Vitamin D is essential for bone and immune health, but a recent study found that not all forms of this vitamin are equally beneficial. Researchers from the Universities of Surrey and Brighton in the U.K. report that supplements that contain vitamin D3 bolster the immune system better than those containing vitamin D2.

“We have shown that vitamin D3 appears to stimulate the type 1 interferon signaling system in the body — a key part of the immune system that provides a first line of defense against bacteria and viruses,” said Colin Smith, the lead author of the study, according to Healthline.  “Thus, a healthy vitamin D3 status may help prevent viruses and bacteria from gaining a foothold in the body.”

Dr. Thomas Carpenter, an endocrinologist at Yale Medicine, explains the difference between D3 and D2.

“Vitamin D2 is the name of the natural plant source of vitamin D,” he tells Healthline. “Vitamin D3 is the form synthesized by most animals. They are nearly identical and have a minor difference in chemical structure.”

Unlike vitamin D2, Vitamin D3 is manufactured in your body when your skin is exposed to sunlight, according to Medical News Today. But most people do not get enough sunlight to fuel this process. Statistics show that 4 in 10 Americans are deficient in this critical vitamin.

Many foods are now fortified to help offset this deficiency.

Spending more time in sunshine, about 10 to 30 minutes a few times each week, is one way to get enough D3. But Carpenter says that some fortified foods have already switched from D2 to D3.

“Years ago, cow’s milk was fortified with D2 and, more recently, most brands fortify with D3 or a mix of the two,” he said, adding that some breakfast cereals, cheeses, and yogurt also fortify but you’d have to check the labels to be sure if they include vitamin D and what form is being used.

The best sources of foods rich in vitamin D3 are the flesh of fatty fish and fish liver oils, say experts at  Harvard T.H. Chan School of Public Health. Smaller amounts are found in egg yolks, cheese, and beef liver.

Dr. Annette Faller, an internal medicine specialist at the Mayo Clinic Health System in La Crosse, Wis., told Parade that she encouraged all of her patients to take vitamin D3 during the COVID-19 pandemic as higher levels of this vitamin “are associated with better outcomes.”

Faller suggests taking 4,000 IU of D3 daily to combat fatigue, boost immunity, and help stave off illnesses.