Why so many women are vitamin D deficient
Scientists are still untangling why the nutrient gap persists—and what it reveals about how women’s health is studied and treated.
Vitamin D is a crucial nutrient that helps the body absorb calcium and maintains bone health, but it’s also one that around a third of Americans are deficient in. And for women, who have slightly higher deficiency rates, it’s an even more complicated story.
For women, who have significantly higher rates of osteoporosis—when bones become weak and brittle—and autoimmune diseases like multiple sclerosis, a vitamin D deficiency can make a big difference. But doctors warn a deficiency can be hard to spot on your own.
Although deficiency is often framed as a fix as simple as stepping into the sun, many women are turning to supplements to meet their nutrient needs. Here’s why that’s important, and how you can figure out if supplements are right for you.
How vitamin D deficiency impacts the body
Vitamin D is especially important for women because bone health is a much bigger problem in women: there are four times more osteoporosis cases in women than men. A lack of vitamin D can lead to softer, weaker bones and eventual fractures, so avoiding a deficiency can be paramount in women, who usually have smaller, thinner bones than their male counterparts.
Although there’s nothing that biologically predetermines that women are more likely to have a vitamin D deficiency, lifestyle factors and major life events may explain the rates. Vitamin D is important all the time, but during pregnancy and after menopause, our bodies need it even more.
(Lifting heavy weights can help you mitigate osteoporosis. Here's how to get started.)
“It can help to shift the balance, especially post-menopause, when there typically is a pretty drastic decline in our ability to build bone,” says Laura Acosta, a registered dietitian at the University of Florida. After menopause, our bodies produce significantly less estrogen, leading to faster bone decline. Vitamin D can help.
But before that, the vitamin is still important: during pregnancy, we need the vitamin “not just for us, but for a growing baby,” she explains.
(The scientific case for eating more bones.)
But emerging research tells us that’s not the only benefit women get from vitamin D. “I think we're just going to find out more and more how important it is, much more than we even realized,” Acosta says.
Emerging research on vitamin D and women’s health
Clemens Bergwitz, an endocrinologist at Yale School of Medicine, puts it simply: “Literature is full of vitamin D effects that don't have anything to do with bone health.”
Recent research has suggested vitamin D could have major impacts on autoimmune diseases, which are far higher in women. Nearly four out of five people with autoimmune diseases are women, a disparity researchers think might be related to the X chromosome, hormone levels, and antibody responses.
In fact, one study found a 22 percent reduction in autoimmune diseases in men and women when taking vitamin D supplements for five years. In another, patients who took the supplements experienced fewer multiple sclerosis symptoms—a disease that’s around three times more prevalent in women.
(Why women are more prone to autoimmune diseases.)
Research is revealing benefits of vitamin D that men and women alike can reap. One study showed vitamin D supplementation could even reduce our biological aging—the decline of our physical function. In the research, patients who took vitamin D had nearly three fewer years of biological aging symptoms, like disease, than their unsupplemented counterparts, thanks to the vitamin’s effects on a material that protects our chromosomes.
Other studies have shown beneficial connections between vitamin D and depression symptoms, insulin resistance, cardiovascular health, and polycystic ovary syndrome.
“The story beyond bones is, I think, going to be autoimmune disease,” Acosta says. “It's going to be mental health, it's going to be metabolic health, cancer prevention, and maybe more that’s not even on our radar right now."
Signs of a vitamin D deficiency
A vitamin D deficiency is “largely silent until it’s really bad,” Acosta says. Although bone and muscle pain may be felt, they’re often indicative of a severe deficiency, not a minor one.
(Want to strengthen your bones? Look beyond vitamin D.)
The best way to know if you have a deficiency is to have your levels checked. Doctors and dietitians can test specifically for vitamin D during a blood test upon request—since it isn’t included in typical blood panels—to assess your levels. From there, you can make a plan with your doctor for how to get more vitamin D.
If you have a family history of osteoporosis, frequent fractures, are pregnant, or are older, getting routine testing may be beneficial. Other factors can compound the problem for some women: Those who live in more northern regions—where they’re exposed to less sunlight—and those with darker complexions are often more vitamin D deficient.
Natural sources of vitamin D
Getting vitamin D naturally is possible, but tricky. And lifestyle factors that are more common in women don’t make it any easier. Primarily vegetarian and vegan diets and sunscreen use—both of which studies have estimated may be higher in women—can limit vitamin D levels. While experts do not suggest changing these habits, they suggest diligent sunscreen users and plant-based communities get tested for a deficiency.
Contrary to popular belief, vitamin D isn’t actually absorbed from the sun’s rays, it’s made in our skin. When ultraviolet rays hit cholesterol molecules in our skin, they trigger our metabolic process that creates vitamin D.
However, even though some research suggests five to 30 minutes of unprotected exposure to sunlight a day may be enough for your vitamin D levels, that’s also enough for UV radiation to be harmful. That’s why sunscreen use is recommended, despite it blocking our ability to make vitamin D from the sun.
(How a warming planet is wreaking havoc on your skin.)
Foods like fatty fish, including salmon and mackerel, eggs, and fortified options like milks are also sources of vitamin D. “There's a very definitive connection with vitamin D, not only helping to increase our absorption of calcium from food but also helping to make sure that calcium gets to the bone,” Acosta says.
(We still don't know why humans started drinking cow's milk.)
“But really, there's only a few foods that are considered good sources of vitamin D," says Anna Maria Merz, a registered dietitian at The Ohio State University Wexner Medical Center.
How to boost your vitamin D
When you consider how many factors affect our vitamin D levels—location, skin tone, season, and diet—finding the right supplement dose may be overwhelming. But there are a few general clinician-recommended rules to give you direction.
Over-the-counter supplements can do the trick in many cases. “The only caveat is that dietary supplements are not regulated by the FDA,” Merz says. Instead, look for supplements with “NSF” or “USP” labels. “They're third parties that will make sure that what the supplement label says is in the product is actually what's in the product,” she says.
The recommended dosage of vitamin D supplements is usually around 600 international units for all adults, and might increase slightly as you age.
But be careful to not overdo it—don’t go beyond 4,000 international units without consulting a clinician. While other vitamins can be expelled through urine, vitamin D is fat-soluble and sticks around in our bodies’ fat cells. Too much of the micronutrient can cause vitamin D toxicity, leading to nausea, kidney stones, and plenty of other undesirable effects.
“That's why I would recommend going through your doctor and having a level checked just to make sure that you're not getting more than you need,” Merz says.
