Vitamin D: food sources, functions, side effects and daily dosing

Vitamin D: food sources, functions, side effects and daily dosing
April 9, 2014 10:01 PM

Vitamins are needed for healthy and perfect life. Without these vitamins life will be dull and handicapped. Vitamin D belongs to a group of fat soluble vitamins and the other group is water soluble vitamins. Mostly human body cannot synthesize vitamins, but vitamin D is unique in this respect as it can be synthesized internally. The most important compounds in vitamin D group are vitamin D1 (cholecalciferol) and vitamin D2 (ergocalciferol).

Vitamin D is nicknamed as “sunshine vitamin” because it is synthesized by the body only on sunlight exposure for adequate time. This is the reason why most people living indoor have vitamin D deficiency. Vitamin D helps in the intestinal absorption of many minerals including calcium, zinc, magnesium and phosphate. It also helps in the maintenance of calcium and phosphorus levels. Absorption of vitamin D occurs in the small intestine.

How can we get vitamin D?


When sunlight falls directly on the skin for some time, body synthesizes vitamin D from cholesterol. Time of exposure should be adequate (ideally 10-15 minutes 3 times a week) as sunlight can damage the skin when duration is prolonged. Vitamin D can also be synthesized by the use of tanning beds, but there is a risk of skin cancers in this.

Food sources

Sources of vitamin D3
  • Lichen
  • Fish liver oil (cod liver oil)
  • Egg
  • Beef liver
  • Fish (salmon, catfish, tuna, eel and sole)
  • Milk
  • Cereals
  • Pork
  • Ricoota cheese
  • Oysters
  • Sausages
  • Ice cream
Sources of vitamin D2
  • Fungi
  • Mushrooms (portabella, shiitake)
  • Alfalfa plants

Why we need vitamin D?

Effects and functions of vitamin D include:

Bone health

Concept of bone health is incomplete without vitamin D. Vitamin D increases the absorption of calcium. Aging increases the risk of osteoporosis in which bones become fragile and prone to fracture. Vitamin D helps in maintenance of bone health. It decreases the risk of bone loss and bone fracture by increasing bone strength. Due to this fact, Vitamin D is recommended to patients suffering from osteomalacia, osteoporosis and elderly. Deficiency of vitamin D in bones causes musculoskeletal pain, which can be relieved by vitamin D supplementation.


Rickets is softening of bones due to vitamin D deficiency. It a disease of childhood mostly occurring in malnourished children, but can occur in adults too. It manifests as deformity of long bones when a child starts to walk. During walking whole weight has to be supported by long bones, so they bow and bend when the affected individual walks. Vitamin D deficiency either by low intake or less sunlight exposure is a major cause of this disease. Rickets often occurs in infants because human milk is poor in vitamin D. Vitamin D fortified milk and vitamin D supplementations can eradicate rickets.


It is a disease of vitamin D deficiency in adults. It is characterized by bowing of legs, bending of spine, bone frangibility, softening of bones and the risk of fractures. There is reduced calcium absorption and increased calcium loss in this disease. As vitamin D increases calcium absorption, it is effective in treating osteomalacia.

Role in pregnancy

Pregnancy demands higher levels of vitamin D than routine levels. Low intake of vitamin D during pregnancy leads to birth defects, including gestational diabetes and small birth weight of infants.

Role in cancer

Vitamin D has some anti-cancer properties as well. It has no significant effect on decreasing the development of cancer but can decrease the mortality rate from cancer. It may protect against prostate, breast and colon cancers.

Role in immunity

Vitamin D acts as an immunity booster. It activates defense systems of the body and increases innate (natural, non specific) immunity. Due to this increased immunity body can fight more powerfully against infections. On the other hand, vitamin D deficiency can provide an opening to the microorganisms to invade the human defenses. Vitamin D deficient people are susceptible to HIV and influenza virus. Insufficient vitamin D increases the risk of autoimmune diseases like multiple sclerosis and diabetes.

Role in cell growth

Regulation of cellular growth requires vitamin D. Its deficiency may disturb cell growth and cause cell injury.

Role in depression

In case of vitamin D deficiency, calcium levels are low, due to which parathyroid gland produces excess of parathyroid hormone (calcium level regulating hormone) for compensation. This overactive parathyroid causes depression. Patients with depression have 14% less levels of calcium. Vitamin D supplementation can treat this depression.

Role in rheumatoid arthritis

Low vitamin D intake increases the susceptibility to develop rheumatoid arthritis. Reduced Vitamin D levels increases the severity of rheumatoid arthritis. Management of rheumatoid arthritis requires vitamin D.

Effect of skin color on vitamin D

Dark skin absorbs fewer sunrays. This leads to inadequate absorption and vitamin D deficiency in people with dark skin. Dark skin color occurs due to the melanin pigment in the skin, which hinders vitamin D synthesis. This fact is one the reason of vitamin D deficiency in Africans.

Role as an anti aging agent

Aging is an inevitable process. Anti aging agents either slow down or reverse the process of aging to some extent. As vitamin D has protective effects against cancer, diabetes and atherosclerosis, it can act as an anti aging agent.

Anti inflammatory actions

Vitamin D acts as an anti inflammatory agent. It inhibits the production and action of prostagladins (pro-inflammatory substances).

Role as an antioxidant

To understand its antioxidant role we need to know what an antioxidant is? Vitamin D protects our body by fighting against certain harmful chemical substances produced in the body called “free radicals”. Basically, free radicals are the reactive molecular derivatives of oxygen that are produced inside the body in a number of metabolic processes. Smoke, pollution, radiations and chemicals also induce the formation of free radicals in the body.

Side effects of vitamin D

The safe upper limit of vitamin D is: 1000-1500 IU for infants. 2500-3000 IU for children. 4000 IU for adults, pregnant and lactating women. Toxicity can occur if this limit is crossed. General signs of vitamin D toxicity are:

  • Anorexia
  • Polyuria
  • Weight loss
  • Weakness
  • Fatigue
  • Sleepiness
  • Metallic taste
  • Nausea, vomiting
  • Dry mouth

Serious complications are:


As vitamin D increases calcium levels, uncontrolled vitamin D can cause hypercalcemia. Hypercalcemia is a condition that occurs when calcium levels are greater than normal. As excess of everything is bad, hypercalcemia too is harmful for body. It can lead to the deposition of calcium in soft tissues of body, including liver, kidney, heart and lungs. Too much vitamin D can cause disorientation and confusion due to its effect on neuronal tissue. It can also cause irreversible kidney damage. Excessive calcium can cause the formation of stones in the kidney. In people with kidney disease vitamin D causes hypercalcemia frequently.

Arrhythmias (irregular heartbeat) can occur due to increased calcium levels caused by excess of vitamin D. If hypercalcemia occurs in a pregnant woman taking vitamin D, it will act as a double edged sword. It will affect mother as well as fetus. Maternal hypercalcemia causes increased sensitivity to vitamin D in the fetus. It may lead to mental retardation and facial abnormalities in fetuses. General symptoms of hypercalcemia are polyuria, polydipsia, constipation, vomiting and weakness.

Effects of hypervitaminosis D in diseased patients


Atherosclerosis is a disease in which artery wall thickens due to the accumulation of calcium and cholesterol. As calcium is already a cause of this disease, excessive vitamin D leading to increased calcium can increase disease severity and worsen it.

Kidney disease

Vitamin D increases the risk of atherosclerosis in patients with kidney disease. Calcium monitoring should be done in those patients taking vitamin D.


Vitamin D, when in excess, can cause formation of stones in these patients.

Histoplasmosis & Lymphoma

Vitamin D increases the level of calcium in these patients. This can lead to stone formation and other effects.

Interactions of vitamin D

Interaction is a kind of action when two things act upon another. Vitamin D can interact with a number of medications.

With steroids: Steroids impair vitamin D metabolism and reduces calcium absorption. This can contribute to osteoporosis in the long run.

With anti-epileptics: These drugs increase vitamin D metabolism and reduce calcium absorption.

With weight lowering drugs: Their combination causes decrease absorption of vitamin D.

With cholesterol lowering drugs: They can reduce absorption of vitamin D.

With digoxin: Vitamin D increase effects of digoxin leading to irregular heartbeat.

With diltiazem: Both drugs can affect the heart. When taken together, vitamin D may decrease its effectiveness.

With diuretics: Taking diuretics with vitamin D can lead to too much calcium in the body. This combination can damage kidneys.

Recommendations for vitamin D

Like all other vitamins, vitamin D has specific daily needs. The exact amount of vitamin D depends upon age. RDA (recommended dietary allowance) tells how much vitamin one should consume on a daily basis.

RDA of vitamin D is:

  • For infants: 0-6 months: 400 IU
  • 7-12 months: 400 IU
  • For children: 1-3 years: 600 IU
  • 4-8 years: 600 IU
  • For adults: 9-70 years: 600 IU
  • Over 70 years: 800 IU
  • For pregnant and lactating women: 600 IU

IU stands for international units.

Share this article
Written by: Michal Vilímovský (EN)
Education: Medical student, 3rd Faculty of Medicine, Charles University, Prague, Czech Republic
Published: April 9, 2014 10:01 PM
Last updated: August 2, 2015 1:45 AM
Next scheduled update: August 2, 2017 1:45 AM
Our site uses cookies to provide services, personalize ads and analyze traffic. By using this site you agree. More info