Vitamin D-3, or cholecalciferol, is the form of vitamin D that is synthesized in the skin when it is exposed to UVB rays from sunlight. Another form of vitamin D, called ergocalciferol, or D-2, is produced by plants. Vitamin D-3 is more potent in humans than D-2 and is the form used by the National Academy of Sciences to establish dietary guidelines, which ostensibly prevent osteoporosis. Some experts, such as Dr. Reinhold Vieth at the University of Toronto, believe the current NAS dietary recommendation for vitamin D-3 is too low and does not meet your physiologic needs.
Vitamin D-3 and Bone Health
Most of your cells and tissues possess receptors for vitamin D, but those that influence bone development have been the most extensively studied. Vitamin D-3 stimulates the absorption of calcium from your intestine and kidneys and encourages calcium deposition in your bones. Within living bone, two types of cells work in concert to keep your bone mass relatively stable: Osteoblasts build bone, while osteoclasts break it down. Vitamin D-3 stimulates osteoblasts and encourages bone growth.
Vitamin D-3 Deficiency Contributes to Osteoporosis
Bone metabolism is influenced by many factors, including vitamin D-3 levels, hormones, genetics, your body weight and your activity levels. Osteoporosis, meaning "porous bones," results from a relative lack of osteoblast activity in comparison to osteoclast activity. Over time, this imbalance leads to a decrease in bone density with a concurrent rise in fracture risk. In adults, vitamin D-3 deficiency leads to a reduction in osteoblast activity, thereby decreasing the rate of bone construction.
National Academy of Science Vitamin D-3 Recommendations
In response to concerns that Americans are consuming too little vitamin D-3, the National Academy of Sciences reviewed its recommendations and offered new guidelines in November 2010. According to the NAS, adults up to age 70 need no more than 600 IU of vitamin D-3 daily to maintain health, and those over 70 need no more than 800 IU. However, many experts, including those at the University of Miami Miller School of Medicine and the University of Toronto, believe that even these recommendations are too low for most age groups and that all elderly adults should receive at least 2,000 IU of vitamin D-3 daily.
Considerations and Recommendations
Many of vitamin D-3's functions, including its influences on immune function and glucose and lipid metabolism, are just beginning to come to light. As new discoveries unfold -- including advances in osteoporosis research -- NAS dietary guidelines for vitamin D-3 may change. Current guidelines reflect an upward adjustment from those devised in 1997, but some researchers still feel these recommendations are inadequate. If you are an adult under age 70 who wishes to prevent osteoporosis, your daily vitamin D-3 intake should be at least 600 IU, and if you are older than 70, 800 IU. Consult your physician about the vitamin D-3 dosage that is best for you.
References
- "The Journal of Nutrition"; Critique of the Considerations for Establishing the Tolerable Upper Intake Level for Vitamin D: Critical Need for Revision Upwards; R. Vieth; April 2006
- National Academy of Sciences Institute of Medicine; New Dietary Intake Levels Established for Calcium and Vitamin D
- "Geriatrics"; Hypovitaminosis D: A Widespread Epidemic; E.P. Cherniack, et al.; April 2008
