Vitamin D is called the sunshine vitamin because the sun’s ultraviolet rays trigger its production within the skin. If your sun exposure and dietary vitamin D intake are limited, you may benefit from a vitamin D supplement. A daily vitamin D intake of 4,000 IU is the tolerable upper intake level for adults, according to the Institute of Medicine. Taking too much vitamin D can cause side effects, and large doses should be taken only under medical supervision.
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Vitamin D Requirement
Vitamin D is required for efficient absorption of dietary calcium, and calcium and vitamin D work together to maintain bone health. The Institute of Medicine's recommended daily allowance for vitamin D is 600 IU for adults through age 70, and 800 IU thereafter. However, Harvard School of Public Health Professor Dr. Edward Giovannucci states that many experts suggest people need an average of 1,000 to 2,000 IU of vitamin D daily. This is a combined amount from sun exposure, food and supplements.
Safe Adequate Intake
Your blood vitamin D level is affected by your sun exposure and dietary intake. A vitamin D level of about 50 nmol/L corresponds to the RDA of 600 IU per day, which is considered adequate for bone and overall health in most people. A study published in the February 2001 issue of "The American Journal of Clinical Nutrition" assessed the efficacy and safety of 1,000 and 4,000 IU per day in middle-aged men and women over a 5-month period. Serum vitamin D levels plateaued at 69 and 96 nmol/L in the low and high intake groups, respectively, without changing serum calcium levels. These data support the adequacy and safety of the Institute of Medicine’s vitamin D intake recommendations.
According to a 2011 report from the Institute of Medicine, vitamin D intakes of less than 10,000 IU daily have not been linked with indicators of toxicity, such as high blood and urinary calcium levels and tissue calcification. A study report published in the January 2003 issue of "The American Journal of Clinical Nutrition" noted that intake of 5,000 to 5,500 IU of vitamin D daily for up to 20 weeks during the winter months among men living in Omaha, Nebraska, produced serum vitamin D levels just below 150 nmol/L. Although this level of vitamin D is higher than what is typically considered normal, the blood calcium concentration among the men in the study remained normal. These research findings support the notion that vitamin D intake levels of around 5,000 IU daily are unlikely to be harmful in the short term.
Your risk for harm increases if you exceed the recommended upper intake level for vitamin D over a prolonged period. Long-term serum vitamin D levels over 125 to 150 nmol/L are linked with potential adverse effects, including increased risk for breast, pancreatic and prostate cancer; cardiovascular disease; falls and bone fractures in the elderly; and death from any cause. The amount of fatty tissue in your body may affect your vitamin D requirement and potential for adverse effects from too much vitamin D. Body fat can serve as a reservoir for vitamin D, which is a fat-soluble vitamin. Elevated body mass index -- an indicator the relative proportion of body fat -- is associated with lower blood vitamin D concentrations in Caucasian populations in North America and Europe, according to a report in the February 2013 issue of "PLoS Medicine."