Calcitriol plays an important role in the maintenance of several organ systems. However, its major role is to increase the flow of calcium into the bloodstream, by promoting absorption of calcium and phosphorus from food in the intestines, and reabsorption of calcium in the kidneys; enabling normal mineralization of bone and preventing hypocalcemic tetany. It is also necessary for bone growth and bone remodeling by osteoblasts and osteoclasts.
Without sufficient vitamin D, bones can become thin, brittle, or misshapen. Deficiency can arise 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. Vitamin D deficiency results in impaired bone mineralization and leads to bone softening diseases, rickets in children and osteomalacia in adults, and possibly contributes to osteoporosis.
Vitamin D plays a number of other roles in human health including inhibition of calcitonin release from the thyroid gland. Calcitonin acts directly on osteoclasts, resulting in inhibition of bone resorption and cartilage degradation. Vitamin D can also inhibit parathyroid hormone secretion from the parathyroid gland, modulate neuromuscular and immune function and reduce inflammation.
Nutrition
Milk and cereal grains are often fortified with vitamin D.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 having 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 outdoors, these cultural changes reduced natural production of vitamin D and caused deficiency diseases. In many countries, such foods 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 a quarter of the estimated adequate intake for adults over age 50.
Deficiency
Deficiency of vitamin D can result from a number of factors: 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. Rickets was first described in the 17th century, by Daniel Whistler and Francis Glisson. The role of diet in the development of rickets was determined by Edward Mellanby between 1918–1920. By altering the diets of dogs raised in the absence of sunlight, he was able to establish unequivocally that rickets was linked with a deficiency of diet, and identified cod liver oil as an excellent anti-rachitic agent. 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[41], cognitive impairment which includes memory loss and foggy brain, and several autoimmune diseases including type 1 diabetes. 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. A resurgence of interest in vitamin D deficiency has led to continued studies on the topic and a focus on educating the consumer on the prevalence and degree of deficiency among the general public.
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