The D vitamins are a group of sterols that have a hormonelike function. A major function of vitamin D is to maintain adequate levels of blood calcium. The parathyroid gland, located in the throat, also functions to raise blood calcium. Low levels of either calcium or vitamin D can stimulate the parathyroid gland to secrete excessive hormone, a condition known as hyperparathyroidism.
Hyperparathyroidism
Tumors are the most common cause of hyperparathyroidism. But if blood calcium levels fall below normal, either because of vitamin D deficiency or other factors, including inadequate calcium intake, normal parathyroid glands become overactive. In this condition, called secondary hyperparathyroidism, lab values indicate high levels of parathyroid hormone, or PTH, and low or normal levels of calcium. Because this condition is associated with kidney stones, weakened bones and constipation, sufferers are said to have "stones, bones and groans."
Deficiencies
Vitamin D deficiency is very common globally. In fact, deficiency has been reported in over a third of otherwise healthy young adults and over half of general medicine inpatients in the United States. Rates of deficiencies are even higher in Europe. A simple blood test can identify levels of this important hormone-vitamin.
Symptoms
Vitamin D deficiency causes a weakening of the bone, resulting in rickets in children and osteomalacia in adults. These conditions are associated with an increased risk of fractures. It is also linked to depression in older adults. Because of vitamin D's role in immune function, deficiencies are believed to be linked with cancer and autoimmune disease.
Sources
The recommended intake of vitamin D for adults is 400 international units per day. Vitamin D is obtainable in the diet naturally in fatty fish, liver and egg yolk, and in fortified foods like milk and cereals. The seaweed kelp is also a source of vitamin D. It is not a dietary requirement in individuals with adequate exposure to sunlight, broadly defined as 15 to 30 minutes every couple of days.
Considerations
Because of a complex interplay of factors, deficiencies can be seen even in those who meet the recommended intake of vitamin D. Blood levels of vitamin D are depressed by the consumption of too much animal protein or too much calcium. Cow's milk is high in both protein and calcium. Therefore milk, even when fortified with vitamin D, contributes to vitamin D deficiencies.
References
- PubMed.gov: High Prevalence of Vitamin D Inadequacy and Implications for Health
- "The China Study"; T. Colin Campbell; 2006
- "Biochemistry"; Pamela C. Champe et al.; 2005
- "Physiology"; Linda S. Costanzo; 2010.
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