation and Total Mortality: A Meta-analysis of Randomized Controlled Trials. Arch Intern Med. 2007;167(16):1730–1737. doi:10.1001/archinte.167.16.1730

10. Keum, N., Lee, D. H., Greenwood, D. C., Manson, J. E., & Giovannucci, E. (2019). Vitamin D supplementation and total cancer incidence and mortality: a meta-analysis of randomized controlled trials. Annals of oncology : official journal of the European Society for Medical Oncology, 30(5), 733–743.

11. The Nutrition Source - Vitamin D. (2012, September 18) [Updated 2022 Nov]. Harvard School of Public Health. Retrieved February 23, 2023, from

12. Pfiffner, M. M. (2022, September 28). Vitamin D health benefits, dosage, safety, side-effects, and more. Examine - Supplements. Retrieved February 23, 2023, from

13. Marcinowska-Suchowierska, E., Kupisz-Urbańska, M., Łukaszkiewicz, J., Płudowski, P., & Jones, G. (2018). Vitamin D Toxicity-A Clinical Perspective. Frontiers in endocrinology, 9, 550.