It’s no secret that vitamin D is an essential nutrient. You can find it in breakfast staples like eggs, milk, and fortified orange juice, as well as in some mushrooms and fatty fish such as halibut, salmon, and herring. (1) Your body can even make it from spending time in the sun. (1)
Still, how much do you really know about what vitamin D can — and can’t — do for your health? Read on to learn what we know so far.
To get a better understanding of vitamin D and scientists’ long-held fascination with its functions, know that not all vitamins and minerals operate in the body alike. “We're discovering that vitamin D behaves much less like a vitamin and much more as a hormone,” says Robin Foroutan, RDN, a health coach at the Morrison Center in New York City and a spokesperson for the Academy of Nutrition and Dietetics. That means vitamin D acts as a messenger rather than a participant in our metabolism, potentially affecting everything from our weight to how our organs function. (2)
Although research on vitamin D abounds, its effect on human health remains uncertain. That’s largely because the majority of vitamin D studies either are on animals or were conducted in small human populations. Most important, the majority of research on the nutrient is observational, meaning that the findings don’t point to a definite cause-and-effect relationship between vitamin D and the potential health benefit being studied.
Studies that do produce likely causal results are conducted with a randomized, controlled model, in which researchers eliminate the risk for bias and account for potentially conflicting factors. (3) Large randomized controlled trials are considered the gold standard for research, and unfortunately, there haven’t been many on vitamin D and its various possible health benefits, perhaps because of the difficulty of conducting these trials, a lack of funding, or the impossibility of such a scenario.
With those limitations in mind, here’s an in-depth look at what vitamin D can, probably can, may, and definitely won't do for your health.
It’s clear that vitamin D aids in the absorption of calcium. Without enough vitamin D in the body, there will not be enough of its active form, the hormone calcitriol. (4) Calcium absorption allows the body to maintain a sufficient level of that element as well as phosphate, both of which promote the growth and maintenance of healthy, strong bones. (1)
That’s why it’s no surprise that getting enough vitamin D is critical for warding off bone diseases, such as rickets in children, osteomalacia in adults, and osteoporosis in the elderly. (4)
Rickets is a rare disease in the United States. It is marked by soft and weak bones in children and is typically associated with developing countries, but an inadequate vitamin D level from lack of sun exposure or diet can affect children anywhere in the world. (5) Signs and symptoms of rickets include pain in the spine, pelvis, and legs, as well as delayed growth and muscle weakness. (6)
Meanwhile, osteomalacia refers to softening bones due to vitamin D deficiency. Its signs include dull, aching pain in the legs, hips, pelvis, ribs, and back, though the condition often doesn’t present symptoms in its early stages. (7)
Osteoporosis is one of the main causes of fractures and broken bones in the elderly. The Mayo Clinic notes that this bone disease results from less creation of new bone and the removal of old bone in the body. Women who are past menopause are at the highest risk of osteoporosis, and as with osteomalacia, people with osteoporosis are often asymptomatic when the disease is in its early stages. Later symptoms may include a stooped posture, declining height, back pain, and an unexpected and immediate bone fracture. (8)
Making sure that you get ample vitamin D through sunlight, food, and supplements can help ward off bone diseases like these. (8)
While vitamin D’s potential role in helping prevent or manage clinical depression is still unclear because of limited research, researchers believe that vitamin D level may indeed play a role in the risk for seasonal affective disorder, or seasonal depression. (9) People with seasonal affective disorder appear to produce less vitamin D, which researchers believe affects the activity of the neurotransmitter serotonin.
Serotonin is the same chemical that your brain pumps out when you go on a long run, eat a piece of chocolate, or hold the hand of the person you love. It’s a feel-good hormone. So when the serotonin level is thrown off, potentially due to a vitamin D deficiency, you may feel blue or be at a greater risk for mood disorders. (10)
Foroutan points out that healthcare providers may prescribe vitamin D supplements to help treat seasonal affective disorder. (9)
Increase your level of vitamin D if you’re deficient and you may find that you get fewer respiratory infections (aka the cold and flu) than usual.
One review of 25 randomized controlled trials involving about 11,300 people suggests that participants who were vitamin D deficient saw a 12 percent reduced risk for respiratory infections after taking a vitamin D supplement, though not every study saw a clear benefit from supplementation. (11)
The main limitations? Researchers weren’t sure whether participants had received a flu vaccine or were diagnosed with the respiratory ailment chronic obstructive pulmonary disease (COPD), which are two confounding factors that may have skewed the results.
An accompanying editorial cautions readers to take the findings with a grain of salt, with the authors arguing against standard year-round vitamin D supplementation and calling for additional research. (12)
Although some were based on randomized controlled clinical trials, the initial aim of that research had been to analyze the relationship between vitamin D and bone growth and strength, and the participants were older (over age 70) and mostly women. Meanwhile, other studies in the review included participants with end-stage kidney failure. These factors suggest that more objective research is needed to determine the real effect that vitamin D may have on heart health and stroke risk — and which groups of people may benefit most.
According to an article, observational studies in cell models suggest that vitamin D may help increase insulin sensitivity, boost beta cell function, and lessen inflammation — all potential benefits for reducing the risk for and helping manage type 2 diabetes. (14)
But here’s the thing with that research and other studies referenced in the article: Most of the research on vitamin D and diabetes (be it type 1 or type 2) is observational, meaning that until there are more randomized controlled trials in large human populations, we won’t know for sure how these factors may affect one another.
“In human studies, the evidence for a potential association is stronger for vitamin D and type 2 diabetes, with much less data on type 1 diabetes,” the authors write. “However, the evidence about type 2 diabetes in humans comes almost exclusively from observational studies, which may be confounded by a variety of factors, and therefore, these studies preclude an assessment of cause and effect.” (14)
Scientists are paying an increasing amount of attention to vitamin D’s possible role in warding off cancer. A review of 63 observational studies that analyzed the potential connection among vitamin D and breast cancer, colon cancer, ovarian cancer, and prostate cancer yielded promising results, suggesting that vitamin D may be an easy and low-cost way to help reduce cancer risk. (15)
Other observational research also suggests that vitamin D may help prevent colorectal cancer, though findings are conflicting. (16) The same goes for vitamin D helping prolong cancer survival. (13)
Because of conflicting science and a lack of randomized, controlled trials, the National Institutes of Health doesn’t yet recommend vitamin D supplements to help reduce the risk for cancer. (1)
Foroutan points out that there are vitamin D receptors in brain tissue, which suggests that the vitamin may play a role in cognitive function and, potentially, the risk for dementia. Research seems to support this notion, with one study suggesting that vitamin D may help clear amyloid plaque, the hallmark of Alzheimer’s disease (the most common form of dementia). (17)
Study authors note that more research is needed to confirm a cause-and-effect relationship between an adequate level of vitamin D and the neurodegenerative disorder.
The Mayo Clinic echoes that opinion, with their experts noting that despite some of the promising findings produced thus far, it’s too soon to tell how vitamin D may affect cognitive decline and dementia risk because studies so far are only observational. (18)
Researchers have long been fascinated with the potential effect that vitamin D may have on autoimmune disorders because of the nutrient’s role in regulating the immune system. (19) Previous reporting at Everyday Health shows that research has been conducted on vitamin D and autoimmune conditions, including rheumatoid arthritis, multiple sclerosis, thyroid disease, inflammatory bowel diseases like Crohn’s, and psoriasis.
But as is the case with much of the research on vitamin D, studies on autoimmune conditions and the nutrient are limited to observational models, so more study is needed. (19)
By now, you realize that scientists are still exploring how exactly vitamin D affects our bodies and what role it might have in disease treatment and prevention. But one thing’s for sure: Vitamin D is no cure-all, so don’t start taking supplements with the hope that all of your health woes will vanish.
There’s no vitamin that can do that.
Vitamin D is an essential nutrient that can help fortify bones and keep them strong while also potentially warding off respiratory infections and keeping your mood sunny. But research regarding its other effects needs to be further investigated before scientists, doctors, and everyone else can make informed decisions about supplementing or upping dietary intake of the vitamin.
Until then, know that most adults need 600 international units of vitamin D daily, while children and the elderly may need different amounts. (1) If you suspect that you have a vitamin D deficiency, talk to your doctor about getting a blood test to check if your level is adequate.
Additional reporting by Melinda Carstensen.