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Vitamin D: What the Latest Research Really Says
There has been much excitement about vitamin D and its potential to prevent anything from the common cold to cancer. Unfortunately, the majority of the research done so far has shown very little. It appears as though there may be a specific place (and a correct dosage) where vitamin D can be beneficial, and researchers are working to determine this.
Vitamin D and the Immune System
It is well known that vitamin D is essential for maintaining strong bones; however, it also supports the immune system through the regulation of immune responses, support of muscle function, support of cell growth, and assistance with glucose metabolism. Because virtually all tissues of the body respond to vitamin D, researchers looked into the relationship of low levels of vitamin D and disease.
Several observational studies have demonstrated associations between low vitamin D levels and increased risks of cancer, infections, autoimmune disorders, diabetes, and cardiovascular disease. One review reported that approximately 36 percent of healthy young adults had low levels of vitamin D, and approximately 58 percent of those individuals receiving medical care had low levels.
Following this research, the Endocrine Society created 2011 guidelines for identifying deficiencies and recommended the use of daily supplements containing up to 2,000 IU of vitamin D for certain populations. These recommendations helped increase the popularity of vitamin D supplements and subsequently grow the global vitamin D market to a predicted $1.5 billion by 2034.
Large-Scale Controlled Trials Show No Benefit
Since 2011, a variety of large-scale controlled trials have failed to demonstrate any benefits associated with the use of vitamin D supplements.
The VITAL study followed over 25,000 adults for five years and found that daily supplementation with vitamin D did not decrease the incidence of heart attacks or cancer.
Another major trial involving over 20,000 adults found similar results regarding cancer. Cancer rates in both the vitamin D treatment group and the placebo group were equivalent.
A 2020 meta-analysis of 33 trials revealed that vitamin D supplementation had no effect on all-cause mortality. Although earlier reviews, such as the 2014 Cochrane analysis of seven trials, found that vitamin D supplementation resulted in a small reduction in the risk of death, the results of the more recent trials raise serious doubts about using supplements to support the general population. For a broader discussion of this issue, please see can vitamin d slow aging? Here’s What Science Says.
Specific Cases Where Vitamin D May Be Beneficial
Although the recent large-scale trials did not show a benefit from using vitamin D supplements, there are specific situations in which vitamin D may provide some benefits. The most promising area of benefit is related to the immune system, specifically in children.
A recent randomized controlled trial conducted in Denmark involved 189 children aged 6 to 11 years. The children received either 1000 IU of vitamin D per day or a placebo. Those receiving the vitamin D had 17 percent fewer sick days from respiratory infections and 43 percent fewer days with fevers compared to those receiving the placebo.
These findings mirror an earlier study conducted in China with 400 infants. Infants in the study received either 1200 IU or 400 IU of vitamin D per day. Higher doses of vitamin D were associated with fewer cases of influenza and shorter time until recovery.
One reason why other studies have not replicated this finding is the presence of seasonal variation. The Danish study demonstrated that vitamin D levels decreased significantly during the winter months in the placebo group. The researchers believe that the decrease in vitamin D levels occurred because of reduced sun exposure. In contrast, the Canadian study, which involved children who already had high levels of vitamin D, found that additional supplementation did not have a positive impact. Therefore, it seems that the benefits of vitamin D supplementation occur because it prevents a decrease in vitamin D levels, rather than increasing the level beyond what is already present.
Additional Situations Where Vitamin D Can Provide Benefits
More recently, the 2024 Endocrine Society guidelines reflect a more balanced view and recommend the use of supplements for several specific groups:
- Pregnant women: Some studies have indicated that supplementing with vitamin D may lower the risk of preeclampsia, preterm labor, and improve newborn health. Recommended dosages are typically around 3000 IU/day.
- People at risk of developing type 2 diabetes: Research suggests that supplementing with an average of 3500 IU/day may delay or prevent the development of type 2 diabetes.
- Adults over 75: Older adults lose their ability to produce and convert vitamin D to its active form. Supplementing with vitamin D may promote better bone and muscle health and reduce mortality risk.
How much Vitamin D can cause a problem?
One of the biggest problems related to Vitamin D is taking too much. In Canada researchers did a clinical trial that lasted for 3 years and had 3 different amounts of Vitamin D (400 IU, 4,000 IU, & 10,000 IU) per day. Results showed that the participants who were given the higher dosages lost bone density at a faster rate than the participants who received the lower dosage (400 IU). Research also found that as levels of Vitamin D rose, so did fall risks. Researchers believe this could be because of the loss of muscle strength associated with excessive Vitamin D intake.
More is Not Always Better
According to the latest recommendations from the National Institutes of Health:
- Adults less than age 50 should take 600 IU of Vitamin D per day.
- Adults greater than or equal to age 50 should take 800 IU of Vitamin D per day.
The recommended amounts will allow people to get what they need to prevent a deficiency but will not provide enough to create an overdose. As a result, some companies which make multivitamins have decided to reduce the amount of Vitamin D in them.
Final Thoughts on Vitamin D
While Vitamin D does play a very significant role in helping to maintain good bone health and assisting with immune function, research has shown that excessive amounts do not produce the broad-based benefits previously suggested. There are certain subgroups of people such as children, elderly, pregnant women, and other at-risk populations that may benefit from targeted supplementation; particularly during periods when there is little sun available to help stimulate Vitamin D production in the skin.
As always, consult with your healthcare professional before changing your regimen of supplements.