Here’s how it works: “Vitamin D, whether ingested or produced in the skin after sun exposure, must be metabolized to its active form—calcitriol—to exert its biological effects,” says study author Louise Wamberg, M.D., Ph.D. To do this, your body uses compounds called enzymes, including some that your fat tissue produces. But that doesn’t mean more fat makes for better vitamin D breakdown, Dr. Wamberg says. In the first part of a new study, published in the International Journal of Obesity, she and her co-authors analyzed fat cells from 20 heavy women and 20 who were normal weight. Fat from obese study participants contained far less of two key enzymes needed to process vitamin D than fat from lean women. “We suggest that fat tissue is not only a passive storage site for vitamin D, but may actively and dynamically metabolize vitamin D,” she says. “These processes may be compromised in obesity.” However, the differences shrink when you do. The researchers also studied 17 obese women who lost 10 percent of their body weight. After shedding pounds, their fat had higher levels of a third important metabolizing enzyme—and their blood levels of vitamin D increased 27 percent. Ongoing research should shed even more light on the links between extra weight and vitamin D status, Dr. Wamberg notes. Currently, the government’s RDA for vitamin D is 600 IU for adults of any weight (read more about the best way to get it). But the Endocrine Society recommends screening obese adults and children for vitamin D deficiency. If you’re heavy and have low levels, you may need two to three times the dose of D supplements to bring them back up, experts there note.
