“We see a lot of associations between vitamin D deficiency and poor health outcomes,” says Mary Byrn, PhD, RN, an associate professor at Loyola University in Chicago, who studies vitamin D. “Although these are relationships and we are unable to conclude cause and effect, taking vitamin D supplements or exposing yourself to the sun in a safe manner to increase vitamin D naturally are easy ways to improve your health and try to reduce your risk of multiple diseases,” Dr. Byrn says. You can also get vitamin D through food, but your options are limited. “Vitamin D is found in few foods naturally, though some foods are fortified with it,” says Sarah Gold Anzlovar, RDN, a dietitian in private practice in Boston. Salmon is one of the few good food sources of vitamin D, Anzlovar says, though the amount can vary depending on the type — wild usually has more than farmed. According to the U.S. Department of Agriculture (USDA), a three-ounce serving of salmon provides 12 micrograms (mcg), which is 60 percent of the daily value. “Egg yolks offer a small amount of vitamin D, she adds, and fortified milks, cereals, and orange juice can also provide some of your daily requirement.” “Because vitamin D isn’t found in a lot of foods, and sun exposure may be limited depending on where you live, I recommend everyone get their vitamin D levels checked at their annual checkup,” says Anzlovar. “Then you can evaluate with your healthcare provider whether a supplement or seeking out more vitamin-D-rich foods is necessary.” Know also that some groups, including individuals with dark skin, those with certain underlying health conditions or who are taking certain medications, and those who live in a city far from the equator, may be more prone to having low vitamin D, according to the Cleveland Clinic. RELATED: 5 Ways to Boost Vitamin D Intake When You Have RA

What Are the Risks of Not Getting Enough Vitamin D?

Not getting enough vitamin D may raise your risk for other diseases and conditions — some of them life-threatening. Here’s a handful.

Vitamin D and Respiratory Illnesses

There has been a lot of talk about preliminary research that found vitamin D supplements may be beneficial in preventing or managing COVID-19, the respiratory disease caused by the novel coronavirus. While studies are still in their early stages, past research does show that vitamin D may help protect people from respiratory illnesses. For example, one study, published in February 2017 in the BMJ, looked at data from 25 clinical trials that examined the impact of vitamin D on respiratory infections including pneumonia, bronchitis, and sinusitis. Of the 11,321 study participants, researchers found that people who took vitamin D were 12 percent less likely to develop respiratory illness compared with people who did not take the vitamin. While that percentage may seem impressive, study authors noted there were limitations. For example, they didn’t have data on who, among the study participants, had received a flu shot, which may have affected their risk for respiratory illness. Speaking of the flu, results are mixed about vitamin D and flu prevention or mitigation. For example, a study published in March 2018 in Nutrients found that taking vitamin D didn’t make the flu shot any more effective. Meanwhile, previous research found that school-age children who took vitamin D, compared with a placebo, were 42 percent less likely to get the flu. Still, researchers say the idea of a connection between vitamin D supplementation and COVID-19 is premature. “It is still too early to draw a clear link between vitamin D deficiency and COVID-19; however, there was a study that indicated that countries with high prevalence of vitamin D deficiency had higher rates of COVID-19 mortality rates,” says Byrn, referring to an unpublished study by researchers at Northwestern University in Chicago. But this study didn’t measure patients’ individual vitamin D levels or their severity of COVID-19 symptoms, Byrn points out. “It is also important to note that this research has not been peer reviewed, so we need to proceed with caution as we make clinical practice decisions with this research evidence,” she adds. The connection is still, potentially, there. “We know that vitamin D plays a role in our immune system and our body’s ability to fight off infection; we know that there are vitamin D receptors on immune cells and that vitamin D deficiency increases our susceptibility to infection,” says Byrn. “So, it is possible that vitamin D deficiency increases the risk of COVID-19 infection, but we don’t have specific research results that allow us to make a definitive conclusion.” RELATED: Can Taking a Vitamin D Supplement Protect You From COVID-19?

Vitamin D Deficiency and Osteoporosis

One of vitamin D’s primary roles is to maintain skeletal health, according to Harvard Medical School: Low levels of vitamin D lead to low bone calcium stores, increasing the risk of fractures. Thus, a vitamin D deficiency may put people at risk for osteoporosis, which happens when new bone doesn’t generate at the same pace as the loss of old bone, according to the Mayo Clinic. Adds Byrn, “Vitamin D plays a large role in bone health including osteoporosis — low vitamin D levels decrease calcium absorption, and calcium absorption is important for bone health.” For healthy people who may not have osteoporosis, the jury is still out whether supplementing with vitamin D can help. A study published in August 2019 in the Journal of the American Medical Association found that among healthy people, supplementing with vitamin D didn’t improve bone health. RELATED: What to Eat and Avoid for Osteoporosis Prevention

Vitamin D Deficiency and Depression

If you’re familiar with seasonal affective disorder (SAD), you may not be surprised to hear that vitamin D deficiency may be linked to a higher risk of depression. After all, the nickname for vitamin D is the sunshine vitamin, and that’s not just because, as Harvard Medical School points out, many people generate much of the vitamin D they need thanks to sun exposure. Vitamin D status is also connected to a sunny (or not-so-sunny) mood. “There is research evidence that shows a relationship between mood and vitamin D levels, where deficient vitamin D levels are related to depression,” says Byrn. One of her studies, published in April 2017 in the Journal of Diabetes Research, found that vitamin D supplementation helped improve the mood of women with type 2 diabetes. All the women in the small study were given a high dose of vitamin D (50,000 IU weekly) for six months. Byrn and her colleagues found there was a significant decrease in depression and anxiety and an improvement in mental health. A meta-analysis published in April 2014 in Nutrients included research that suggested vitamin D supplements may in some cases be as effective as antidepressant medication, though more studies are needed. If you’re exhibiting symptoms of depression — such as feelings of emptiness or hopelessness, irritability, fatigue, appetite changes, and thoughts of suicide, per the National Institute of Mental Health — it may be worth a conversation with your healthcare team. “Low vitamin D may play a role [in depression], and it would be good to advocate and ask your provider to check your vitamin D level to see if a deficiency could be contributing to your symptoms,” advises Byrn. RELATED: 6 Depression Symptoms You Shouldn’t Ignore

Vitamin D and Risk of Schizophrenia

Schizophrenia is a severe brain disorder that affects between 0.25 and 0.64 percent of American adults, according to the National Institute of Mental Health. Symptoms of schizophrenia, which commonly appear between ages 16 and 30, include hallucinations, incoherent speech, withdrawal from others, and trouble focusing or paying attention. People who are vitamin D deficient may be twice as likely to be diagnosed with schizophrenia compared with people with sufficient vitamin D levels, suggests a review published in October 2014 in the Journal of Clinical Endocrinology and MetabolismResearchers reviewed findings from 19 observational studies that analyzed the possible relationship between schizophrenia and vitamin D deficiency, and observed a link between the two factors. While they noted randomized controlled trials would be needed to determine whether treatment for low vitamin D may help prevent schizophrenia, they explained that the condition is more prevalent in places with high latitudes and cold climates, and that studies suggest children who relocate to colder climates appear to be at a higher risk of developing the condition compared with their parents. Considering what scientists know about the role of vitamin D in mental health, the researchers’ findings may have merit. Although there is no cure for schizophrenia, treatments for schizophrenia include medication, psychosocial therapy, cognitive behavioral therapy, and family education and support groups. RELATED: What Is Schizoaffective Disorder?

Dementia and Vitamin D Deficiency

A study published in August 2014 in the journal Neurology found that moderate and severe vitamin D deficiency in older adults was associated with a doubled risk for some forms of dementia, including Alzheimer’s disease. Dementia involves a decline in thinking, behavior, and memory that negatively affects daily life. Alzheimer’s disease is the most common form of dementia, accounting for as many as 80 percent of dementia cases, according to the Alzheimer’s Association. The aforementioned study analyzed more than 1,600 people ages 65 or older who did not have dementia at the beginning of the study. Compared with people who had normal vitamin D levels, those with low levels of the vitamin had a 53 percent increased risk of developing all-cause dementia, while those who were severely deficient had a 125 percent increased risk, researchers observed. Also, study authors found people who had lower levels of vitamin D were about 70 percent more likely to develop Alzheimer’s disease specifically, and that those who were severely deficient were over 120 percent more likely to develop this neurodegenerative disorder. Considering the devastating toll that dementia can have on patients and their families alike, those findings may seem alarming. But researchers noted their study was observational, meaning they didn’t prove a direct cause-and-effect relationship with vitamin D deficiency and dementia and Alzheimer’s. Nonetheless, they theorized that the sunshine vitamin might help clear plaques in the brain that are linked to dementia. Regardless of the relationship between vitamin D and dementia, know that following tried-and-true health advice, like eating a healthy diet (the MIND diet is specifically linked to lower dementia risk), exercising regularly, and tending to your mental health can help reduce your risk of dementia, notes the Alzheimer’s Association. RELATED: What Are the Causes and Risk Factors of Alzheimer’s Disease?

Vitamin D Deficiency and Diabetes

The connection between low levels of vitamin D and diabetes is clear. “Again we see a relationship between vitamin D deficiency and diabetes,” says Byrn. Just why there’s a link is still unknown, but researchers have hypotheses. “Some researchers believe the link is related to the role of vitamin D in insulin sensitivity and resistance; however, randomized controlled trials don’t all support evidence that increasing vitamin D levels through vitamin D supplements results in improvements in insulin sensitivity,” says Byrn. Previous research points to these mixed results. “Another possibility of the link is related to the role of vitamin D in inflammation, because people with type 2 diabetes also have higher chronic inflammation,” notes Byrn. A review published in March 2017 in Biochemical Journal found that when vitamin D is deficient, many cellular processes in the body begin to break down, and this sets the stage for the onset of diseases such as diabetes. Still, people at risk for diabetes shouldn’t start an aggressive supplement regime. A large, multicenter study called D2d, which was funded in part by the National Institutes of Health, found that it’s still unclear that supplementing with vitamin D prevents type 2 diabetes, since the findings in that well-designed study, which was published in August 2019 in the New England Journal of Medicine, were not statistically significant. RELATED: Is Vitamin D Helpful for Preventing or Treating Type 2 Diabetes?

Prostate Cancer and Low Vitamin D

A study published in May 2014 in the journal Clinical Cancer Research supplementing with vitamin D can help. A found a link between low blood levels of vitamin D and aggressive prostate cancer in European American and African American men. Researchers looked at vitamin D levels in 667 men ages 40 to 79 who were undergoing prostate biopsies. The connection between vitamin D and prostate cancer seemed especially strong in African American men, with results suggesting that African American men with low vitamin D levels were more likely to test positive for the cancer than the other men with normal vitamin D levels. Though these findings were observational — that is, the study didn’t prove that low vitamin D led to prostate cancer, only that the two factors may be associated — the research does suggest you may help reduce your risk of the disease by ensuring you get adequate vitamin D. You should also make regular doctor’s visits and watch for common prostate cancer symptoms to receive a prompt diagnosis and treatment if you’re affected. Prostate cancer occurs mostly in older men, with the average age of diagnosis being about 66, according to the American Cancer Society. Other than skin cancer, it’s the most common cancer in men, and the second most common cause of cancer death in American men, per the American Cancer Society. RELATED: 11 Potential Signs of Prostate Cancer

Severe ED Linked to Low Vitamin D

A small study of 143 subjects published in August 2014 in the Journal of Sexual Medicine found that men with severe erectile dysfunction (ED) had significantly lower vitamin D levels than men with mild ED. Study authors theorized that vitamin D deficiency may contribute to ED by impeding the arteries’ ability to dilate — a condition called endothelial dysfunction and a heart-disease marker that has been associated with vitamin D deficiency in other research. For instance, a study published in the Journal of the American College of Cardiology suggested that lack of vitamin D was indeed linked with arterial stiffness in healthy people. One of the requirements for achieving an erection is proper function of the arteries, which are responsible for supplying the penis with blood so it can become engorged. ED is the most common sexual problem among men, affecting up to 30 million American men, according to the National Institute of Diabetes and Digestive and Kidney Diseases. ED can stem from other health conditions like diabetes, prostate cancer, and high blood pressure. Common ED treatments include hormone replacement therapy, counseling, and lifestyle changes like quitting smoking, limiting alcohol, and eating a balanced diet. RELATED: What Are the Symptoms and Causes of Erectile Dysfunction?

Vitamin D Deficiency and Heart Disease

Numerous studies have shown an association between low vitamin D blood levels and heart disease and related complications, according to a review published in January 2014 in Circulation Research, but science has not clearly established whether supplementation can reduce these risks. The review cites research that points to vitamin D levels as a potential culprit for health problems related to heart disease, including atherosclerosis, high blood pressure, diabetes, and stroke. You can reduce your risk of heart disease by maintaining a healthy weight, exercising regularly, and eating a diet rich in lean meat, nuts, and fruits and veggies, according to the American Heart Association. RELATED: Heart-Healthy Diet Makeovers

Vitamin D Deficiency and Breast Cancer

It’s not just prostate cancer that shares a link with low vitamin D levels. “We have also seen a link with vitamin D deficiency and breast cancer,” says Byrn. A review published in December 2017 in Breast Cancer: Basic and Clinical Research found that “most of the vitamin D studies support the inverse association between vitamin D level and breast cancer risk.” That means that a vitamin D deficiency was associated with a higher breast cancer risk. Another preliminary study published in July 2019 in Breast Cancer Research found that breast cancer cells exposed to high concentrations of vitamin D were associated with reduced severity. Studies in humans are needed.