While there is a correlation between obesity and vitamin D deficiency, as the NIH notes, there’s no scientific proof that taking a vitamin D supplement will help trim your waistline, says Michael Holick, MD, PhD, an endocrinologist and the director of the Bone Health Care Clinic at Boston University Medical Center.One study found that when obese and overweight women brought their vitamin D levels up to a normal level with a supplement, while also eating a low-calorie diet and exercising, they lost more weight than women who were also on a low-calorie diet and exercising but were unable to raise their vitamin D up to the ideal level. Still, the researchers agreed that it’s too early to draw any conclusions that taking vitamin D can help you shed pounds because it’s still unclear whether a low vitamin D status is a consequence of obesity or in some way involved in its cause.Having sufficient vitamin D, though, may help people live healthier lives, which could in turn help a person lose weight. “If bone aches and pains from a vitamin D deficiency keep you from exercising, and if vitamin D supplementation allows you to exercise more because you don’t have those aches, that could help you be more active,” says Dr. Holick.6. Fact: Vitamin D Helps Support Your Immune System To keep your immune system functioning at its best, you’ll want to make sure you’re getting enough vitamin D.That’s because, according to Harvard T.H. Chan School of Public Health, vitamin D may help mitigate the harmful inflammatory response of certain white blood cells, as well as increase the immune cells’ production of microbe-fighting proteins. Adults who have low vitamin D levels are more likely to report having experienced a recent cold, cough, or upper respiratory tract infection.“Clinical trials have found that vitamin D supplementation can reduce the amount of illnesses that children develop,” says McTiernan, referring to research on influenza. And a meta-analysis yielded two points: First, people who took vitamin D supplements were 12 percent less likely to develop acute respiratory infections compared with people who didn’t; and second, the authors found that for those with a severe vitamin D deficiency, taking vitamin D supplements lowered their respiratory infection risk by 70 percent.Because of the potential benefits to the immune system, some people have been quick to assume vitamin D can work as a potential COVID-19 prevention tool or therapy. While there is not yet enough research to support this, some studies from early in the pandemic did find a correlation.One study noted that more than 80 percent of people with COVID-19 in a hospital in Spain had a vitamin D deficiency. And another study that analyzed 235 individuals infected with the virus concluded that improving vitamin D status in both the general population as well as those hospitalized with COVID-19 had the potential to reduce the severity of symptoms and death associated with the disease.If you have questions about taking vitamin D and proper dosing, it’s best to discuss them with your primary care provider or a registered dietitian.7. Myth: Everyone Should Get Their Vitamin D Levels TestedFor maintaining good health, it’s important to have your vitamin D levels in the adequate range. According to the NIH, adults need between 20 and 30 nanograms per milliliter (ng/mL) of vitamin D in their blood. While there’s no way to know if your levels are in that range without a test, that doesn’t make it necessary to get one, unless, as the Cleveland Clinic notes, you have a condition such as osteoporosis.“Testing is becoming more common, but there is no need for widespread screening,” says Holick. “If you take the recommended amounts of vitamin D, your levels will be sufficient.”It’s probably a good idea to talk to your doctor before you start supplementing. “I support seeing your doctor or other provider for a check of vitamin D level, and then taking a supplement if the test shows you’re lacking,” says McTiernan.Also, according to MedlinePlus, your doctor may order a vitamin D blood test if you have low-vitamin-D symptoms, which could include bone weakness, bone softness, and fractures. Other risk factors that may signal to your doctor that you need a vitamin D test include obesity, Crohn’s disease, ulcerative colitis, previous gastric bypass surgery, older age, a dark complexion, and lack of sunlight in your day (for example, if you’re on bed rest or have an injury that keeps you indoors), MedlinePlus notes.8. Fact: Getting Enough Vitamin D May Improve Blood Sugar Levels If you’re trying to prevent or manage type 2 diabetes, it might be time to ask your doctor about your vitamin D levels.“We think vitamin D helps with insulin resistance,” says Holick. For example, a 2023 review and meta-analysis of three randomized clinical trials found that for adults diagnosed with prediabetes, vitamin D supplementation was effective in decreasing the risk of their condition progressing to type 2 diabetes.Also, a 2023 study of diabetics found that lower levels of vitamin D correlated with higher levels of A1C, one indicator of poor diabetes management. However, the study was small, so more research is needed.Given the mixed results, ask your healthcare team about possibly adding a vitamin D supplement to your diet for preventing or treating diabetes.RELATED: What Vitamin D Is Good for When You Have or Are at Risk for Diabetes9. Fact: Adequate Vitamin D Is Linked With a Lower Risk of Certain Cancers“Individuals with very low vitamin D levels are thought to be at increased risk for breast, colon, kidney, lung, and pancreatic cancers,” says Carol Fabian, MD, an oncologist and the director of the Breast Cancer Prevention and Survivorship Research Center at the University of Kansas Cancer Medical Center in Kansas City. The Cleveland Clinic notes, too, that vitamin D may help lower the risk for prostate cancer.The most at-risk groups are individuals who are obese or have darker-pigmented skin and have lower levels of vitamin D but are not supplementing, notes Dr. Fabian. Other factors can also drive up the risk. “Most people who are obese do not exercise much, and obesity and a sedentary lifestyle are also risk factors for breast, colon, kidney, lung, and pancreatic cancers,” says Fabian. While vitamin D deficiency may not be directly upping cancer risk, an association between the two exists in some cases.That said, a review from 2022 noted that there is strong data supporting a protective effect of vitamin D (due to its role in regulating cells) against several types of cancer.Furthermore, research shows the potential positive effects of maintaining adequate levels of vitamin D. Overall, individuals with the highest levels (greater than or equal to 35 ng/mL of vitamin D) may have about a 30 percent lower risk of breast cancer compared with those with the lowest blood levels of vitamin D (less than 15 ng/ml), says Fabian. And missing out on important UVB rays during the crucial sunny months may also up your odds of cancer. “A very large observational study with many years of follow-up suggests that women who have very low levels of vitamin D measured in the spring and summer are at increased risk for breast cancer,” says Fabian, referring to the Nurses’ Health Study.Right now, there are challenges to vitamin D and cancer studies, one review noted. For example, the amounts of vitamin D provided to study participants varies greatly from study to study, and men and women respond to vitamin D supplementation in different ways.As for utilizing vitamin D to treat cancer? The research and technology aren’t there yet. “High doses of drugs with a structure similar to vitamin D may someday be used to treat some forms of breast cancer, but doses of vitamin D that are high enough to cause death of cancer cells are likely to cause very high blood levels of calcium, which can be toxic to the body,” says Fabian.10. Myth: All Adult Women Need the Same Amount of Vitamin D