Is Kate Middleton an Unhealthy Weight?

April 29, 2011— -- A radiant, but wisp-thin Kate Middleton stepped into the public eye today, emerging from the royal state car -- a 1978 Rolls-Royce -- and walking into Westminster Abbey to take her wedding vows.

The lace-bodice dress, designed by Alexander McQueen's Sarah Burton, clung gracefully to her lean frame, possibly adding to questions about the newly titled Duchess of Cambridge's health.

If the tabloid reports are correct, then Kate Middleton has dropped a significant amount of weight since the announcement of her engagement to Britain's Prince William in November.

While it couldn't be confirmed, British papers said the future queen lost two dress sizes, going from a U.S. size 6 to a size 2.

Watch a special 2-hour edition of "20/20" at 9 p.m. ET tonight for a look back at all the highlights from the royal wedding with Barbara Walters.

"Basically, this could be about 10 to 14 or more pounds of weight loss, and not just body fat. She is lean to begin with, so some of the loss has been lean mass," said Leslie Bonci, director of sports nutrition at the University of Pittsburgh Medical Center.

Losing this amount of weight, however, isn't unhealthy.

"If it took her 5 months to lose 20 pounds, she lost a pound every 7.5 days. Safe weight loss recommendations for most folks is no more than 1 pound per week, so even if she lost 20 pounds, she was within the guidelines for a healthy rate of weight loss," said Carla Wolper, obesity researcher at St. Luke's Hospital and assistant professor at the Eating Disorders Research Center at Columbia University Medical Center, both in New Yorkq.

While the number of pounds she lost isn't unhealthy, her actual weight could be. Buckingham Palace has remained mum about how much Kate weighs or how tall she is, but rumors put her at 5-foot-10 and about 120 pounds.

"At a weight of 120 for 5-foot-10, that puts her at 80 percent of ideal body weight, or definitely underweight," said Bonci.

Royal watchers believe stress is partly to blame for Kate's shrinking frame. ABC royal contributor Duncan Larcombe said recently that in each of Kate's successive official appearances since her engagement, she appeared "thinner and thinner."

"I would imagine she probably can't eat," Larcombe said.

Nutrition experts in the U.S. say the stress is likely a contributing factor, but believe there are other reasons behind Kate's weight loss.

"This appears to be a case of voluntary, go-on-a-diet, quick-fix weight loss," said Dr. David Katz, director of the Yale University Prevention Research Center in New Haven, Conn.

But to some experts, it's all part of a very normal pre-marital ritual.

"Almost everyone in the U.S. who is getting married, male or female, unless very slim from the get-go, goes on a diet before their wedding. Perhaps the Brits do as well," said Wolper.

Will She Keep It Off?

There have been rumors that Kate lost weight on the Dukan Diet, a low-carbohydrate, high-protein diet developed by a French physician. The palace, however, has denied the tabloid reports.

"She is no different than actresses and models who are watched by the whole world," said Dr. Mitchell Roslin, a bariatric surgeon at Lenox Hill Hospital in New York. "Of course, it will not be easy to always be in the public eye, but I think she is well prepared and doubt will have any eating disorders, etc."

Although she will continue to be a very public figure, experts hope with the wedding behind her, Kate will start to gain weight again.

"Relaxing the rules once life returns to normal can easily include less attention to portions and preparation methods and thus taking in more calories than needed, leading to weight gain," said Wolper.

"Hopefully, she will show up at the dinner table with her prince, sit down, eat and enjoy," said Bonci.

ABC News' Cathy Becker, Gabriel O'Rorke and Jane Kurtzman contributed to this report.