by Julie Rafferty, Tufts University

Credit: Unsplash/CC0 Public Domain

How much vitamin D do we really need to preserve healthy bones and overall health? Do higher levels of vitamin D supplementation, as some studies suggest, reduce the risk of cardiovascular disease, cancer, diabetes, autoimmune disorders, and respiratory infections? Or is the evidence still too limited?

With the use of vitamin D supplements and testing for vitamin D levels now common practice for many in the U.S. and other developed nations, an expert panel established by the Endocrine Society and co-chaired by faculty at Tufts University School of Medicine set out to examine existing high-quality research on the issue to update recommendations, which had not been reviewed by the Endocrine Society in more than a decade.

After four years of study, the panel released new guidelines earlier this year that begin to fine-tune our understanding of how much vitamin D people at different ages and stages of their lives need to consume daily, what effects it may have on preventing disease, and whether people should have their vitamin D levels tested regularly.

According to Anastassios G. Pittas, GBS06, co-chair of the guideline-writing panel and professor at Tufts University School of Medicine, the rigorous examination of dozens of research studies on the effects of vitamin D indicates that for most people who are otherwise healthy, 600 IU per day of vitamin D is sufficient. Because there are relatively few sources of naturally occurring vitamin D, most people should consider supplements to meet their daily needs.

The new guidelines recommend that pregnant women, children and adolescents ages 1–18, seniors aged 75 and older, and people with prediabetes increase their vitamin D consumption to levels higher than everyone else. The guidelines do not recommend specific dosages, however, because research thus far is insufficient to clearly identify an optimal dosage for any of these groups.

The National Academy of Medicine (formerly the Institutes of Medicine) had previously established the Recommended Daily Allowance (RDA) to be 600 IU for people ages 1–70 and 800 IU for people older than 70. The RDA for women during pregnancy was 600 IU.

The new guidelines also state that regular testing for vitamin D levels is not necessary for most individuals, including the four groups who should consume more vitamin D, as there is insufficient evidence to determine the optimal target level people should aim for.

The new guidelines have evoked controversy, particularly around the efficacy of testing and whether evidence is sufficient to say vitamin D provides additional health benefits beyond those identified in the new guidelines. Authors of the new guidelines say their review pinpoints areas where further research is still needed.

Why more vitamin D for certain groups

The panel concluded that people in four specific groups can achieve specific health benefits from taking vitamin D at doses higher than those recommended by the National Academy of Medicine:

• Pregnant women: Research studies examined provided evidence to suggest that vitamin D reduces the risk of preeclampsia, which causes very high blood pressure and blood protein levels that can risk the health of mother and fetus. Vitamin D also lowers the risk of intrauterine mortality, preterm birth, and neonatal mortality. The dosages used in the studies examined ranged from 600–5,000 IU daily, with an average of 2,500 IU.

• Children and adolescents ages 1–18: This group benefits from higher intakes of vitamin D to potentially lower the risk of respiratory infections, research shows. Dosage in the studies examined ranged from 300 to 2,000 IU daily, with an average of 1,200 IU.

• Seniors 75 or older: This group benefits from a lowered risk of premature mortality with vitamin D. In the clinical trials included in the systematic review that reported on the mortality outcome, vitamin D dosage ranged from 400 to 3,333 IU daily. The estimated weighted average from all the studies was approximately 900 IU daily.

• People with prediabetes: Vitamin D may be a new weapon to reduce the risk of progression to diabetes in adults with prediabetes. In the clinical trials included in the systematic review, the vitamin D dosages varied widely from 842 to 7,543 IU daily. The estimated weighted average from all the studies was approximately 3,500 IU of vitamin D daily.

For all these groups, supplementation is recommended without testing for vitamin D levels.

Vitamin D and prediabetes

"The evidence regarding prediabetes came as a surprise to many," says Pittas, who is chief of endocrinology, diabetes, and metabolism at Tufts Medical Center and an expert in vitamin D. "This may change the treatment plan we recommend for people with prediabetes."

Pittas noted that a large part of the evidence that vitamin D reduces the risk of diabetes in adults with prediabetes came from the D2d study, a large, multi-center clinical trial supported by the National Institutes of Health (NIH) that he led at Tufts. Similar clinical trials were conducted in Japan and Norway that showed nearly identical results.

"When our team at Tufts did a meta-analysis combining the data from the D2d study with those from Japan and Norway, a significant result emerged showing that vitamin D lowered the relative risk of developing diabetes in people with prediabetes by 15%."

Sources and benefits of vitamin D

Vitamin D helps regulate calcium levels in the body, which are essential for healthy bones, teeth, and muscles. Without enough vitamin D, the body can't absorb dietary calcium, which can lead to bone deformities in children and bone pain in adults.

The chief sources of vitamin D include synthesis in the skin upon exposure to sunshine and in many countries, foods fortified with vitamin D such as cereals, orange juice, and milk.

"Our recommendations are not saying 'don't take vitamin D supplements,'" says Pittas. "There is very little vitamin D that naturally occurs in foods, and synthesis from sun exposure is unpredictable. We are saying that everyone needs to take 400–600 IU per day to maintain good calcium absorption and bone health."

"There is tremendous variability in how people respond to the vitamin D they may get from the sun," Pittas notes. For example, sunscreen and darker complexions may decrease the amount of vitamin D synthesized. "It's simpler to advise everyone to continue taking a supplement with 400–600 IU per day, but certain groups would benefit from more vitamin D."

"The approach taken was more evidence-based than previously used," notes Bess Dawson-Hughes, senior scientist at the Jean Mayer USDA Human Nutrition Research Center on Aging (HNRCA) at Tufts and professor of medicine at Tufts University School of Medicine.

"We should keep in mind that vitamin D deficiencies are widespread in the developing world and in China, India, and the Middle East," adds Dawson-Hughes, who is an expert on bone health and vitamin D but was not part of the guideline-revising panel.

The evidence on vitamin D testing

The new guidelines also recommend that routine testing for vitamin D levels using the 25-hydroxyvitamin D or 25(OH)D test is not warranted for healthy people because there is little clear scientific evidence as to what should be considered an adequate test result.

"We are not saying that testing is without merit," Pittas says. "Rather, we are saying that more research is needed to determine the levels for which we should aim. What levels specifically are necessary for overall good health and what levels signal a need for even more supplementation in certain groups?"

Testing is recommended if there are other specific clinical indications, such as hypocalcemia, or low calcium levels. The guidelines do not specify what is considered to be established indications for testing other than hypocalcemia, Dawson-Hughes notes.

Underlying conditions that can substantially alter vitamin D physiology include inflammatory bowel disease or renal disease, or conditions that decrease absorption, such as gastric bypass. The guidelines around both vitamin D supplementation and testing do not specifically address these conditions because the guidelines are meant to apply to the general population.

"Testing is expensive and may not be available to people of limited means," says Dawson-Hughes. "In a country like the U.S. where many people are taking supplements and consuming foods fortified with vitamin D, testing has become excessive. In areas of the world where vitamin D deficiency is widespread, it would seem reasonable to start with supplementation and then consider testing."

When released in June, doctors and other readers responded with concerns that the panel had ignored high-quality studies showing additional benefits to vitamin D supplementation outside fields related to endocrinology, or that their stance on vitamin D testing might immediately result in insurers no longer covering the test, even if a clinician believes it is warranted.

"In our final recommendations, the panel identified a number of areas where the strength of the evidence simply isn't high enough yet to warrant making recommendations," says Pittas. "The testing question is among the areas that requires more study."

Provided by Tufts University