Vitamin D is necessary for bone metabolism, helping the body to absorb calcium and maintain adequate levels of calcium and phosphate in the blood. Starting in 2002, there was an uptick in research on vitamin D.
A 2011 Institute of Medicine (IOM) report suggested the benefits of vitamin D with regards to bone health only. There are some ongoing trials looking at possible benefits of vitamin D on other health parameters. The IOM was clear on the impact of taking too much vitamin D (including hypercalcemia and calcifications in soft tissue and blood vessels) which makes it important to take the right dose.
A June 2017 Journal of the American Medical Association study suggests that many individuals are taking doses that are too high. In other words, this is a case of "too much of a good thing." This study noted marked increased intake of vitamin D that occurred between 1999 and 2014. The doses ranged between 1000 IU and 4000 IU. Data was collected from the National Health and Nutrition Examination Survey (NHANES), a program of studies that was designed to evaluate the health and nutrition status of children and adults in the U.S. The date in the NHANES was obtained in the form of questionnaires. It is a well-regarded data set, and used or referenced in numerous studies.
The researchers analyzed data involving 39,243 subjects with a mean age of 46.6 years old. About half the participants were women, and 69.7 percent self-reported as non-Hispanic white. The analysis also showed:
- Prevalence of taking 1000 IU or more of vitamin D was 18.2 percent during 2013-2014
- Prevalence of taking 1000 IU or more of vitamin D was 0.3 percent during 1999-2000
- Prevalence of taking 4000 IU or more of vitamin D was 3.25 percent during 2013-2014
- Prevalence of taking 4000 IU or more of vitamin D was less than 0.1 percent during 2005-2006
- In 2013 – 2014, the intake of 4000 IU or more was highest among women (4.2 percent), among non-Hispanic white (3.9 percent), and among individuals 70 years or older (6.8 percent).
Among all participants, about 3 percent exceeded the 4000 IU “safe” limit. Overall, the increased doses noted in the later years tabulated in the study were found in all age groups, all ethnicities, and across genders.
Currently, the recommended dietary allowance (RDA) of vitamin D is 800 IU for adults aged 70 years or younger and 600 IU for adults over age 70. The upper limit for daily intake is 4000 IU. At this level, an individual may begin to develop complications.
It should be noted that this study did not take into account the “updated” Daily Value (DV) for vitamin D, which is 800 IU. This new DV will be used by vitamin makers when they meet the new — and unfortunately delayed — deadline for the labels, mandated by the Federal Drug Administration (FDA).
To be clear, RDA is the “recommended daily allowance” or intake that a healthy individual should follow. RDAs are developed by the Food and Nutrition Board at the Institute of Medicine of the National Academies. RDAs vary by age, gender, and life circumstance (a pregnant or nursing mother).
DV or Daily Value is established by the FDA and are used on food and dietary supplement labels as a percentage. There is one DV for each nutrient for everyone over the age of four years old. DVs usually exceed RDAs. If the DV for vitamin D is 800 IU, then a vitamin that offers 400 IU of vitamin D is offering 50 percent of the DV.
The researchers stress that, despite the perceived benefits and the need for specific vitamin D daily goals, taking higher than recommended doses in combination with calcium can encourage kidney stones. Another study suggests that higher than recommended doses of vitamin D can increase risk of fractures and falls.
On the other hand, too-low levels of vitamin D interfere with the bone metabolism of otherwise healthy women in their childbearing years. Too-low levels are linked to an increased risk of diabetes, neuromuscular disorders, and immune, skin, renal, and heart disease.
Limitations of the study include the fact that it was not a controlled research project designed to look at the risks and benefits of different amounts of vitamin D, and the fact that the accuracy of the data depended on subjects' recall.
The best sources of vitamin D include: Sunlight, fatty fish, canned tuna, Portobello mushrooms, fortified milk or milk alternatives, fortified orange juice, egg yolks, fortified cereals, cod liver oil, and supplements.
It’s important to have a discussion with your health professional regarding the optimal level of vitamin D for you. Make sure to account for food sources before using supplements. In most cases, taking 1000 - 2000 IU daily is considered safe for most adults.