vitamin d in milk
Vitamin D is a group of fat-soluble prohormones, the two major forms of which are vitamin D2 (or ergocalciferol) and vitamin D3 (or cholecalciferol). The term vitamin D also refers to metabolites and other analogues of these substances. Vitamin D3 is produced in skin exposed to sunlight, specifically ultraviolet B radiation.
Vitamin D plays an important role in the maintenance of organ systems.
- Vitamin D regulates the calcium and phosphorus levels in the blood by promoting their absorption from food in the intestines, and by promoting reabsorption of calcium in the kidneys, which enables normal mineralization of bone and prevents hypocalcemic tetany. It is also needed for bone growth and bone remodeling by osteoblasts and osteoclasts..
- In the absence of vitamin K or with drugs (particularly blood thinners) that interfere with Vitamin K metabolism, Vitamin D can promote soft tissue calcification.
- It inhibits parathyroid hormone secretion from the parathyroid gland.
- Vitamin D affects the immune system by promoting phagocytosis, anti-tumor activity, and immunomodulatory functions.
Vitamin D deficiency can result from inadequate intake coupled with inadequate sunlight exposure; disorders that limit its absorption; conditions that impair conversion of vitamin D into active metabolites, such as liver or kidney disorders; or, rarely, by a number of hereditary disorders. Deficiency results in impaired bone mineralization and leads to bone softening diseases rickets in children and osteomalacia in adults, and possibly contributes to osteoporosis. However, sunlight exposure, to avoid deficiency, carries other risks, including skin cancer; this risk is avoided with dietary absorption, either through diet or as a dietary supplement.
Mechanism of action
After vitamin D is produced in the skin or consumed in food, it is converted in the liver and kidney to form 1,25 dihydroxyvitamin D, (1,25(OH)2D) the physiologically active form of vitamin D (when "D" is used without a subscript it refers to either D2 or D3). This physiologically active form of vitamin D is known as calcitriol. Following this conversion, calcitriol is released into the circulation, and by binding to a carrier protein in the plasma, vitamin D binding protein (VDBP), it is transported to various target organs.
The physiologically active form of vitamin D mediates its biological effects by binding to the vitamin D receptor (VDR), which is principally located in the nuclei of target cells. The binding of calcitriol to the VDR allows the VDR to act as a transcription factor that modulates the gene expression of transport proteins (such as TRPV6 and calbindin), which are involved in calcium absorption in the intestine.
The Vitamin D receptor belongs to the nuclear receptor superfamily of steroid/thyroid hormone receptors, and VDR are expressed by cells in most organs, including the brain, heart, skin, gonads, prostate, and breast. VDR activation in the intestine, bone, kidney, and parathyroid gland cells leads to the maintenance of calcium and phosphorus levels in the blood (with the assistance of parathyroid hormone and calcitonin) and to the maintenance of bone content.
The VDR is known to be involved in cell proliferation and differentiation. Vitamin D also affects the immune system, and VDR are expressed in several white blood cells including monocytes and activated T and B cells.
Nutrition
Vitamin D is naturally produced by the human body when exposed to direct sunlight. Season, geographic latitude, time of day, cloud cover, smog, and sunscreen affect UV ray exposure and vitamin D synthesis in the skin, and it is important for individuals with limited sun exposure to include good sources of vitamin D in their diet. Extra vitamin D is also recommended for older adults and people with dark skin. Individuals with a high-risk of deficiency should consume 25 μg (1000 IU) of vitamin D daily to maintain adequate blood concentrations of 25-hydroxyvitamin D.
As civilization and the Industrial Revolution enabled humans to work indoors and wear more clothes when in the sun, these cultural changes reduced natural production of vitamin D and caused deficiency diseases. In many countries, foods such as milk, yogurt, margarine, oil spreads, breakfast cereal, pastries, and bread are fortified with vitamin D2 and/or vitamin D3, to minimize the risk of vitamin D deficiency. In the United States and Canada, for example, fortified milk typically provides 100 IU per glass, or one quarter of the estimated adequate intake for adults over the age of 50. Supplementation of 100 IU (2.5 microgram) vitamin D3 raises blood calcidiol levels by 2.5 nmol/litre (1 ng/ml).
Natural sources of vitamin D include:
- Fish liver oils, such as cod liver oil, 1 Tbs. (15 mL) provides 1,360 IU (one IU equals 25 ng)
- Fatty fish species, such as:
- Herring, 85 g (3 ounces (oz)) provides 1383 IU
- Catfish, 85 g (3 oz) provides 425 IU
- Salmon, cooked, 100 g (3.5 oz]) provides 360 IU
- Mackerel, cooked, 100 g (3.5 oz]), 345 IU
- Sardines, canned in oil, drained, 50 g (1.75 oz), 250 IU
- Tuna, canned in oil, 85 g (3 oz), 200 IU
- Eel, cooked, 100 g (3.5 oz), 200 IU
- One whole egg, provides 20 IU
- Beef liver, cooked, 100 g (3.5 oz), provides 15 IU
- UV-irradiated mushrooms (Vitamin D2)
The practical reality is that on average, the U.S. diet provides 100 IU/day. One problem is that much of the fortified milk is not fortified to the required amount (Vit D Dietary Reference Intakes, pages 256-57).
Adequate Intake is defined as 200 IU/day for ages infant to 50, 400/day for 51-70, and 600/day for >70. The 100% Daily Value used for product labels is 400 IU. The safe upper limit is set at 2000 IU. The Institute Of Medicine is revisiting vitamin D and calcium recommendations. The report is expected Spring 2010.
Measuring nutritional status
A blood calcidiol (25-hydroxy-vitamin D) level is the accepted way to determine vitamin D nutritional status. The optimal level of serum 25-hydroxyvitamin D is 35–55 ng/mL (or 90-140 nmol/L); with some debate among medical scientists for the slightly higher value.
For instance, a later classification is:
- 0-14.9 ng/mL = Severely deficient
- 15.0-31.9 ng/mL = Mildly deficient
- 32.0-100.0 ng/mL = Optimal
- >100.0 ng/mL = Toxicity possible
Deficiency
Main article: Hypovitaminosis DDeficiency of vitamin D can result from a number of factors including: inadequate intake coupled with inadequate sunlight (UVB) exposure, disorders that limit its absorption from the gastrointestinal tract, conditions that impair conversion of vitamin D into active metabolites, such as liver or kidney disorders and body characteristics such as skin color and body fat. Rarely deficiency can result from a number of hereditary disorders. Deficiency results in impaired bone mineralization, and leads to bone softening diseases including:
- Rickets, a childhood disease characterized by impeded growth, and deformity, of the long bones. The role of diet in the development of rickets was determined by Edward Mellanby between 1918–1920. In 1921 Elmer McCollum identified a substance found in certain fats that could prevent rickets. Prior to the fortification of milk products with vitamin D, rickets was a major public health problem. In the United States the fortification of milk with 10 micrograms (400 IU) of vitamin D per quart in the 1930s led to a dramatic decline in the number of rickets cases.
- Osteomalacia, a bone-thinning disorder that occurs exclusively in adults and is characterized by proximal muscle weakness and bone fragility. The effects of osteomalacia are thought to contribute to chronic musculoskeletal pain. A number of reports thus indicate that vitamin D deficiency may be related to various types of pain, but of the five small double-blind randomized controlled trials, only one found a reduction in pain after supplementation, and there is no persuasive evidence of lower vitamin D status in chronic pain sufferers compared to controls.
- Osteoporosis, a condition characterized by reduced bone mineral density and increased bone fragility.
Vitamin D malnutrition may also be linked to an increased susceptibility to several chronic diseases such as high blood pressure, tuberculosis, cancer, periodontal disease, multiple sclerosis, chronic pain, seasonal affective disorder , peripheral artery disease, cognitive impairment which includes memory loss and foggy brain, and several autoimmune diseases including type 1 diabetes (see role in immunomodulation). There is an association between low vitamin D levels and Parkinson's disease, but whether Parkinson's causes low vitamin D levels, or whether low vitamin D levels play a role in the pathogenesis of Parkinson's disease has not been established.
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