Vitamin D – What You Need to Know

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Understand the role of vitamin D in your overall well-being and learn guidelines for maintaining healthy levels of this essential vitamin.

Vitamin D plays an important role in women’s health, supporting essential bodily functions and contributing to disease prevention. Often referred to as the “sunshine vitamin” because production is induced in the skin when we absorb sunlight (UVB rays), maintaining sufficient levels of vitamin D is a key step toward supporting our overall wellbeing.

“Vitamin D is really important for both bone health and muscle function, and it helps maintain immune function,” says JoAnn Manson, MD, DPH, a physician at Brigham and Women's Hospital and Michael and Lee Bell Professor of Women's Health at Harvard Medical School.

Bone health, specifically, is an important women’s health issue directly linked to vitamin D. Our bodies need vitamin D to be able to efficiently absorb calcium, which is necessary to prevent bone loss and fractures. Women are more prone to bone loss, or osteoporosis, because our bones are generally smaller and less dense, we live longer than men do (and bone loss increases with age), and as estrogen levels decrease with menopause, we lose the protective benefit of the hormone on bone density.

Vitamin D is also essential for muscle function, and low levels of vitamin D are associated with and increased risk of falls and muscle weakness.

Women’s reproductive health can also be affected by vitamin D, notes Dr. Manson. “There’s evidence that vitamin D status is important for optimal fertility and healthy pregnancy, including avoiding preeclampsia and hypertension during pregnancy and in avoiding preterm birth.”

Data from observational studies in humans and animals has shown low levels of vitamin D to be associated with “impaired fertility, endometriosis, and polycystic ovary syndrome,” as well as “higher rates of preeclampsia, preterm birth, bacterial vaginosis, and gestational diabetes in women with low vitamin D levels.”

The connection with healthy pregnancy is “an additional reason women should be sure to take prenatal vitamins, which include vitamin D,” Dr. Manson says, adding, “if you’re at increased risk of pregnancy complications, you may need more than what’s in prenatal vitamins; speak with your obstetrician about whether a supplement of 1000-2000 IUs might be appropriate.”

Many factors affect an individual’s blood level of vitamin D, including your diet, where you live, how much time you spend in the sun, your skin color, your age, and your weight.

We do get some vitamin D through food, so diet is a contributing factor to vitamin D levels. “In general, most people get about 300-400 IUs daily from food sources,” Dr. Manson says. “In the US, we have a fortified food supply, so milk and other dairy products tend to be fortified with vitamin D, as are many cereals. Fatty fish, like salmon, sardines, or tuna, are also natural sources of vitamin D, as are eggs and sundried mushrooms.”

Nutrition labels now require listings of vitamin D content of packaged foods, and Dr. Manson notes that “monitoring how much vitamin D you’re getting through the foods you choose can be really helpful in avoiding deficiency.”

Where you live also plays a role in how much vitamin D your body produces. Northern areas of the US receive less ultraviolet B (UVB) sunlight than do southern parts, and above about 37 degrees latitude (around San Francisco, California, St. Louis, Missouri, or Philadelphia, Pennsylvania) UVB is too low in late fall through early spring to produce vitamin D.

If you live in an area that receives less UVB sunlight, it’s important to consider how you can replace that source of vitamin D. “People who are getting very little, if any, sun exposure, as well as having very low vitamin D intake in the diet through fortified foods, are at risk for deficiency,” Dr. Manson says.

But keep in mind that you don’t need to spend an entire day in the sun to reap the benefit of UVB exposure. “Even 15 minutes in the sun without sunscreen, three times a week, is usually enough to avoid vitamin D deficiency,” Dr. Manson notes.

Age also plays a role in your vitamin D level, as production of the vitamin in skin gets becomes less efficient as you age. Overweight and obesity can also be factors in lower levels of vitamin D in the blood because the vitamin can be taken up and held by fat cells.

Your skin color will also affect your vitamin D levels. “People who have more skin pigmentation are likely to make less vitamin D in the skin so will often have lower vitamin D levels in the blood,” Dr. Manson says. Darker skin produces less vitamin D because melanin, the substance the gives skin its color, basically acts as a UVB sunscreen. This is reflected in research revealing higher rates of vitamin D deficiency among Black and Hispanic people.

As lead author of the VITAL study, which followed more than 25,000 people age 50 and older who took daily supplements containing vitamin D, omega-3 fatty acids, or placebo to determine the impact on risk of developing cancer, heart disease, and stroke in people who did not have a prior history of these illnesses, and a researcher in the field of women’s health and chronic disease epidemiology, Dr. Manson shares further insight into the role of vitamin D in women’s health.

Dr. Manson: There's variation in people’s requirements for vitamin D; thus, no single cutpoint defines vitamin D deficiency for all people. Guidelines specifying sufficient levels of vitamin D have been the subject of debate in the past because of different interpretations of “deficiency.”

Some professional societies say that you're deficient or insufficient if you’re below 20 nanograms per milliliter (ng/mL) and others that say you need to be at or greater than 30 ng/mL. If you go by the 30 ng/mL guideline, we could say that approximately 40 percent of the US population would be classified as deficient.

However, according to the National Academy of Medicine, formerly the Institute of Medicine, most people’s needs for vitamin D to maintain bone health are met at 20 ng/mL and those with levels less than 12 ng per mL tend to be deficient. If you use that range, the percentage of people who are actually deficient is a much smaller number, maybe 10 to 15 percent of the population. So, the percentage of the population that’s “deficient” depends on what definition or cutpoint is being used.

In addition, it’s important to point out that you can't say that any given individual is deficient just based on a blood level because the needs of individuals vary widely. Some people can maintain normal vitamin D status—and all the biological functions of vitamin D—even with a low blood level because they're just very efficient at using vitamin D. So, how much vitamin D we need varies by the individual.

Dr. Manson: Often, vitamin D deficiency does not have major or specific symptoms. A person with vitamin D deficiency might experience subtle symptoms like fatigue, difficulty concentrating, or muscle or bone aches, but nothing you would immediately identify as related to vitamin D deficiency. If it's more severe, a person may have significant bone pain or muscle aching, and there may be an increased risk of fracture, difficulty getting pregnant, and other conditions.

Dr. Manson: In 2011, the National Academy of Medicine released guidelines outlining that most Americans and Canadians up to age 70 need no more than 600 international units (IUs) of vitamin D per day to maintain health, and adults 71 and older may need as much as 800 IUs.

If you have concerns that you may not be getting enough vitamin D from food and incidental sun exposure, you may want to take a supplement. Aiming to supplement with 1,000-2,000 IUs a day is reasonable. The National Academy of Medicine recommends keeping supplementation below 4,000 IUs a day. Vitamin D supplements are available in most drug stores or supermarkets, so they’re easy to find and are generally safe and well-tolerated.

It’s important to note that while maintaining sufficient vitamin D levels that align with standard guidelines is warranted, there’s no benefit to be gained (and there are actual risks) from “mega dosing” with vitamin D and calcium. This can lead to deposits of calcium in the soft tissue and the blood vessels, as well as kidney stones.

Dr. Manson: Neither the National Academy of Medicine nor the US Preventive Services Task Force recommend routine screening for vitamin D in the general population. However, a clinician may recommend screening patients who have been diagnosed with certain medical conditions.

Conditions that might lead to a screening recommendation include bone health conditions, like osteoporosis, and malabsorption conditions, which impact the body’s ability to absorb vitamins and minerals, such as inflammatory bowel disease (including Crohn’s disease and ulcerative colitis. Individuals who have undergone gastric bypass surgery, which could lead to some malabsorption of vitamin D and other fat-soluble vitamins, might also be advised to have screening.

Some clinicians may be more likely to screen for vitamin D deficiency in individuals with darker skin pigmentation, as they are synthesizing less vitamin D in the skin. And, older adults, who have an elevated risk for osteoporosis and bone fractures, and those living in institutions may also be screened more routinely.

Finally, some medications, including anticonvulsants, interfere with absorption of vitamin D, and a clinician may recommend screening and, if necessary, supplements in for individuals taking those medications.

Dr. Manson: In the VITAL trial, which was the largest trial of vitamin D supplementation in the world and included more than 25,000 participants nationwide, we tested the impact of 2,000 IUs of daily vitamin D supplementation over 5.3 years. Our study found that cancer death was reduced among the participants who supplemented for many years. Since the VITAL trial, there have been additional studies and meta-analyses that have found that vitamin D supplementation appears to reduce the risk of dying from cancer. Vitamin D actually affects the biology of cancer, making it less likely to become metastatic.

Dr. Manson: There is ongoing research related to the connection between vitamin D and the immune system, and specifically the potential for a vitamin D to help protect against respiratory infections.

Another area that's looking promising is research into vitamin D and type 2 diabetes. A meta-analysis suggested that vitamin D may have benefits in impacting glucose levels and preventing progression to type 2 diabetes. However, it’s important to note that a very high body mass index appears to counteract some of the benefits of vitamin D, so the benefit was seen only in participants who did not have obesity.

Dr. Manson: We’re still learning a lot about COVID-19 and the potential connection to vitamin D. Vitamin D can tamp down inflammation, so that may play a role in how a person’s vitamin D levels contribute to a response to COVID-19 infection. Some of the adverse effects of the infection most closely related to mortality result from inflammation in the lungs and respiratory distress. We’re studying the effect of vitamin D supplementation in preventing severe COVID-19 illness in one of our ongoing trials.

Some but not all studies suggest that patients with lower vitamin D levels are more likely to test positive for COVID-19, that those who are diagnosed tend to have worse outcomes, and that people hospitalized for COVID-19 who were deficient in vitamin D were more likely to die from COVID-19.

Dr. Manson: I think the most significant point here is that vitamin D is essential to normal biological function but that it doesn’t take large, “mega” doses to meet those needs. Most studies related to the significant health benefits we’ve discussed here suggest that you just need moderate intake, likely easy to obtain from a combination of a healthy, balanced diet, incidental sun exposure, or taking 1000-2000 IUs in a daily supplement. 

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