By Diane L. Schneider, MD
As we head into spring after a long winter, your vitamin D level right now is likely at the lowest point it will be all year, unless you are taking a vitamin D supplement or a multivitamin containing vitamin D. This is because sunshine is the main source of vitamin D, and during the winter exposure to the sun does not produce sufficient vitamin D—even in sunny Southern California, where I live. Vitamin D has a seasonal variation, with highest levels in the summer and typically 20 percent lower in the winter.
Why Is Vitamin D Important?
The main function of vitamin D is to preserve calcium balance. Vitamin D regulates how well calcium is absorbed from the intestine. With adequate vitamin D, you absorb about 30 to 40 percent of the calcium that you take in from foods, drinks, and supplements. If your vitamin D level is low, the efficiency of calcium absorption drops to 10 to 15 percent. When not enough calcium is consumed, bone releases calcium to keep everything running. Over long periods of time, “borrowing calcium from your bone” will result in bone loss and the risk of breaking a bone.
Benefits beyond Bone Health
In frail older men and women who are at risk of vitamin D deficiency, supplementation may decrease the risk of falls. Beyond fall prevention, recent research has suggested that vitamin D has other benefits, including reduced risk of heart disease, cancer, diabetes, autoimmune diseases, and infectious respiratory diseases. Most of these findings were based on observational studies rather than randomized, controlled trials with individuals assigned to vitamin D or a placebo pill. Several large trials are under way to investigate the effects of vitamin D on heart disease and cancer. We will have to wait for the results of those trials to provide evidence of benefit.
How Much Vitamin D Do You Need?
How much vitamin D you need is a hot topic, with different groups providing different recommendations. The public health recommendation for the Institute of Medicine is 600 international units (IU) each day for children (older than one year) and adults. The amount increases to 800 IU per day for adults ages 70 years and older. Individual needs may vary, however, depending on many factors, such as body size, age, sunscreen use, skin color, geographical location, and time of year. For example, to maintain your blood level of vitamin D above 30 nanograms per milliliter (ng/ mL, the unit of measurement for this test) may require at least 1,000 to 2,000 IU per day of supplemental vitamin D.
While you might assume that food would be the ideal source of vitamin D, the reality is that few foods naturally contain vitamin D. The main natural food sources of vitamin D are oily fish, like salmon, mackerel, and sardines. Egg yolks contain only small amounts of vitamin D. In the United States, all milk is fortified with vitamin D. An 8-ounce glass of milk contains 100 IU. Other fortified foods include cereals and some brands of orange juice and yogurt.
Can You Take Too Much Vitamin D?
Vitamin D is a fat-soluble vitamin that is stored in fat. Vitamin D taken at recommended doses is generally not of a concern. Classic toxicity with high blood levels of calcium and kidney and liver damage occurs with blood levels in the 200 to 400 ng/mL range as a result of high vitamin D intake. In general, daily intake up to 10,000 IU is thought to be safe. You should consult your healthcare provider to individualize the right dose for you.
Should You Be Screened for Vitamin D?
Most organizations do not recommend universal screening for assessing vitamin D blood levels. The US Preventive Services Task Force concluded this year that the benefits as well as any potential harm from vitamin D screening and early interventions cannot be determined. Individuals at high risk of low vitamin D, however, such as those with obesity, osteoporosis, celiac disease, or Crohn’s disease, are advised to have vitamin D levels checked. Winter and early spring, when sun exposure is lowest, is the best time of year to check.
Despite all the interest and research in vitamin D, many unanswered questions remain. Although blood levels are the best indicator of vitamin D status, universal screening is not recommended. Its role in health and disease beyond bone health is conflicting. To determine your best course of action, bring up the issue of vitamin D with your primary care provider.
Diane L. Schneider, MD, is a geriatrician, epidemiologist, author, and co-founder of 4BoneHealth.org. An experienced writer and public speaker, Dr. Schneider is frequently featured as an osteoporosis expert on numerous television and radio shows and in Internet articles, magazines, newspapers, and medical publications. Dr. Schneider is the author of The Complete Book of Bone Health (Prometheus Books, 2011; $21).