As vitamins go, D has earned its right to be in the celebrity spotlight. After all, vitamin D is crucial to keeping so many of your organs and body systems in good working order, including your bones, brain, heart and lungs. Some studies suggest that having too little vitamin D can put you at increased risk for cancer, heart disease and a weakened immune system.

Getting your D level checked and taking a supplement to make up for any deficit is the perfect solution, right? The latest scientific answer is: maybe, maybe not.

Recently the U.S. Preventive Services Task Force released its position on screening for vitamin D deficiency. That group is an independent, volunteer panel of experts in prevention and evidence-based medicine. After reviewing the best available data, it concluded that there’s not enough evidence to suggest what levels of D are optimal for health or what levels constitute a deficiency — which makes it tricky to decide who needs a supplement. 

The medical community is working to make things clearer. Brigham and Women’s Hospital is in the middle of a randomized control trial (a high-quality study that carries scientific weight) that includes more than 25,000 men and women. One group will take vitamin D supplements; the other group will get a placebo. Researchers will compare cancer, stroke and heart disease risk between the two groups. The results of this study won’t be out until 2017. Until then, here are some answers to basic questions.

Who may need vitamin D supplements?

Certain groups are more likely to be deficient in vitamin D and may need a supplement. These include elderly people, pregnant and breastfeeding women, people with dark skin and anyone who has chronic kidney disease or parathyroid disease or who takes steroids long-term. People with celiac disease or Crohn’s disease and those who are obese may also be deficient in D. 

Some experts think even more people could benefit from supplements. “Most Americans aren’t outside enough without sunscreen to make adequate vitamin D,” says John J. Cannell, MD, founder and executive editor of the non-profit Vitamin D Council. “If you’re like many people who spend 40 hours a week inside working and either avoids the sun or applies sunscreen when heading out, you should get your vitamin D level checked,” says Cannell.

If you do spend time in the sun without sunscreen, with at least your arms and legs exposed, you can probably skip the test because your body is likely making enough D. (Of course, most experts advise against exposing yourself to sun without sunscreen for more than a few minutes because of the risk of skin cancer.) Worried about your levels over the winter? Humans are able to store the vitamin in their liver and fat. 

What exactly is a low vitamin D level?

How you define low vitamin D levels depends on whom you ask. The Institute of Medicine’s Food and Nutrition Board considers a blood level greater than 20 ng/ml (nanograms per liter) adequate, while the Endocrine Society deems anything above 30 ng/ml adequate, and the Vitamin D Council recommends that your levels be above 40 ng/ml. Why so much variation? It’s partially the lack of high-quality studies. But it’s also because these routinely available tests only measure the level of 25-hydroxy circulating in our blood. To get a completely accurate picture of your body’s D levels, you’d need to measure D3 levels. These tests aren't available yet. This shouldn’t deter you from requesting a 25-hydroxy vitamin D test, considered the most accurate test available right now.

If you are low in vitamin D, your doctor will help you figure out how much to take. Vitamin D3 is the form to choose because it’s the type of D your body makes after being out in the sun. Most organizations agree that between 1,000 and 2,000 IUs a day is safe. But if you are low, this may not be enough, says Cannell. Eight to 12 weeks after you start taking D3, ask your doctor to retest you again to see if your levels are increasing. 

How much sunlight do I need to get enough vitamin D?

There’s no magic number of minutes. How much vitamin D you will produce depends on your skin color, the time of day, cloud coverage and where you live in the world. You don’t need to burn or tan to produce vitamin D. The National Institutes of Health’s vitamin D fact sheet says they have no official guidelines on exactly how much sun you should get to make enough vitamin D. But they do state that some vitamin D researchers suggest the following: 5 to 30 minutes of sun exposure between 10 a.m. and 3 p.m. at least twice a week to the face, arms, legs or back without sunscreen. The more skin you expose, the more vitamin D you produce. Even sunscreen users will likely make at least some vitamin D because most aren’t diligent enough with reapplying or slathering it on as thickly as they should, says Cannell. 

Do any foods have vitamin D?

Food sources provide small amounts of vitamin D. Fatty fish such as salmon, tuna, and mackerel are some of the best sources. Three ounces of sockeye salmon, for instance, has 447 IUs of vitamin D. Small amounts of D are also found in beef liver, cheese, mushrooms, milk and egg yolks. But if you generally stay out of the sun most of the year, foods alone likely aren’t enough.

Jessica DeCostole is a licensed registered dietitian and health writer living in Baltimore, MD.