The 25-year-old reality TV star is in an undisclosed private facility, her publicist exclusively tells E! News, "to undergo diagnostic treatment to determine why she's not been putting on any weight."
"She is working with a team of doctors and specialists whose focus is nutrition," says the rep. "It is important to Nicole that she achieve this goal in a healthy way, [but] this is not a treatment for an eating disorder."
Richie's treatment means work on the new season of The Simple Life, which will find Richie and recently reconciled pal Paris Hilton as camp counselors, is temporarily on hold.
In a joint statement, show producers 20th Century Fox and Bunim/Murray said, "Everyone...supports Nicole and wishes her well. We consider her a part of our family and look forward to resuming production on The Simple Life when she is ready."
Meanwhile, E!, which televises the series, added, "Nicole is an important member of the E! Networks family. We send her our very best wishes and look forward to working with her on the new season of The Simple Life." (E! Online is a division of E! Networks.)
In recent months, Richie has addressed rampant tabloid speculation over her weight loss. Last spring, she told People that "stress" over her breakup with former fiancé Adam "DJ AM" Goldstein briefly affected her eating habits.
In June, she admitted to Vanity Fair that she was working on her body image. "I know I'm too thin right now, so I wouldn't want any young girl looking at me and saying, 'That's what I want to look like,' " Richie told the magazine. "I do know that they will, which is another reason I really do need to do something about it. I'm not happy with the way I look right now."
Then last month, Richie issued a statement vehemently denying a CNN report that she had begun treatment for anorexia.
"Contrary to CNN's false accusations, I did not check myself into an eating disorder rehab," she said. "I don't know why or how this rumor started, but I am home, in L.A., and very happy. I do not have an eating disorder, and I don't know how many times I have to say it."
The disputed report even led her father, Lionel Richie, to chime in on controversy, saying stress, not an eating disorder, prevented her from gaining weight. "Believe me," Richie told 20/20. "I've checked it out three different ways to Sunday. We went to a couple of doctors and the doctors said, 'Well, it's not an eating disorder. She's just—her nerves affect her weight.' "
Hoping to silence her critics once and for all, Richie subsequently appeared on the Tyra Banks Show. When asked straight out if she suffered from anorexia, Richie replied, "Absolutely not. One hundred percent not. I am not anorexic. I am not bulimic. I do not have an eating disorder."