Your Guide To Vitamin D

Vitamin D is a fat-soluble vitamin responsible for helping the absorption of calcium, zinc, magnesium, phosphate and iron. Vitamin D is classed as a secosteroid (meaning it is in a sub-class of steroids or derived from steroids). A lot of people see the word steroid and assume it is something bad for you, this is simply not the case. Hundreds of distinct steroids are found in animals, fungi, plants, and many steroids are necessary to life at all levels. Research is now showing us that Vitamin D acts more as a hormone than an actual vitamin.

Fat-soluble vitamins include vitamins A, D, E, K, F. Absorbed with ingested dietary fat, fat-soluble vitamins are stored in moderate amounts from the gastrointestinal tract. Present in minute amounts in various foods, these vitamins are essential to maintaining normal metabolism and biochemical functions; fat malabsorption may result in fat-soluble vitamin deficiencies. Fat-soluble vitamins are found in many types of foods and may be supplemented with pharmaceutical formulations.

A vitamin is an essential organic compound which cannot be synthesized by the body and has to be ingested by the individual. Vitamin D is synthesized by the body through sunlight and acts as a prehormone. There are five forms of vitamin D, the most important to humans are vitamin D2 and vitamin D3. Both forms can be ingested through diet and supplementation, however the major source of the vitamin will be through sunlight (specifically D3). Many studies have found D3 to be superior due to it being more biologically active, and have shown that D3 raises the blood vitamin D levels more than its counterpart.


Many studies have compared D2 and D3 directly, and some have reported that D3 is more biologically active; that D3 raised 25(OH)D more than D2 at the same dose. A meta-analysis of several of these studies in 2012 concluded that this depended on the way that vitamin D was delivered. The conclusion was that in large single doses, D3 was more effective at raising 25(OH)D; however, in smaller, consistent daily doses (1000-4000 IU), D2 and D3 worked equally well.2 Since this meta-analysis was published, new evidence now suggests that even in a daily dose, D3 is more effective at raising 25(OH)D levels. In the first study, adults were randomly assigned to 1000 IU of vitamin D2, 1000 IU D3 or placebo daily for 25 weeks. At the end of the study, 25(OH)D concentrations in the D3 group were higher than those in the D2 group.3 A second study using 2000 IU doses of D2 or D3 compared to placebo ran for 8 weeks, and similarly found that 25(OH)D increased significantly more in the D3 group compared to the D2 group. These studies suggest that vitamin D3 is a higher quality supplement than vitamin D2.

Reference Intakes

The Food and Nutrition Board (FNB) has provided the intake reference values for Vitamin D and other nutrients by developing the Dietary Reference Intakes (DRIs). DRI is the general term for a set of reference values used to plan and assess nutrient intakes of healthy people. These values, which vary by age and gender, include:

  • Tolerable Upper Intake Level (UL): maximum daily intake unlikely to cause adverse health effects.
  • Adequate Intake (AI): established when evidence is insufficient to develop an RDA and is set at a level assumed to ensure nutritional adequacy.
  • Recommended Dietary Allowance (RDA): average daily level of intake sufficient to meet the nutrient requirements of nearly all (97%–98%) healthy people.

An RDA was put forth by the FNB establishing the daily intake of Vitamin D that is sufficient to maintain normal calcium metabolism and bone health in healthy individuals. RDAs for vitamin D are listed in both micrograms (mcg) and International Units (IUs); the biological activity of 40 IU is equal to 1 mcg. Even though sunlight may be a major source of vitamin D for some, the vitamin D RDAs are all set on the basis of minimal sun exposure. Keep in mind that sun exposure is the fastest way to overcome a vitamin D deficiency.

Age Male Female Pregnancy
0–12 months 400 IU
(10 mcg)
400 IU
(10 mcg)
1–13 years 600 IU
(15 mcg)
600 IU
(15 mcg)
14–18 years 600 IU
(15 mcg)
600 IU
(15 mcg)
600 IU
(15 mcg)
19–50 years 600 IU
(15 mcg)
600 IU
(15 mcg)
600 IU
(15 mcg)
51–70 years 600 IU
(15 mcg)
600 IU
(15 mcg)
>70 years 800 IU
(15 mcg)
800 IU
(15 mcg)

Vitamin D Deficiency

Nutrient deficiencies are usually the result of dietary inadequacy, impaired absorption and use, increased requirement, or increased excretion. A vitamin D deficiency can occur when usual intake is lower than recommended levels over time, exposure to sunlight is limited, the kidneys cannot convert 25(OH)D to its active form, or absorption of vitamin D from the digestive tract is inadequate. Vitamin D-deficient diets are associated with milk allergy, lactose intolerance, ovo-vegetarianism, and veganism.


Photo Credit: Brian Moss

Rickets and osteomalacia are the classical vitamin D deficiency diseases. In children, vitamin D deficiency causes rickets, a disease characterized by a failure of bone tissue to properly mineralize, resulting in soft bones and skeletal deformities. Rickets was first described in the mid-17th century by British researchers. In the late 19th and early 20th centuries, German physicians noted that consuming 1–3 teaspoons/day of cod liver oil could reverse rickets. The fortification of milk with vitamin D beginning in the 1930s has made rickets a rare disease in the United States, although it is still reported periodically, particularly among African American infants and children. This is because melanin in the skin hinders vitamin D synthesis in dark-skinned individuals.

Prolonged exclusive breastfeeding without the AAP-recommended vitamin D supplementation is a significant cause of rickets, particularly in dark-skinned infants breastfed by mothers who are not vitamin D replete. Additional causes of rickets include extensive use of sunscreens and placement of children in daycare programs, where they often have less outdoor activity and sun exposure. Rickets is also more prevalent among immigrants from Asia, Africa, and the Middle East, possibly because of genetic differences in vitamin D metabolism and behavioral differences that lead to less sun exposure.

In adults, vitamin D deficiency can lead to osteomalacia, resulting in weak bones. Symptoms of bone pain and muscle weakness can indicate inadequate vitamin D levels, but such symptoms can be subtle and go undetected in the initial stages. In a recent study doctors gave hemodialysis patients (people being treated for kidney failure) doses of vitamin D to determine if it would effect their muscle size and strength. All of the patients being supplemented with vitamin D saw increases is muscle size and strength, the studies conclusion “Treatment with active vitamin D was associated with greater muscle size and strength in this cohort of HD patients.”

Sources of Vitamin D

Very few foods in nature contain vitamin D which is why sunlight is always the best source. Wether or not you are getting enough sunlight it is still a good idea to get some vitamin D from your diet. Fish (such as salmon, tuna, and mackerel) and fish liver oils are among the best sources. Small amounts of vitamin D are also found in beef liver, and egg yolks. Other dairy products such as milk and cheese also contain Vitamin D. Contained in these foods is primarily vitamin D3 and its metabolite 25(OH)D3. Some mushrooms provide vitamin D2 in variable amounts. Mushrooms with enhanced levels of vitamin D2 from being exposed to ultraviolet light under controlled conditions are also available.


Fortified foods provide most of the vitamin D in the American diet. For example, almost all of the U.S. milk supply is voluntarily fortified with 100 IU/cup. (In Canada, milk is fortified by law with 35–40 IU/100 mL, as is margarine at ≥530 IU/100 g.) In the 1930s, a milk fortification program was implemented in the United States to combat rickets, which was a major health problem back then. Other dairy products made from milk, such as cheese and ice cream, are generally not fortified. Ready-to-eat breakfast cereals often contain added vitamin D, as do some brands of orange juice, yogurt, margarine and other food products. Even infant formula is fortified with vitamin D in both the US and Canada.

Sun Exposure

The majority of people get at least a small percentage of their vitamin D from sunlight. Season, skin melanin content, length of day, smog, cloud cover, time of day, and sunscreen all affect UV radiation exposure and vitamin D synthesis. Your body stores whatever it can that it produces from sunlight in the warmer months and stores it in the liver and fat. Cloud cover and shade (even produced by pollution) can reduce UV energy by 50-60%. It it also important to note that the specific radiation that produces vitamin D (UVB) cannot penetrate glass. This means you will not produce any of the vitamin sitting by the window on a sunny day, a major misconception most people have.

All vitamins are important, however when it comes to athletic performance vitamin D seems to at the top of the food chain. From helping strengthen bones to building muscle and strength, every individual should make sure they are getting ample amounts of vitamin D, specifically D3. It is one of the few substances that is natural and actually proven to help build a stronger healthier body.


1.Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academy Press, 2010.
2. Tripkovic L, Lambert H, Hart K, et al: Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr 2012.
3. Logan VF, Gray AR, Peddie MC, et al: Long-term vitamin D3 supplementation is more effective than vitamin D2 in maintaining serum 25-hydroxyvitamin D status over the winter months. Br J Nutr 2013, 109:1082-1088.
4. Lehmann U, Hirche F, Stangl GI, et al: Bioavailability of vitamin D(2) and D(3) in healthy volunteers, a randomized placebo-controlled trial. J Clin Endocrinol Metab 2013, 98:4339-4345.
5.U.S. Department of Agriculture, Agricultural Research Service. 2011. USDA National Nutrient Database for Standard Reference, Release 24. Nutrient Data Laboratory Home Page,
6.Ovesen L, Brot C, Jakobsen J. Food contents and biological activity of 25-hydroxyvitamin D: a vitamin D metabolite to be reckoned with? Ann Nutr Metab 2003;47:107-13.
7.Mattila PH, Piironen VI, Uusi-Rauva EJ, Koivistoinen PE. Vitamin D contents in edible mushrooms. J Agric Food Chem 1994;42:2449-53.
8.Bailey RL, Dodd KW, Goldman JA, Gahche JJ, Dwyer JT, Moshfegh AJ, et al. Estimation of total usual calcium and vitamin D intakes in the United States. J Nutr 2010;140:817-822.
9.Picciano MF. Nutrient composition of human milk. Pediatr Clin North Am 2001;48:53-67.
10.Holick MF, Binkley NC, Bischoff-Ferrari HA, Gordon CM, Hanley DA, Heaney RP, Murad H, Weaver CM. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab 2011;96:1911-30.
11.The Relationship between Vitamin D and Muscle Size and Strength in Patients on Hemodialysis

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