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Vitamin D

IMAGE Vitamin D is a fat-soluble vitamin. Fat-soluble vitamins are stored in the body's liver and fatty tissues. Vitamin D acts as both a vitamin and a hormone.

Vitamin D is found in some foods, but the main sources are vitamin D-fortified milk and sunlight. The ultraviolet rays of the sun react with cholesterol present on the skin and create previtamin D3. This compound goes through a series of reactions in the kidneys and the liver, and the final product is vitamin D.

Functions

Vitamin D's functions:

  • Plays a crucial role in the growth and maintenance of strong, healthy bones
  • Maintains normal blood levels of calcium and phosphorus

In children with low vitamin D levels, the supplement can improve bone mineral density. While the evidence does not give a clear answer, it has also been suggested that vitamin D supplementation may reduce the risk of osteoporosis and seasonal flu in children, and high blood pressure and some forms of cancer.

Vitamin D has also been found to improve pain symptoms in patients with low vitamin D levels.

Recommended Intake

Here are the guidelines for vitamin D intake:

Age Group Recommended Dietary Allowance or Adequate Intake (IU/Day)
0-12 months 400
1-70 years 600
71+ years 800
Pregnant or nursing women 600

IU: international units

The American Academy of Pediatrics recommends supplementation for all children who do not receive at least 400 IU of vitamin D daily. Breastfed babies may require a supplement within the first few days of life. Bottle-fed babies who do not consume enough vitamin-D fortified formula may also need the supplement, as well as any child who does not get plenty of vitamin D in their diet.

As seen above, requirements for pregnant women are the same as for healthy adults, though some believe that pregnant mothers should take more vitamin D than recommended. Furthermore, some experts believe that people at highest risk for vitamin D deficiency, such as older adults or those with limited sun exposure during the winter months, should take 1,000 IU or more daily. However, since the risk of vitamin D toxicity increases with higher doses, such recommendations ought to be discussed individually with a physician.

Vitamin D Deficiency

Symptoms of overt vitamin D deficiency are rare today, but can include the following:

  • Rickets —in children, a disease in which the bones become soft and weak
  • Osteomalacia —in adults, a disease in which the bones become soft and weak
  • Muscle weakness

More mild vitamin D deficiency is thought to be relatively common, especially in higher latitudes, and may lead to increased risk of osteoporosis.

Vitamin D Toxicity

Since vitamin D is stored in the body and not excreted in the urine like most water-soluble vitamins, it is possible for it to accumulate and reach toxic levels. Here are safe upper level intakes for vitamin D:

Age Group Upper Level Intake (IU/Day)
0-6 months 1,000
7-12 months 1,500
1-3 years 2,500
4-8 years 3,000
9 years and older 4,000
Pregnant or nursing women 4,000

Symptoms of toxicity:

  • Nausea
  • Vomiting
  • Poor appetite
  • Constipation
  • Weakness
  • Weight loss
  • High blood pressure
  • Raised levels of calcium in the blood, which can cause:
    • Confusion
    • Heart rhythm abnormalities
    • Deposits of calcium in soft tissues, like the kidney, heart, and lungs

It is unlikely for sunlight and diet to cause vitamin D toxicity.

If you have problems with any symptoms related to vitamin D toxicity, take less vitamin D supplements.

Major Food Sources

Fortified foods provide the most vitamin D. Examples of foods that may be fortified with vitamin D are:

  • Milk
  • Cereal
  • Orange juice
  • Yogurt
  • Margarine
  • Soy beverages

There are not many foods that are natural sources of vitamin D. Of those foods that have vitamin D naturally are (most to least):

  • Fatty fish, such as salmon, tuna, and mackerel
  • Beef liver
  • Cheese
  • Egg yolks
  • Mushrooms

A relatively small amount of sun exposure can provide adequate vitamin D. In a study of naval personnel in submarines, six days of sun exposure proved capable of supplying enough vitamin D for 49 sunless days. However, the actual synthesis of vitamin D through sunlight is affected by season, latitude, time of day, cloud cover, smog, use of sunblock, and skin pigmentation.

Health Implications

Populations at Risk for Vitamin D Deficiency

The following populations may be at risk for vitamin D deficiency and may require a supplement:

  • Breastfeed babies—Human milk does not have enough vitamin D. Breastfed babies should receive a 400 IU vitamin supplement each day to make up for this.
  • Older adults—Studies suggest that adults over age 65 have less ability to synthesize vitamin D through sunlight exposure than adults aged 20-30. They are also likely to spend less time out in the sun. Elderly women with vitamin D deficiency may benefit from taking D3 supplements. Talk to your doctor, though, before taking these.
  • Locales with limited sun exposure—People who live above latitudes of approximately 40°N and below latitudes of approximately 40°S are at risk for deficiency during most of the winter months.
  • People with dark skin—Those with darker skin are less able to make vitamin D from the sun.
  • People who are obese —Body fat can bind to some vitamin D preventing it from getting into the blood where it can be used by the body.
  • People with a reduced ability to absorb dietary fat—Because vitamin D is a fat-soluble vitamin, fat is required for its absorption from foods. Some conditions that can cause fat malabsorption include Crohn's disease , cystic fibrosis , celiac disease , pancreatic enzyme deficiency, and liver disease.

Tips For Increasing Your Vitamin D Intake

Here are tips to help increase your intake of vitamin D:

  • If you take a vitamin supplement, make sure it contains vitamin D.
  • Drink vitamin D-fortified milk.
  • Get sun exposure, but be careful to watch for sunburn. Sunlight is a major cause of skin cancer .
  • Academy of Nutrition and Dietetics

    http://www.eatright.org

  • Office of Dietary Supplements

    http://ods.od.nih.gov

  • Dietitians of Canada

    http://www.dietitians.ca

  • Health Canada

    http://www.hc-sc.gc.ca

  • Bischoff-Ferrari HA, Willett WC, Wong JB, et al. Fracture prevention with vitamin D supplementation. JAMA. 2005;293(18):2257-64.

  • Calcium and vitamin D for treatment and prevention of osteoporosis. EBSCO DynaMed website. Available at: https://dynamed.ebscohost.com/about/about-us . Updated June 25, 2013. Accessed October 1, 2013.

  • Duplessis CA, Harris EB, Watenpaugh DE, et al. Vitamin D supplementation in underway submariners. Aviat Space Environ Med. 2005;76:569-75.

  • Vitamin D. Office of Dietary Supplements. National Institutes of Health website. Available at: http://ods.od.nih.gov/factsheets/vitamind.asp . Updated June 24, 2013. Accessed October 1, 2013.

  • Vitamin D intake and supplementation. EBSCO DynaMed website. Available at: https://dynamed.ebscohost.com/about/about-us . Updated June 25, 2013. Accessed October 1, 2013.

  • Wagner CL, Greer FR, American Academy of Pediatrics Section on Breastfeeding, American Academy of Pediatrics Committee on Nutrition. Prevention of rickets and vitamin D deficiency in infants, children, and adolescents. Pediatrics. 2008;122:1142-1152.

  • 4/5/2010 DynaMed's Systematic Literature Surveillance https://dynamed.ebscohost.com/about/about-us : Urashima M, Segawa T, Okazaki M, Kurihara M, Wada Y, Ida H. Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. Am J Clin Nutr. 2010;91(5):1255-1260.

  • 2/11/2011 DynaMed's Systematic Literature Surveillance https://dynamed.ebscohost.com/about/about-us : Winzenberg T, Powell S, Shaw KA, Jones G. Effects of vitamin D supplementation on bone density in healthy children: systematic review and meta-analysis. BMJ. 2011;342.

  • 7/28/2011 DynaMed's Systematic Literature Surveillance https://dynamed.ebscohost.com/about/about-us : Bjelakovic G, Gluud LL, Nikolova D, et al. Vitamin D supplementation for prevention of mortality in adults. Cochrane Database Syst Rev. 20116;(7):CD007470.

  • 3/6/2013 DynaMed's Systematic Literature Surveillance https://dynamed.ebscohost.com/about/about-us : Schreuder F, Bernsen RM, van der Wouden JC. Vitamin D supplementation for nonspecific musculoskeletal pain in non-Western immigrants: a randomized controlled trial. Ann Fam Med. 2012 Nov-Dec;10(6):547-55.