Vitamin D is the only vitamin we can make by standing in the sun. We can also eat it in fish, beef liver and mushrooms. Most milk, cereal and orange juice is fortified with vitamin D, so many people supplement their vitamin D without even thinking about it. Yet there is a growing debate among doctors over whether Americans are getting enough vitamin D.
Vitamin D helps the body absorb calcium and is crucial to building healthy bones. So crucial, in fact, that scientists think lighter skin tones are an adaptation for producing more vitamin D in less-sunny climates. Skin low in the pigment melanin can produce more vitamin D from less sun exposure than darker skin.
Vitamin D deficiency can cause health problems, and perhaps the best known of these is rickets, which is marked by weak, bent bones. Just as scientists were discovering vitamins at the turn of the 20th Century, a scourge of rickets hit the children of factory workers. Urban children were spending most of their time indoors or under polluted skies. With the help of scientist Harriette Chick in Vienna, researchers clearly showed that taking cod liver oil and getting more sun exposure treated rickets. Adolf Otto Reinhold Windaus won the Nobel Prize for his contribution to isolating vitamin D in 1928.
Now, scientists are finding links between vitamin D and a whole spectrum of other health conditions. Both muscles and nerves need vitamin D to function well. Vitamin D may also help the immune system to reduce inflammation and moderate cell growth.
Preliminary studies show a link between unhealthy vitamin D levels and cancer of the colon, prostate and breast, according to the National Institutes of Health's Office of Dietary Supplements. Vitamin D may help prevent diabetes, hypertension and autoimmune disorders such as multiple sclerosis. But, according to the NIH, research in these areas is somewhat preliminary and hasn't conclusively determined exactly how vitamin D influences people's risk of these conditions.
Today, there's debate about how much vitamin D people really need. The Institute of Medicine (IOM) sets the nutritional guidelines used by the CDC and other government organizations, and based on IOM guidelines, the CDC estimates 8 percent of Americans are deficient in vitamin D, and another 24 percent have inadequate vitamin D levels. However, several medical associations such set the cutoff for healthy vitamin D levels higher than the IOM, and classify more of the population as vitamin D deficient. [Infographic: New Rules for Vitamin D and Calcium: Most People Get Enough]
How much vitamin D do you need?
Much of the debate about how much vitamin D we need comes down to the question of which studies to use to guide vitamin recommendations. A smaller part of the vitamin D controversy lies in how associations prefer to measure vitamin D as it binds to molecules and changes forms in the body.
The IOM updated its vitamin D standards in 2013 based on "nearly 1,000 published studies as well as testimony from scientists and stakeholders," according to the institute. The IOM noted there are many studies on vitamin D's benefits, "such as protection against cancer, heart disease, autoimmune diseases, and diabetes." But the IOM judged that only the evidence on bone health was strong enough to sway vitamin D recommendations.
Doctors measure vitamin D levels by the concentration in the blood, often with the units of nanomoles per liter. The most popular test measures a form of vitamin D called 25-hydroxyvitamin D, which binds to a certain protein circulating in the blood. The IOM sets optimal vitamin D blood levels at 50 nanomoles per liter. However other medical associations have said the scientific literature on vitamin D is strong enough to support recommending higher vitamin D levels. The U.S. Endocrine Society says 72 nanomoles per liter is optimal, and the International Osteoporosis Foundation recommends seniors maintain at least 75 nanomoles per liter in their blood to avoid fractures. The IOM says a person with anything less than 30 nanomoles per liter is at risk for being vitamin D deficient; the U.S. Endocrine Society estimates anything less than 50 nanomoles per liter may be deficient.
To further complicate matters, a recent study of more than 10,000 people in the National Health and Nutrition Examination Survey found that vitamin D's benefits in lowering people's risk of heart disease and dying hit a ceiling at about 52 nanomoles per liter. Beyond that, more vitamin D didn't help, according to the study published in June 2013 in the American Journal of Medicine.
The majority of Americans have blood levels lower than 75 nanomoles per liter, according to the NIH.
Do Vitamin D supplements work?
Many medical associations and organizations recommend eating most of your vitamin D, either from food rich in vitamin D, food supplemented with vitamin D or, in some cases, vitamin D supplements. Because of the risk of skin cancer, The American Academy of Dermatology "does not recommend getting vitamin D from sun exposure."
The IOM bases its recommendations for daily intakes on the assumption that most adults aren't getting enough vitamin D from the sun or their diets. The IOM recommends infants get 400 International Units (IU) a day, adults get 600 IU, and people older than 70 get 800 IU. But because medical associations disagree on what counts as optimal vitamin D levels, the recommended daily intakes also vary.
Certain populations who are at risk of vitamin D deficiencies may need more vitamin D than the average person. Older people are prone to vitamin D deficiencies because skin becomes less efficient at synthesizing vitamin D with age, and older kidneys are less efficient at converting vitamin D into an active form. Human milk is a poor source of vitamin D, so the American Academy of Pediatrics recommends parents give exclusively breastfed infants vitamin D supplements.
Vitamin D is fat-soluble, so people with Crohn's disease, celiac disease or other conditions that interfere with fat digestion may have lower vitamin D levels. Obesity can contribute to low vitamin D because body fat binds to vitamin D and prevents is from getting into the blood, according to to the NIH. People with inflammatory bowel disease and those who have had gastric bypass surgery may need more vitamin D to maintain healthy levels, according to the U.S. endocrine Society.
Some medications may reduce vitamin D levels. Oral steroids, which are typically prescribed to reduce inflammation, can lead to vitamin D deficiency. One study of more than 31,000 people found people taking oral steroids were twice as likely to have vitamin D deficiency that the general population, according to the study published in December 2011 in the Journal of Clinical Endocrinology and Metabolism.
People with darker skin tones may be at more risk for vitamin D deficiencies. The CDC estimates 41 percent of the non-hispanic black population is at risk for inadequate vitamin D levels, and 32 percent is at risk for deficiency.
Yet new research has questioned whether the tests for vitamin D are accidentally inflating the estimated number of black Americans with a vitamin D deficiency. A study of more than 2,000 black and white adults found that black Americans typically had lower levels of 25-hydroxyvitamin D in their blood, less vitamin D binding protein and less parathyroid hormone, which converts vitamin D into an active, bioavailable form. However black Americans had better bone mineral density than white Americans in the study. Furthermore, the researcher's calculations showed white and black Americans had similar levels of bioavailable vitamin D. The study, published November 2013 in the New England Journal of Medicine, may lead to more research into the different forms of vitamin D in the body.
Are Vitamin D supplements safe?
While doctors may not agree on optimal vitamin D levels, there is a greater consensus on what vitamin D levels are unsafe. The IOM's safe upper limit for daily vitamin D intake is 4,000 IU. A person who ingests too much vitamin D may feel nauseated, weak and lose their appetite. Getting 10,000 to 40,000 IU of vitamin D in a day could lead to acute toxicity. Weight loss, heart arrhythmias, kidney stones and polyuria (excessive urination) are all linked to vitamin D toxicity. However it is rare for a person to reach this level with a normal sun exposure and diet. Most unsafe vitamin D levels come from taking too many supplements. Over the counter supplements may contain 150 IU to 800 IU of vitamin D per serving.
Taking more than the daily upper limit of vitamin D could harm a fetus during pregnancy. High levels of vitamin D can also worsen atherosclerosis, or the hardening of the arteries. Taking vitamin D supplements alongside medications that increase calcium levels — such as diuretics, digoxin or diltiazem — can lead to a build up of too much calcium in the body, which can affect the heart. Vitamin D supplements can exacerbate the side effects of the psoriasis drug calcipotriene, and decrease the effectiveness of the cholesterol drug atorvastatin (Lipitor).