Media superstar Oprah Winfrey’s well-publicized battle with weight loss took an added twist this month when her medical advisor revealed that the broadcast mogul suffers from vitamin D deficiency.

“She had Vitamin D deficiency,” Dr. Mehmet Oz told CNN’s Larry King Live. “Why would that be shocking for a darker-skinned person who spends a lot of time in Chicago? If you’re watching this show right now, if you live in a Northern state, you are probably Vitamin D deficient. And then what does that cause?

Oz continued, “It causes auto-immune problems. So for her, it takes your thyroid gland — which is like your thermostat in your house and it shifts it off kilter, so you can’t quite get the right temperature in your body. And those things tend to lead to guess what — changes in the metabolism and weight gain.”

Winfrey’s vitamin D deficiency was revealed as the star dealt with weight-gain issues. According to Winfrey’s web site, www.Oprah.com, “It’s no secret that Oprah has struggled with her weight for decades. Over the years, she’s spoken candidly about bouncing from one diet to the next, emotional eating and her drug of choice, potato chips. In 1988, The Oprah Show received its highest ratings of all time when Oprah celebrated a 67-pound weight loss by wheeling a wagon full of fat onto the stage. Soon after, her size 10 jeans didn’t zip and the pounds began to add up again. In 1992, Oprah met personal trainer Bob Greene. At the time, she weighed 237 pounds, the heaviest she says she’s ever been. She made a commitment to her health, began to lose weight and even ran a marathon…but the weight loss roller coaster continued.”

The site continues, “When Oprah turned 50 in 2004, she seemed to have her health and weight under control. For years, she maintained her figure with diet and exercise. She says she thought she had it all figured out, but in 2007, as Oprah dealt with emotional and medical issues, the number on the scale began to creep closer to 200. To begin living her best life, Oprah’s speaking candidly about what she calls ‘the brown elephant in the room’ — her weight gain.”

Dr. Oz is a vitamin D advocate, and puts knowing your vitamin D levels on his Ultimate Health Checklist for 2009 — encouraging people to get 15 minutes a day of sun or taking 1,000 IU of vitamin D. The vitamin D reseach community actually suggests 2,000 IU of vitamin D.