Role of vitamin D: Key roles are to promote the absorption of calcium and phosphorus by the intestine to maintain intracellular calcium homeostasis for normal functioning of the heart, muscles, lungs, and other organs and deposit calcium and phosphorus in bones and teeth to strengthen them.
Sources of vitamin D: Vitamin D is either produced endogenously, through skin exposure to sunlight, or obtained exogenously from ingestion of foods and supplements. When skin is exposed to sunlight (ultraviolet light) it penetrates into the skin and converts a precursor into vitamin D. A key reason for Vitamin D deficiency today is sun avoidance. There's an increase in the use of sunscreen and covering up from sunlight—mainly due to concerns surrounding skin cancer.
Vitamin D, in the forms of ergocalciferol (D2) or cholecalciferol (D3) are in fortified milk (8 oz = 100 IU) and breakfast cereals. Small amounts are contained in cheese, eggs, canned salmon, and some fatty fish.
Between the decreasing intake of milk in all age groups and decreased exposure to sunlight, it’s easy to see why an increasing number of people can be both vitamin D and calcium deficient. Keep in mind that these nutrients are metabolically interrelated.
Vitamin D DRI: The Dietary Reference Intake (DRI) for vitamin D was set in 1997. The DRI from infancy to age 50 is 5 mcg/day (200 IU) (1 mcg of cholecalciferol = 40 IU vitamin D), from 51-70 years, 10 mcg/day (400 IU) and 70 years on, 15 mcg/day (600 IU).
Some researchers believe the DRI is not set high enough and are pushing for higher recommended levels, especially for older people who don’t make vitamin D from sunlight as easily. The tolerable upper intake (UL) is 50 mcg/day or 2,000 IU. Keep in mind that vitamin D is a fat-soluble vitamin.
Vitamin D Deficiencies: Several studies show that a large percent of adults, particularly older adults and African Americans, are vitamin D deficient. Vitamin D status can be determined by measuring serum concentrations of 25-hydroxyvitamin D. Vitamin D deficiency in adults is defined as 50 nmol/L and vitamin D insufficiency as 50 to 80 nmol/L.
Sufficient Vitamin D Intake: Some experts are encouraging people of all ages to seek more sunshine by spending from 5 to 15 minutes a day in the sun without sunscreen, increasing the intake of fortified milk and the possible use of a vitamin D supplement.
Diseases in Discussion: The connection between insufficient vitamin D and rickets is well known and surprisingly, rickets is reappearing in vitamin D deficient children. Beyond the role of vitamin D in type 1 and 2 diabetes, a role has been conjectured in osteoporosis, kidney failure, several types of cancer, psoriasis, and the autoimmune diseases: rheumatoid arthritis, and multiple sclerosis.
A recent meta-analysis by Zipitas et al. (Archives of Diseases of Childhood – 2008), showed that the risk of type 1 diabetes was significantly reduced (29%) in infants who were supplemented with vitamin D compared to those who weren’t. There was also a dose-response effect, where higher intake of vitamin D resulted in a lower risk of developing type 1.
Some research suggests that the geographic and seasonal distribution of type 1 diabetes may relate to vitamin D deficiency. Children in Finland (further from equator) are about 400 times more likely than a child in Venezuela (closer to equator) to develop type 1. And a type 1 diagnosis is more frequent in the fall and winter when vitamin D deficiency may be more prevalent and lowest during the warm months of summer.
Some research has shown lower levels of serum vitamin D at the the time of diagnosis of type 1 compared to children without diabetes. The exact mechanism by which vitamin D plays a role is not known. However, receptors for the active form of vitamin D have been found in beta and immune cells. A research group from University of California in San Diego recently (Diabetologia, June 2008) showed higher serum levels of vitamin D associated with reduced incidence rates of type 1 worldwide and suggested there is widespread vitamin D deficiency in children. They recommend a higher intake of vitamin D (1,000 IU of D3 and 5 to 10 minutes of sunshine daily, around noon.
Of note is the just released (10/13/08): Vitamin D guidelines from the American Academy of Pediatrics (AAP). In brief, AAP now recommends that infants, children and adolescents receive a minimum of 400 IU of Vitamin D.
(Due to the lack of consumption of this amount of Vitamin D from dietary sources a supplement may be required.) This is double the recommended 200 IU per day from the 2003 guideline. AAP changed their guideline due to the concerns noted in this NutriZine. Read the complete AAP guidelines (http://www.aap.org/new/VitaminDreport.pdf).
Type 2 and vitamin D: Data from observational studies is accumulating to link deficient vitamin D intake (which may also be tied to its interrelationship with calcium) as an independent factor in the increased incidence of metabolic syndrome, insulin resistance and type 2 diabetes. Low concentrations of serum vitamin D have been associated with beta cell dysfunction and impaired insulin secretion. This relationship has been reported in the Third National Health and Nutrition Examination Survey (NHANES III) (Diabetes Care. 28, 2005). Research suggests this relationship may be linked to the beneficial effect of dairy foods (good sources of calcium and vitamin D) on body weight and insulin sensitivity. Vitamin D deficiency has also been shown to impair insulin synthesis and secretion in humans and animals.
A recent prospective (22 years) observational study from Finland (Knekt et al, Epidemiology, September 2008) followed (22 years) men and women ages 40 to 74 who didn’t have diabetes when the study began. They compared baseline vitamin D levels and showed that men with the highest serum vitamin D levels were least likely to develop type 2. This finding was not documented in women. One of the many published papers from the 20 year observational Nurses’ Health Study dissects the association between vitamin D and calcium intake and type 2. The authors found a potentially beneficial role for both vitamin D and calcium in reducing the risk of type 2. |
