July 22nd, 2011
08:58 AM ET

How much vitamin D do I need?

Every weekday, a CNNHealth expert doctor answers a viewer question. On Friday, it's Dr. Melina Jampolis, a physician nutrition specialist.

Question asked by Terry of Dodge City, Kansas

I have some 2,000 IU vitamin D capsules that I am about to take. I'm a 60-year-old male, in pretty good health and not overweight or underweight. I'm also active. I've read that 2,000 is the upper limit for daily dosage, and I don't think I need to take that much anyway. The capsules I have cannot be split, so could I use them every other day and be OK? Thank you very much, and have a great week. I appreciate your site and the info.

Expert answer:

Hi Terry. I wrote about vitamin D awhile back but since the RDA (recommended daily allowance) has changed since then, I thought it would be worthwhile to update my response.

Vitamin D is a fat soluble vitamin that is mainly produced by exposure of the skin to ultraviolet light.

Your ability to produce vitamin D in the skin decreases with age, and since it is present in very few foods naturally, consuming fortified foods or taking a vitamin D supplement is necessary for many people, particularly people with osteoporosis and those at higher risk of deficiency due to limited sun exposure, breast fed infants, overweight/obese individuals, older adults, dark-skinned people, and people with fat malabsorption.

Vitamin D is essential for bone and muscle health in both adults and children. In addition, vitamin D has numerous other roles in the body, and a growing body of research is finding that deficiencies may be associated with an increased risk of certain types of cancer (especially colon cancer), type 2 diabetes, heart disease and autoimmune diseases like multiple sclerosis.

Because these studies are mainly observational, extensive research is under way to determine the causal relationship between vitamin D levels and disease as well as the role of increasing vitamin D intake.

Vitamin D levels can be checked by a simple blood test called 25OHD, which is considered the best indication of vitamin D status. While optimal levels have not been firmly established, levels above 50 nmol/L (20 ng/ml) are considered adequate by the Institute of Medicine.

A report published in 2007 suggested that greater than or equal to 75 nmol/L (30 ng/ml) may be a better target, and last April, the International Osteoporosis Foundation issued a position paper in which most of the experts felt that the higher target blood levels (75 nmol/L or 30 ng/ml) were more appropriate for older adults to decrease their risk of fractures and falls.

As with most things in nutrition, more is not always better. Research suggests that levels above 150 nmol/L (60 ng/ml) may have adverse health effects.

The latest RDAs of vitamin D increased to 600 IU for adults up to the age of 70 and 800 IU for adults older than 70, and the tolerable upper limit increased from 2,000 to 4,000 IU per day for adults. The IOF estimated that doses of 800 to 1,000 IU (20-25 mcg) or higher may be required in older adults to prevent fractures and falls but stated that more research is necessary.

In your case, you do not appear to have any risk factors for vitamin D deficiency, but if you are concerned, I recommend having a blood test done.

If your blood level is low, you will need a higher dose to raise and keep levels in the normal range. This is best done under the supervision of your doctor, as responses to vitamin D supplements are variable and there are dosage guidelines that your doctor can utilize.

If your levels are normal, taking 2,000 IU every other day or every third day would be fine, according to Dr. Bess Dawson-Hughes, senior scientist and director of the Bone Metabolism Laboratory at the Jean Mayer USDA HNRCA at Tufts University.

In fact, similar blood level responses are seen when equivalent doses of vitamin D are taken daily, weekly, or monthly.

Follow Dr. Melina on Twitter.

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Get a behind-the-scenes look at the latest stories from CNN Chief Medical Correspondent, Dr. Sanjay Gupta, Senior Medical Correspondent Elizabeth Cohen and the CNN Medical Unit producers. They'll share news and views on health and medical trends - info that will help you take better care of yourself and the people you love.