Vitamin D
Vitamin D is unusual because our bodies can make it when sunlight reaches the skin. But vitamin D levels often decline as we get older, partly because aging skin becomes less efficient at producing it and many older adults spend less time in strong sunlight.
Vitamin D is essential for calcium absorption and bone health, and it also plays important roles in muscles, the immune system and other tissues.
The basic case for preventing vitamin D deficiency is strong. The more interesting question is whether “enough to avoid deficiency” is the same as an optimal level for health.
Could maintaining a higher vitamin D level provide additional benefits for our bones, muscles, immune system and long-term health as we age? Researchers don't yet agree on the answer.
That question makes vitamin D particularly interesting for healthy aging.
Why Are Low Vitamin D Levels So Common?
Although our bodies can make vitamin D from sunlight, getting substantial vitamin D this way may be harder than it sounds. Few older adults regularly spend time outdoors with large areas of skin exposed to strong sunlight, and clothing, shade, sunscreen, season and latitude can further reduce vitamin D production.
Age also matters. Older skin is less efficient at producing vitamin D from sunlight.
People with darker skin generally need more sun exposure to produce the same amount of vitamin D, because melanin reduces the skin's production of vitamin D from UVB light.
Body weight can matter as well. People with obesity tend to have lower blood levels of vitamin D and may need more vitamin D to achieve the same circulating level.
All of this helps explain why relying on sunlight alone may not provide enough vitamin D for many older adults. Fortunately, vitamin D status can be measured with a simple blood test.
What Does the Research Show?
Vitamin D is clearly important for bone health and calcium metabolism, and treating vitamin D deficiency is well established.
But research suggests that the story may extend beyond preventing deficiency.
Large randomized trials have generally not found that vitamin D prevents heart attacks or substantially reduces the number of people who develop cancer. But some important positive findings have emerged.
In the large VITAL trial, vitamin D supplementation was associated with a 17% reduction in advanced cancers—cancers that became metastatic or fatal. Several analyses of randomized trials have also found evidence that daily vitamin D supplementation may modestly reduce cancer mortality, even though it doesn't appear to prevent most cancers from developing.
And for people 75 and older, the Endocrine Society now suggests vitamin D supplementation because research indicates that it may reduce the risk of death.
These findings raise an interesting question: Is merely avoiding vitamin D deficiency enough, or might maintaining a good vitamin D status provide additional benefits as we age?
The answer isn't settled, but there is enough positive evidence to make vitamin D particularly worth following.
Vitamin D, Immunity and Respiratory Infections
Vitamin D plays an important role in the immune system, which has led researchers to ask whether supplementation might reduce respiratory infections such as colds and influenza.
Some randomized trials and large analyses have found a modest reduction in acute respiratory infections among people taking vitamin D regularly. The benefit has often appeared greatest in people who started with low vitamin D levels.
One randomized trial in Japanese schoolchildren found substantially fewer cases of influenza A among children taking vitamin D during the winter, although other studies have produced mixed results.
The evidence isn't strong enough to say that vitamin D prevents the flu or common cold. But it provides another reason why maintaining an adequate vitamin D level may be important for immune health, particularly in people whose levels are low.
Vitamin D and Breast Cancer
There is an intriguing association between vitamin D levels and outcomes after a breast cancer diagnosis.
A meta-analysis of 12 studies involving 8,574 women with breast cancer found that women with lower 25-hydroxyvitamin D levels had poorer overall survival, breast-cancer-specific survival, and disease-free survival. Another meta-analysis found that women with the highest vitamin D levels had about 42% lower breast-cancer mortality than those with the lowest levels.
Earlier-stage breast cancer studies have also associated low vitamin D levels with a greater risk of recurrence.
These are encouraging findings, but most come from observational studies. They show that women with higher vitamin D levels tend to have better outcomes; they don't prove that taking vitamin D supplements after diagnosis produces those benefits.
For women with breast cancer, however, the evidence provides another reason to know their vitamin D level and avoid deficiency.
Should You Test Your Vitamin D Level?
Vitamin D can be measured with a simple blood test called 25-hydroxyvitamin D [25(OH)D]. This is the standard test for determining vitamin D status.
Interestingly, major medical guidelines generally do not recommend routine vitamin D testing in healthy adults. The reason isn't that the test doesn't work. Rather, researchers haven't established exactly what blood level produces the best long-term health outcomes.
This creates an interesting situation for older adults. The Endocrine Society recommends vitamin D supplementation for people 75 and older because it may reduce mortality, yet it does not recommend routinely measuring their vitamin D levels.
For people interested in healthy aging, there is an obvious reason to consider testing: without knowing your vitamin D level, you don't know whether it is low, adequate—or perhaps already higher than it needs to be.
What Is an Optimal Vitamin D Level?
This is one of the most interesting debates about vitamin D.
The National Academies considers a blood level of 20 ng/mL or higher adequate for most people. But some physicians and vitamin D researchers argue that being high enough to prevent deficiency is not necessarily the same as having an optimal level for long-term health.
Dr. Mark Hyman, for example, generally recommends maintaining 25-hydroxyvitamin D levels around 50–70 ng/mL. Other physicians and researchers have suggested targets of 30 ng/mL or higher.
The difficulty is that randomized trials have not established that maintaining 50–70 ng/mL produces better long-term outcomes than maintaining lower adequate levels. The large, long-term trials needed to answer that question directly would be difficult and expensive—and may never be done.
So there are really two different questions:
What level is high enough to prevent vitamin D deficiency?
and
What level might be optimal for healthy aging?
Medicine has a much clearer answer to the first question than the second.
A Practical Question
When evidence is uncertain, it is also reasonable to consider cost, potential benefit and potential harm.
Vitamin D3 is inexpensive and, at appropriate doses, generally safe. There is strong evidence for correcting deficiency, along with suggestive evidence of additional benefits involving cancer mortality, respiratory infections and healthy aging.
At the same time, more is not always better. Very high doses can cause excessive calcium levels and other problems.
For me, this makes vitamin D less a question of “Should everyone take as much as possible?” and more a question of “What blood level do I want to maintain, and how much vitamin D do I need to maintain it?”
How Much Vitamin D3?
Vitamin D supplements are available as D2 (ergocalciferol) and D3 (cholecalciferol). Both can raise vitamin D levels, but D3 generally raises and maintains blood levels more effectively and is usually the preferred form.
The amount needed varies considerably from person to person. Age, body weight, sun exposure, diet, absorption and starting vitamin D level can all affect how someone responds to supplementation.
That is another argument for measuring 25-hydroxyvitamin D rather than assuming that the same dose produces the same blood level in everyone.
The National Academies recommends 800 IU per day for adults over 70. This is intended to provide adequate vitamin D for most older adults; it is not a dose established to produce an optimal blood level.
With vitamin D, the more useful question may be not simply “How many IU should I take?” but “What does that dose actually do to my blood level?”
Is Vitamin D Safe?
Vitamin D3 is inexpensive and generally safe at appropriate doses. Vitamin D toxicity is uncommon and usually results from taking excessive amounts of supplements over time.
Too much vitamin D can raise calcium levels in the blood, potentially causing nausea, weakness, kidney stones and, in severe cases, kidney damage or abnormal heart rhythms.
The National Academies sets 4,000 IU per day as the tolerable upper intake level for adults, although higher doses are sometimes used under medical supervision.
This is another advantage of measuring vitamin D levels when taking substantial amounts over long periods: you can see what the supplement is actually doing rather than simply guessing.
The Bottom Line
Vitamin D is unusual among supplements. Our bodies can theoretically make it from sunlight, but in everyday life many people may not produce enough, particularly as they get older. A simple blood test can measure vitamin D status, and vitamin D3 is inexpensive and generally safe.
Its importance for bones and calcium metabolism is well established. Research also suggests possible benefits involving immune function, respiratory infections, cancer outcomes and longevity, although the evidence is not equally strong in all of these areas.
The biggest unanswered question may be whether the vitamin D level needed to avoid deficiency is also the level that is optimal for healthy aging.
We don't yet know the answer. But for older adults, vitamin D seems worth paying attention to—and knowing your 25-hydroxyvitamin D level is a useful place to start.
Learn More
A comprehensive review of vitamin D, including recommended intakes, blood levels, deficiency, supplementation, health effects and safety.
The Endocrine Society's 2024 guideline on vitamin D. Of particular interest for older adults, it suggests vitamin D supplementation for people 75 and older because of the potential to reduce mortality.
A readable overview of vitamin D sources, deficiency, research and the continuing debate over how much vitamin D is optimal.
Videos
Dr. Mark Hyman explains why he believes vitamin D deficiency is common and often overlooked. He discusses vitamin D testing, sun exposure and supplementation, and argues that vitamin D may have important effects beyond bone health, including immune function and overall health.