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Vitamin D

Description

Although vitamin D is typically classified as a fat-soluble vitamin, it actually functions as a hormone in the body. Because it can be manufactured by the body (formed in the skin following exposure to the ultraviolet rays of the sun), vitamin D is not technically classified as an essential nutrient at all. In the skin, exposure to ultraviolet rays converts vitamin D precursors (compounds with structures similar to cholesterol) into an inactive form of vitamin D. This inactive form is then converted to the active form by enzymes located in the liver and kidneys. Regular sunlight exposure is the primary way that most of us get our vitamin D. Food sources of vitamin D include only a few such as vitamin D-fortified milk (100 IU per cup), cod liver oil, and fatty fish such as salmon and small amounts are found in egg yolks and liver.

Functions

The primary effect of vitamin D is to maintain calcium levels in the blood. To do this, vitamin D promotes both the absorption of calcium from the intestines into the blood and the removal of calcium from the bones into the blood. Vitamin D also reduced calcium loss in the urine. In most cases, the increased calcium absorption results in an increase in bone density and bone strength, which can help reduce the risk of osteoporosis.

Vitamin D deficiency

  • Vitamin D plays a central role in maintenance of bone health. It is essential for calcium absorption and it regulates
  • Vitamin D deficiency is relatively common in the U.S. Studies have demonstrated that nearly 60% of people over 70 years are vitamin D deficient. Furthermore, the average daily intake of vitamin D in the United States is only 30% of the RDA.
  • A vitamin D deficiency can result from: 1) inadequate dietary intake, 2) reduced endogenous production due to limited exposure to sunlight, 3) kidney and/or liver dysfunctions, which inhibit conversion of vitamin D to its metabolically active forms, or 4) fat-malabsorption syndromes, such as cystic fibrosis and cholestatic liver diseases.
  • Symptoms of deficiency include: Rickets in children and osteomalacia in adults resulting from poor ossification of bone tissue, resulting in weak bones that bend readily. Skeletal deformities and muscular weakness are common symptoms of rickets. Osteomalacia develops in women with closely spaced, multiple pregnancies or in people confined indoors with no exposure to sunlight. Symptoms in adults include painful softening and bending of bones, low serum calcium concentrations and tetany. Less severe vitamin D deficiency can result in hyperparathyroidism and increased bone turnover leading to bone loss and osteoporosis.
  • Osteoporosis: The disease results in fractures of the vertebra, hip and wrist that occur either spontaneously or with minimal daily activities such as opening a window. The prevention and treatment of osteoporosis depends on optimal intake of both calcium and vitamin D. In addition, gene-regulating vitamin D receptors on cell membranes influence bone density and result in a genetic predisposition to osteoporosis.

Dietary sources

The only frequently consumed dietary source of vitamin D is vitamin D-fortified milk. Muscle meat, milk (nonfortified), fruits and vegetables are negligible sources of vitamin D. Liver, butter, cream, mushrooms, fatty fish oil and egg yolk contains varying amounts of vitamin D, and some ready-to-eat cereals are fortified with the vitamin. Cod liver oil and fatty fish oils are excellent sources of vitamin D.

Required Intakes

The new DRI for vitamin D has been set at 5 - 10 mcg (200 - 400 IU). However, research has indicated that a daily intake of 5 to 20 mcg (200 to 800 IU) could benefit most adults. This higher concentration would be sufficient to maintain healthy serum concentrations of vitamin D. This concentration could also provide many additional benefits to bone health, multiple sclerosis, diabetes mellitus, syndrome X and for cancer prevention.

 Age  mg/day IU/day  Tolerable Upper Intake Levels
 0 to 6 months  5  200  25
 7 to 12 months  5  200  25
 1 to 3 years  5  200  50
 4 to 8 years  5  200  50
 9 to 18 years  5  200  50
 19 to 50 years  5  200  50
 51 to 70 years  10  400  50
 70 + years  15  600  50
 Pregnancy  5  200  50
 Lactation  5  200  50
 

Cautions

  • Prolonged exposure to sunlight does not cause vitamin D toxicity.
  • The NOAEL (the No Observed Adverse Effect Level) is 50 mcg (2000 IU).
  • The majority of dietary supplements include 400 IU of vitamin D. There are no reports of adverse effects at this concentration of intake.
  • There is no evidence of any benefit in taking more than 800 IU.
  • The effects of excessive vitamin D intake include hypercalcemia and hypercalciuria, leading to deposition of calcium in soft tissue and irreversible renal and cardiovascular damage.
  • Symptoms of toxicity include loss of appetite, excessive thirst, nausea, vomiting, irritability, weakness and weight loss.