12.1 Vitamin D

Vitamin D

Vitamin D is unique among the vitamins in that it is part vitamin, part hormone. It is considered part hormone for two reasons: (1) we have the ability to synthesize it, and (2) it has hormone- like functions. The amount synthesized, however, is often not enough to meet our needs. Thus, we need to consume this vitamin under certain circumstances, meaning that vitamin D is a conditionally essential micronutrient.

There are two major dietary forms of vitamin D: the form produced by plants and yeast is vitamin D₂ (ergocalciferol), and the form made by animals is vitamin D₃ (cholecalciferol). The structures of these two forms are shown below. Notice that the only difference is the presence of a double bond in D₂ that is not in D₃.

Figure 12.11 Structure of vitamin D₂ (ergocalciferol) and vitamin D₃ (cholecalciferol)1,2

We synthesize vitamin D₃ from cholesterol, as shown below. In the skin, cholesterol is converted to 7-dehydrocholesterol. In the presence of UV-B light, 7-dehydrocholesterol is converted to vitamin D₃. Synthesized vitamin D will combine with vitamin D-binding protein (DBP) to be transported to the liver. Dietary vitamin D₂ and D₃ is transported to the liver via chylomicrons. Once in the liver, vitamin D₃ is converted into calcitriol (shown by its chemical abbreviation, 1,25(OH)₂D, in Figure 12.12), which is the circulating form of vitamin D. The synthesis and activation of vitamin D is shown in the figures below.

Figure 12.12 Vitamin D synthesis and activation3

For more information on vitamin D, see the Required Web Link below.

Required Web LinkVitamin D Fact Sheet for Health Professionals

Subsections:

• 12.11 Environmental Factors That Impact Vitamin D3 Synthesis
• 12.12 Sources of Dietary Vitamin D
• 12.16 Vitamin D Deficiency, Toxicity, & Insufficiency

References & Links

• http://en.wikipedia.org/wiki/File:Ergocalciferol.svg
• http://en.wikipedia.org/wiki/File:Cholecalciferol.svg
• http://commons.wikimedia.org/wiki/File:Liver.svg

Links

Vitamin D Fact Sheet for Health Professionals – https://ods.od.nih.gov/factsheets/VitaminD- HealthProfessional/

Environmental Factors That Impact Vitamin D3 Synthesis

There are a number of environmental factors that affect vitamin D₃ synthesis: Latitude, Season, Time of Day, Skin Color, Age, and Clothing.

Latitude

The latitude a person is at affects that person’s ability to synthesize vitamin D₃. There is an inverse relationship between distance from the equator and UV light exposure. Thus, with increased distance from the equator (increased latitude), there is decreased UV light exposure and vitamin D₃ synthesis. The link below shows the latitude and longitude lines of the United States.

Required Web LinkUnited States Latitude and Longitude Lines

Seasons

Seasons also make a difference in vitamin D₃ synthesis. In Boston (42० N), vitamin D synthesis only occurs from March-October, because during late fall and winter not enough UV-B reaches the earth’s surface to synthesize vitamin D₃. However, in Los Angeles (34० N), vitamin D₃

synthesis occurs year round2. The difference is the angle of the sun relative to latitude and how many UV-B photons are absorbed before they reach the earth’s surface1.

Time

Time of day is also an important factor in affecting vitamin D₃ synthesis. Vitamin D₃ synthesis increases in the morning before peaking at noon, then declines the rest of the day1.

Skin pigmentation

Another factor that plays an important role in vitamin D₃ synthesis is skin pigmentation. Skin pigmentation tends to be darker around the equator to help protect inhabitants from the harmful effects of sun exposure. Skin color is the result of increased production of the pigment melanin, which is the pigment responsible for all skin colors.

Very dark skin color can provide a sun protection factor (SPF) 8-30 for those individuals who never burn2. These individuals will require approximately 5- to 10-times greater sunlight exposure than a light-skinned, white person to synthesize the same amount of vitamin D₃2,3.

Age

Age also plays a factor in vitamin D₃ synthesis. Aging results in decreased 7-dehydrocholesterol concentrations in the skin, resulting in an approximately 75% reduction in the vitamin D₃ synthesis capability by age 703.

Clothing

Clothing is another factor that influences vitamin D₃ synthesis. More clothing means that less sun reaches your skin, and thus less vitamin D₃ synthesis.

References & Links

• Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, editors. (2006) Modern nutrition in health and disease. Baltimore, MD: Lippincott Williams & Wilkins.
• Holick M. (2008) Vitamin D: A D-lightful health perspective. Nutr Rev 66(10 Suppl 2): S182.
• Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.

Links

US Latitude and Longitude Lines – http://modernsurvivalblog.com/survival-skills/basic-map- reading-latitude-longitude/

Dietary Sources of Vitamin D

Because of the possible double-edged sword of sun exposure for synthesizing vitamin D₃, consuming vitamin D from the diet or supplements is the alternative.

However, there are a limited number of food naturally rich in vitamin D. Good sources of vitamin D are fatty fish (salmon, tuna, etc.) and their oils (such as cod liver oil). The amount in fatty fish varies greatly with wild-caught salmon being the highest. One study showed that farmed salmon contained almost 75% less vitamin D than wild-caught salmon1. It is not known whether this disparity exists between other types of farmed and wild-caught fish varieties.

Table 12.121 Vitamin D content of fish1

Thus, since not many foods contain vitamin D, many brands of milk have been fortified with vitamin D₂ or D₃ (100 IU/8 oz) since the 1930s2. However, the actual measured amount of vitamin D in many brands of milk is far less than stated on their labels3,4. Part of this problem stems from a lack of a standardized method for measuring vitamin D in the past. Without standardized analysis, there inevitably was a wide range of variation from lab to lab in the reported amount of vitamin D.

References & Links

• Lu Z, Chen TC, Zhang A, Persons KS, Kohn N, et al. (2007) An evaluation of the vitamin D3 content in fish: Is the vitamin D content adequate to satisfy the dietary requirement for vitamin D? J Steroid Biochem Mol Biol 103(3-5): 642.
• Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.
• Holick MF, Shao Q, Liu WW, Chen TC. (1992) The vitamin D content of fortified milk and infant formula. New England Journal of Medicine, the 326(18): 1178.
• Faulkner H, Hussein A, Foran M, Szijarto L. (2000) A survey of vitamin A and D contents of fortified fluid milk in ontario. J Dairy Sci 83(6): 1210.

Vitamin D Deficiency, Toxicity & Insufficiency

Rickets is a vitamin D deficiency in infants and children. A lack of vitamin D leads to decreased bone mineralization, causing the bones to become weak. The bones then bow under pressure, leading to the characteristic bowed legs, as seen in Figure 12.131.

Figure 12.131 Children suffering from rickets1

Osteomalacia is a vitamin D deficiency in adults and results in poor bone mineralization. The bone becomes soft, resulting in bone pain and an increased risk of fractures2. While rickets and osteomalacia are fairly rare in the United States, it is believed that vitamin D insufficiency might be much more widespread. Insufficiency means that the level of intake, or body status, is suboptimal (neither deficient nor optimal). Suboptimal/insufficient means intake, or status, is higher than deficient, but lower than optimal. Thus, higher intake levels will provide additional benefits. The functions of vitamin D are growing by the day due to increased research discoveries. These functions now include benefits beyond bone health, further supporting the importance of vitamin D. In late 2010, an RDA for vitamin D was established (was an Adequate Intake before). This made it, along with calcium, the first micronutrients to have their DRIs revised3. The RDA for vitamin D is 3-times higher than the previous AI. Many believe these are more reasonable levels, while others think that the new RDA is still not high enough. This belief, that many people’s vitamin D intake/status is suboptimal, is challenged by a recent review described in the link below that found that vitamin D did not reduce osteoporosis risk. In addition, a recent meta-analysis (second link) concluded, “there is probably no benefit to

expect from vitamin D supplementation in normally healthy people.”

Required Web LinksVitamin D Ineffective for Preventing Osteoporosis Limits of Vitamin D Supplements

Vitamin D from supplements can become toxic. You cannot develop vitamin D toxicity from sun exposure, because the sunlight degrades a precursor of vitamin D₃ in the skin4. Vitamin D toxicity results in hypercalcemia or high blood calcium levels. These become problematic because it can lead to the calcification of soft tissues.

References & Links

• http://en.wikipedia.org/wiki/File:Rickets_USNLM.gif
• Whitney E, Rolfes SR. (2011) Understanding nutrition. Belmont, CA: Wadsworth Cengage Learning.
• http://iom.nationalacademies.org/Reports/2010/Dietary-Reference-Intakes-for-Calcium-and- Vitamin-D.aspx
• Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, editors. (2006) Modern nutrition in health and disease. Baltimore, MD: Lippincott Williams & Wilkins.

Links

Vitamin D Ineffective for Preventing Osteoporosis – http://well.blogs.nytimes.com/2013/10/17/vitamin-d-ineffective-for-preventing-osteoporosis/? Limits of Vitamin D Supplements – http://well.blogs.nytimes.com/2013/12/11/limits-of-vitamin- d-supplements/?