Vitamin D plays an important role in the formation and maintenance of healthy bones and teeth. Vitamin D enhances calcium absorption in the intestines and maintains calcium and phosphorus levels within narrow limits. Vitamin D is also critical for proper bone mineralization. Secondary roles of vitamin D include regulation of the immune system, reducing the effects of inflammation and regulation of cell growth, cell differentiation and cell death.
There are three different forms of vitamin D found in the body; cholecalciferol, calcidiol, and calcitriol. Cholecalciferol, or vitamin D3, is the only natural form of vitamin D because it is produced in the skin from exposure to sunlight. Calcidiol, or 25-hydroxyvitamin D, is produced from vitamin D3 and is the form of vitamin D found in the bloodstream. Calcitriol, or 1,25-dihydroxyvitamin D, is produced from calcidiol and is a potent steroid hormone. Calcitriol is the active form of vitamin D and has a significant role in health maintenance and disease prevention.
Vitamin D deficiencies are common and can result from lack of sunlight exposure, low vitamin D intake and conditions that hinder vitamin D absorption from the gastrointestinal tract. When vitamin D is deficient, calcium and phosphate levels decline as well. Low calcium and phosphate levels can result in rickets in children and osteomalacia in adults because bone mineralization is hindered. Osteoporosis may also result from vitamin D deficiency. In response to low calcium levels, the body will produce more parathyroid hormone. However, high parathyroid hormone levels leech calcium out of bones, thereby weakening them and making them susceptible to fracture.
Vitamin D deficiency has been linked to numerous other chronic disease conditions. Vitamin D deficiency is associated with neuromuscular conditions such as muscle weakness, falls, low back pain, multiple sclerosis and fibromyalgia. Vitamin D insufficiency increases susceptibility to diabetes by decreasing insulin sensitivity and hindering beta cell function. Low vitamin D levels have also been implicated in heart failure and certain cancers including breast, colon, ovarian, and prostate. It is important to note that the link between vitamin D and these diseases has been reported in animal studies and observational human studies. Therefore, it is not possible to determine that low vitamin D levels actually cause these diseases. Only prospective randomized trials can give us the answer.
Vitamin D deficiency can only be determined by measuring the levels of 25-hydroxyvitamin D in the bloodstream. Levels below 20 ng/mL indicate a deficiency, 33-90 ng/mL is normal, and levels greater than 100 ng/mL represent excess (Grant 2005).
Although the recommended daily intake of vitamin D is 400 international units (IU), these levels are not adequate to prevent or treat vitamin D deficiency. They are only adequate to prevent metabolic bone diseases. Vitamin D3 dosages of 1,000-2,000 IU are generally recommended for optimal health and even higher dosages of 2,000-7,000 units have been deemed sufficient to maintain 25-hydroxyvitamin D levels within normal ranges (Cannell, 2008).
For those people who may be at risk for deficiency, vitamin D supplementation may be necessary. Vitamin D can be ingested in two forms: D2 (ergocalciferol) and D3 (cholecalciferol). The best supplemental form of vitamin D is vitamin D3. The body is very efficient at converting D3 into usable forms of vitamin D in the body. Vitamin D3 is three times more effective in raising 25-hydroxyvitamin D levels in the bloodstream and D3 maintains these elevated levels for a longer period compared to vitamin D2. The metabolites of vitamin D3 also have a greater affinity for the vitamin D binding proteins. Supplemental vitamin D3 has also been shown to reduce mortality risk whereas D2 has shown no benefit to mortality risk.
The best way to avoid a vitamin D deficiency is to eat foods rich in vitamin D, get regular exposure to sunlight and to consider supplementation with vitamin D3. There are few foods that naturally contain high levels of vitamin D. The best sources include salmon, tuna, mackerel and fish liver oils. Smaller amounts are found in cheese, eggs, yogurt and liver. Yogurt can be blended with fruit for a homemade phytonutrients and
liquid vitamins rich smoothie. Most of the vitamin D in the American diet is attained from fortified foods (Calvo 2004). Milk, breakfast cereals, orange juice and yogurt are commonly fortified with vitamin D.
Sunlight exposure is another way to obtain vitamin D and represents the major source of vitamin D in most people (Stroud 2008). Ultraviolet radiation penetrates the skin and converts 7-dehydrocholesterol in the skin to pre-vitamin D3, which is ultimately converted to vitamin D3. The ideal amount of sunlight to achieve sufficient vitamin D formation is 5-30 minutes at least twice a week (Holick 2007). However, excessive sunlight exposure is not recommended because it will not further enhance vitamin D formation and will greatly increase the risk of skin cancer.
Overall, most people do not achieve adequate intakes of vitamin D. Some experts now believe that the ideal daily intake of vitamin D may be 4 to 10 times greater than the recommended daily allowance. Vitamin D3 supplementation is the only practical way to attain these daily levels that have been shown to reduce chronic disease risk.
Dr Linda Kennedy, MS, ND, SLP
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