Unit 9 – Vitamins and Minerals Part 2

9. 5 Vitamin D: Important to Bone Health and Beyond

Vitamin D is unique among the vitamins because we can synthesize most of what we need in our skin. Sunlight is an essential ingredient in this process, so vitamin D is sometimes called the “sunshine vitamin.” However, the amount of vitamin D synthesized in the body is often insufficient to meet our needs, so many people also need to consume dietary sources.

Metabolism and Functions of Vitamin D

Vitamin D synthesis in the skin begins with the conversion of cholesterol to previtamin  D. Then, in the presence of ultraviolet (UV) rays from sunlight,  it’s converted to vitamin D, which is transported to the liver.

Did you wonder the difference  between  vitamin D2 vs D3

Vitamin D2 (ergocalciferol) is the plant source of the vitamin derived from yeast and some plants, such as mushrooms. Humans and other animals make Vitamin D3 (cholecalciferol).

To be used by the body, vitamin D must be activated by enzymes located in our livers and kidneys, as noted in the diagram below.

Vitamin D can be synthesized in the skin (vitamin D3) or provided in the diet (vitamin D2 or D3). It is converted by reactions occurring first in the liver (making calcidiol) and then kidney (making calcitriol, the active form). Once active, vitamin D works in several ways to ensure blood calcium homeostasis and enhance the availability of calcium for bone mineralization. Active vitamin D will increase the absorption of calcium and phosphorus in the intestines. It also increasers bone breakdown which releases calcium and phosphorus into the blood; at the kidneys calcium retention is stimulated reducing the amount lost in the urine. Normal levels of blood calcium and phosphorus support mineralization of the bones

Figure 9.9. Vitamin D can be synthesized in the skin or provided in the diet. It is converted by reactions occurring first in the liver (making calcidiol) and then the kidney (making calcitriol, the active form). Once active, vitamin D works in several ways to ensure blood calcium homeostasis and enhance the availability of calcium for bone mineralization.

Beyond its role in bone health, vitamin D has many other functions in the body.  Some studies show that the vitamin reduces risks of cancer, diabetes, cardiovascular disease, and multiple sclerosis. However, it has been difficult to determine if a lack of vitamin D actually contributes to the cause of these diseases, and research in this area is ongoing.1

Sunlight as a Source of Vitamin D

Exposure to sunlight provides the majority of their body’s needs, and a little sun exposure can go a long way. Vitamin D researchers suggest that most people need between 5 and 30 minutes of sun exposure between 10 AM and 3 PM, at least twice per week, to synthesize adequate vitamin D. However, any factor that decreases exposure to UV rays can interfere with vitamin D synthesis.2

With so many factors affecting UV radiation exposure, many people cannot synthesize enough vitamin D for at least part of the year. Because vitamin D is fat-soluble, liver and adipose storage can supply the body for a while. Beyond that, dietary sources and supplements may be needed to meet the vitamin D requirement.2

Dietary Sources of Vitamin D

Only a few foods are naturally good sources of vitamin D. These include fatty fish such as salmon, tuna, and mackerel, as well as fish liver oil (e.g., cod liver oil). Smaller amounts are found in egg yolks, cheese, and beef liver. Additionally, some mushrooms grown in UV light can be a good source of vitamin D.

Most cow’s milk is fortified with vitamin D in the U.S. and Canada, but other dairy products such as ice cream and cheese are not. Fortified orange juice, soymilk and other plant-based beverages, and breakfast cereal can all contribute to dietary vitamin D intake, although amounts added vary significantly between products.2

Bar graph showing dietary sources of vitamin D compared with the RDA for adults of 600 IU. Top sources include cod liver oil, trout, salmon, mushrooms, and fortified milk, plant-based milk and cereal. Food sources pictured include salmon, milk, soy milk, mushrooms, and fortified cereal.

Figure 9.10. Dietary sources of vitamin D. Source: Examples of good sources pictured include salmon, milk, mushrooms, fortified soy milk, and fortified cereal. NIH Office of Dietary Supplements

 Human breast milk doesn’t contain adequate vitamin D, so the American Academy of Pediatrics recommends that breastfed infants receive a supplement with 400 IU of vitamin D per day until they have weaned to vitamin D-fortified formula or cow’s milk. A vitamin D supplement may also be recommended for older children and adults, depending on dietary intake and sun exposure, but this should be discussed with a healthcare provider.

Vitamin D Deficiency and Toxicity

In children, vitamin D deficiency causes rickets, a disease in which the bones are soft, weak, and deformed. Rickets was very common in the U.S. until the 1930s when FDA implemented the milk fortification program.  Milk fortification has virtually eliminated rickets from the U.S. and other developed countries. However, rickets does still occur in breastfed infants and children raised on vegan diets who aren’t provided with other sources of vitamin D, particularly if they have darker skin pigmentation.1,3

A black-and-white photograph shows three children with rickets, each with legs deformed to varying degrees. The child on the left has legs that bow inwards, her knees touching. The child on the right has legs that are bowed, with knees far apart.

Figure 9.11. Vitamin D deficiency in children causes rickets, a disease in which inadequate vitamin D leads to soft, weak, and deformed bones.

In adults, vitamin D deficiency causes osteomalacia, characterized by softening of bones, reduced bone mineral density, and increased risk of osteoporosis. Because bones are continuously remodeled throughout the lifespan, inadequate vitamin D limits the calcium available to rebuild bone tissue. Vitamin D deficiency can also cause bone pain and muscle weakness and pain, symptoms that can increase the risk of falling and fractures, particularly in older adults.2

Although vitamin D toxicity is rare, taking excessive amounts of vitamin D in supplement form can lead to hypercalcemia or high blood calcium. Hypercalcemia can cause kidney damage and calcium deposits to develop in soft tissues such as the kidneys, blood vessels, or other parts of the cardiovascular system. Synthesis of vitamin D from the sun does not cause vitamin D toxicity because vitamin D synthesis is tightly regulated and decreases if the body has abundant vitamin D.





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Introduction to Nutrition and Wellness Copyright © 2022 by Janet Colson; Sandra Poirier; and Yvonne Dadson is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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