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Vitamin D can be obtained through sunshine, certain foods, and supplements. Olga Shumitskaya/Getty Images
  • Researchers say vitamin D3 bolsters the immune system better than vitamin D2.
  • This goes against previous research that rated both forms of vitamin D about the same.
  • Experts say you can obtain a sufficient amount of vitamin D by being out in the sun for up to 30 minutes.
  • Certain foods, such as milk, cheese, and breakfast cereals, are fortified with vitamin D.

Vitamin D is an essential nutrient for bone and immune system health, but not all forms of the vitamin are created equal, new research suggests.

Looking at two forms of vitamin D supplements — those containing vitamin D2 and those containing vitamin D3 — scientists from the Universities of Surrey and Brighton in the United Kingdom report that only vitamin D3 induced an effect on the body that might bolster the immune system.

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

This is significant, because previous research indicated both forms of vitamin D were equally effective, while this latest study suggests foods fortified with vitamin D should prioritize D3 over D2.

“The lack of impact we found when looking at vitamin D2 means that a larger study is urgently required to clarify the differences in the effects,” said Susan Lanham-New, PhD, a professor and head of the department of nutritional sciences at Brighton and a study co-author.

Vitamin D is different than other vitamins in that it’s naturally synthesized by the cholesterol cells in the body when your skin is exposed to sunlight.

But most people don’t get enough sunshine or eat enough vitamin D-rich foods to get an adequate supply. More than 4 in 10 Americans get insufficient levels of vitamin D, studies show.

Hence, many foods are fortified with vitamin D to help increase daily intake — milk being one of the most common.

“Vitamin D2 is the name of the natural plant source of vitamin D. Vitamin D3 is the form of vitamin D synthesized by most animals,” Dr. Thomas Carpenter, an endocrinologist at Yale Medicine in Connecticut, told Healthline. “They are nearly identical and have a minor difference in chemical structure.”

As for vitamin D’s potential immunoprotective effects, the jury is still out.

“This is not well understood, but some data to indicate stimulation of certain antibacterial proteins has been shown, but clinical efficacy is not always evident in studies,” Carpenter said.

Besides spending more time in the sunshine — you only need about 10 to 30 minutes’ worth a few times per week — certain foods are going to be your best source of vitamin D.

With vitamin D fortified milk, the good news is that many producers have already switched over from using D2 to D3, Carpenter said.

“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.

“Other foods in which vitamin D is added as fortification include breakfast cereals, cheeses, yogurts, although this is not always the case and one would need to review the food label to know if the particular product is fortified,” Carpenter said.