Vitamin D has long been known as essential for healthy bone formation. However, vitamin D became the superstar of vitamins thanks in large part to its #1 promoter, Dr. Michael Holick. Now there is growing reason to believe Dr. Holick’s enthusiasm for promoting vitamin D as a super vitamin is not warranted. Dr. Holick’s claim that many people are deficient in vitamin D and need to take a supplement may not be entirely objective (1). Dr. Holick maintained that the RDA levels for vitamin D were inadequate. He also claimed that most people should be taking vitamin D supplements well in excess of the RDA level, as this would help reduce the risk of numerous ills. However, as with prior nutrients that reached what in hindsight appears to be fad status, there is now growing evidence that a lot of Dr. Holick’s enthusiasm for the many possible health benefits from taking more than the RDA for vitamin D have long been questioned. Indeed, many of the supposed benefits remain unverified and there is growing evidence that supplements well in excess of the RDA of vitamin D may be harmful.
To determine if more than the RDA of vitamin D would impact the risk of cancer or cardiovascular disease, Dr. JoAnn Manson at Harvard and colleagues conducted a large randomized, placebo-controlled clinical trial called VITAL. This large VITAL study randomly assigned over 25,000 subjects to supplements of vitamin D or omega-3s. They examined the impact of taking several times the RDA (2000 IUs) of vitamin D for several years. Unfortunately, this large clinical trial found that supplementation with vitamin D did not result in a significantly lower incidence of invasive cancer or of cardiovascular events compared to a placebo (3).
Is More Vitamin D Better for Bone Health?
Vitamin D is best established as necessary for optimal bone health. People with low vitamin D levels in their tissues (<12ng/dl of 25-OH-D) are more prone to rickets as children and weakening bones as adults (osteomalacia). But would extra vitamin D above the RDA level in people who were not initially deficient help slow the decline in bone mineral density (BMD) seen as part of the aging process? To find out, researchers gave 311 healthy adults (mean age 62) supplements of either 400 IU (RDA), 4,000 IU or 10,000 IU vitamin D per day. This was a three year double-blind controlled clinical assessment of the effects of these 3 different doses of vitamin D supplementation on volumetric bone mineral density (BMD) and bone strength. The subjects taking the two higher doses of vitamin D experienced more episodes of hypercalciuria and hypercalcemia -- a serum calcium level more than 10.22 mg/dL (2.55 mmol/L). Other adverse events, such as renal or hepatic dysfunction, falls, low-trauma fractures, and cancer, did not differ among the dosage groups. Dr. Boyd and colleagues concluded: "There is growing evidence that for bone, the benefit of vitamin D supplementation is only seen in the treatment of vitamin D deficiency." There is also evidence that very high intermittent (monthly or annual) doses of vitamin D may be harmful, with increased risk of falls or fracture" (4).
One possible explanation for these findings may be an increased plasma marker of bone resorption (CTx) paired with suppression of parathyroid hormone. This was particularly true among the highest 10,000 IU/day group who saw sharp decline in parathyroid hormone levels, as well as a sharp increase in plasma CTx levels during the first 18 months of being on high-dose vitamin D, according to the authors. The negative dose-response relationship between volumetric BMD and increased vitamin D supplementation suggests this may be part of the way too much vitamin D may weaken bones over the long term.
Dr. Steven Boyd stated that these findings were unexpected, and that the outcomes were in fact the opposite of what they expected to find. But he cautioned that "this evidence of high-dose vitamin D having a negative effect on bone should be regarded as hypothesis generating, requiring confirmation with further research." Therefore, the appropriate interpretation of this study is that for maintenance of bone quality in healthy, vitamin D-sufficient adults, these results do not support a skeletal benefit of vitamin D doses well above the RDA.
Bottom Line: Many Americans have been led to believe that the RDA for vitamin D is too low. This has led some doctors and many people on their own to take well in excess of the current RDA of vitamin D. These results indicate claims that the current RDA for vitamin D are likely inadequate appear to be incorrect. People taking vitamin D supplements in excess of the RDA in hopes of strengthening their bones are more likely to end up with weaker bones and likely a greater risk of osteoporotic fracture. These recent findings do not support a benefit of high-dose vitamin D supplementation for bone health. Further research would be needed to determine the most likely toxic effects of taking vitamin D supplements well in excess of the current RDA levels of 400-600IU for adults.
By James J. Kenney PhD, FACN
- Wolpowitz D, Gilchrest BA. The vitamin D questions: how much do you need and how should you get it? J Am Acad Dermatol. 2006 Feb;54(2):301-17.
- Burt LA, et al "Effect of high-dose vitamin D supplementation on volumetric bone density and bone strength" JAMA 2019; DOI: 10.1001/jama.2019.11889.
Stephanie Ronco has been editing for Food and Health Communications since 2011. She graduated from Colorado College magna cum laude with distinction in Comparative Literature. She was elected a member of Phi Beta Kappa in 2008.