Nicole Richie has gone on a crusade – a health crusade!

She’s vowed to boost the health of two of her closest friends – actress Mischa Barton and celeb stylist Rachel Zoe.

The health-conscious reality star – who has overcome her own eating disorder and drug problems – is encouraging pudgy Mischa, 24, to lose weight and Rachel to add 15 much-needed pounds
"Nicole has battled weight issues, but finally figured out how to balance her life, and is determined to lead Mischa and Rachel to a happier and healthier lifestyle," an insider told The ENQUIRER.

"Mischa has been down lately because her career has been in limbo, and rather than returning to the party scene, she has turned to food."

The 5-foot-8 star of The O.C. – who once weighed a slender 125 pounds – recently ballooned to about 165, according to the insider.

"Mischa’s career has been foundering. Her last series The Beautiful Life was canceled – and she’s been eating everything in sight for solace.

"Tiny Rachel is at the other end of the spectrum. She works so hard dressing her A-list actress clients that she literally forgets to eat."

The Rachel Zoe Project
star – who at 38 stands just 5 feet tall and has weighed about 100 pounds at her healthiest – recently slipped to an emaciated 82 pounds.

"Nicole has stepped in and had one-on-one interventions with Mischa and Rachel," revealed the insider.

"She’s counseled both on a healthier way of life and how to incorporate nutritious organic food into the stress-filled schedule of a celebrity."

Nicole, 28, follows a strict organic diet of protein, fruits and veggies, and she regularly does yoga
 "Nicole overcame her body-image and substance-abuse problems and she wants to help her friends Mischa and Rachel have the same success," the insider divulged.