When ingested, vitamin D2 (ergocalciferol) and D3 (cholecalciferol) are absorbed in the small intestine. From there, they enter the bloodstream and travel to the liver where they’re converted by enzymes into a new form. They then travel to the kidneys where they’re converted by other enzymes into a biologically active form. Once all of this has happened, Dr. Holick says the vitamin D molecule travels throughout the body to perform a variety of functions.
“The main difference between the two forms of vitamin D is in their structure,” Holick says. The two compounds have different side chains (chemical groups attached to the molecule). This difference in structure makes vitamin D3 easier to convert into a usable form after you ingest it.
“The enzyme that converts vitamin D in your body prefers vitamin D3 over vitamin D2,” says Qianzhi Jiang, PhD, R.D., founder of The Nutrition Changer, a nutrition services company in Natick, Massachusetts, and adjunct professor at Framingham State University.
So, although vitamins D2 and D3 perform the same function, the body metabolizes vitamin D3 more efficiently than D2.
