Role of Vitamin D

Vitamin D refers to a group of fat-soluble vitamins derived from cholesterol. Vitamins D2 (ergocalciferol) and D3 (calcitriol) are the only ones known to perform biological actions in the human body. The skin synthesizes vitamin D when exposed to sunlight. In fact, for most people, more than 90 percent of their vitamin D3 comes from casual exposure to UVB rays in sunlight. Anything that reduces your exposure to the sun’s UVB rays decreases the amount of vitamin D3 your skin synthesizes. That would include long winters, your home’s altitude, whether you are wearing sunscreen, and the color of your skin (including tanned skin). Do you ever wonder about an increased risk of skin cancer by spending too much time in the sun? Do not fret. Less than thirty minutes of sun exposure to the arms and legs will increase blood levels of vitamin D3 more than orally ingesting 10,000 IU (250 micrograms) of vitamin D3.

Activated vitamin D3 (calcitriol) regulates blood calcium levels and bone health in concert with parathyroid hormone by regulating calcium and phosphorus absorption in the small intestine. With an inadequate vitamin D intake, less than 15 percent of calcium is absorbed from foods or supplements. The effects of calcitriol on calcium homeostasis are critical for bone health. Vitamin D can also maintain blood calcium levels by working with the kidney to excrete more or less calcium in the urine. Finally, in conjunction with parathyroid hormone, vitamin D stimulates osteoclasts to break down bone when calcium is needed elsewhere in the body.

Health Benefits

Observational studies have shown that people with low levels of vitamin D in their blood have lower bone mineral density and an increased incidence of osteoporosis. In contrast, diets with high intakes of salmon, which contains a large amount of vitamin D, are linked with better bone health. A review of twelve clinical trials, published in the May 2005 issue of the Journal of the American Medical Association, concluded that oral vitamin D supplements at doses of 700–800 international units per day, with or without coadministration of calcium supplements, reduced the incidence of hip fracture by 26 percent and other nonvertebral fractures by 23 percent.^[1] A reduction in fracture risk was not observed when people took vitamin D supplements at doses of 400 international units.

Many other health benefits have been linked to higher intakes of vitamin D, including decreased risk of infections and cardiovascular disease. Furthermore, evidence from laboratory studies conducted in cells, tissues, and animals suggests vitamin D prevents the growth of certain cancers, blocks inflammatory pathways, reverses atherosclerosis, increases insulin secretion, and blocks viral and bacterial infection.

Dietary Reference Intake for Vitamin D

The Institute of Medicine RDAs for vitamin D for different age groups are listed in Table 12.3 “Dietary reference intakes for vitamin D”. For adults, the RDA is 600 international units (IUs), which is equivalent to 15 micrograms of vitamin D. Slightly higher levels are recommended for adults aged fifty and older. It is recommended for them to get between 800 and 1,000 international units of vitamin D every day. The tolerable upper intake level (UL) for vitamin D is 4,000 international units per day. Toxicity from excess vitamin D is rare, but certain diseases such as hyperparathyroidism, lymphoma, and tuberculosis make people more sensitive to the increases in calcium caused by high intakes of vitamin D.

Dietary Sources of Vitamin D

Consuming Too Little Vitamin D

A deficiency of vitamin D in children causes a bone disease known as nutritional rickets. Rickets is very common among children in developing countries and is characterized by soft, weak, deformed bones that are exceptionally susceptible to fracture. In adults, vitamin D deficiency causes a similar disease called osteomalacia, which is characterized by a low BMD. Osteomalacia has the same symptoms and consequences as osteoporosis and often coexists with osteoporosis. Vitamin D deficiency is common, especially in the elderly population, dark-skinned populations, and the many people who live in the northern latitudes where sunlight exposure is low during the long winter season.

Figure 12.6 serves as a visual representation of the consequences of vitamin D deficiency in children, highlighting the importance of proper nutrition and sunlight exposure for healthy bone development. The child is depicted standing with bowed legs, a common symptom of the disease. The bones appear weak and soft, leading to deformities such as an enlarged head and prominent joints.

Vitamin D deficiency has been linked to an increased risk of autoimmune diseases. Immune diseases, rheumatoid arthritis, multiple sclerosis, and Type 1 diabetes have been observed in populations with inadequate vitamin D levels.

Consuming Too Much Vitamin D

Although vitamin D toxicity is rare, too much can cause high calcium concentrations or hypercalcemia. Hypercalcemia can lead to a large amount of calcium being excreted through the urine, which can cause kidney damage.  Calcium deposits may also develop in soft tissues such as the kidneys, blood vessels, or other parts of the cardiovascular system. Symptoms of hypercalcemia include weakness, loss of appetite, mental confusion, diarrhea, vomiting, excessive urine output, and excessive thirst. However, it is important to know that the synthesis of vitamin D from the sun does not cause vitamin D toxicity due to the skin production of vitamin D3 being a tightly regulated process.