Shannon Frost Greenstein had rules about food: No butter on her toast. No red meat. At Thanksgiving, she nibbled on mashed potatoes and bread. At Starbucks, her go-to drink was a fat-free, sugar-free hot chocolate.

"I'll eat later," she would tell friends. "I'm not hungry right now." Greenstein, a 5-foot-8 former dance major, became fixated on her weight as it rose from 115 pounds to 130 in college, then dropped to 110 by her late 20s.

"My scale would dictate my mood for the day," she says. Losing weight "felt like success to me. Even if I felt bad at work or like a bad friend, this was something I was good at."

Dangerously good. By her early 30s - though married and trying to conceive a child - Greenstein was limiting herself to one meal a day, typically a Lean Cuisine entrée or cereal with milk. She weighed 98 pounds when her husband gave her an ultimatum: "If you want to be with me, if you want to be a mother, you have to find help."

The words "eating disorder" may conjure images of an anorexic teenaged girl. But today, those who experience eating disorders - including bulimia, binge eating, and anorexia, the least common form - are a far more varied group.

"The face of eating disorders today is any face: a young face, an old face, a white face, a black face. It's no longer an adolescent or college-aged girls' problem; it's everybody's problem," says Margo Maine, a clinical psychologist and author of Pursuing Perfection: Eating Disorders, Body Myths, and Women at Midlife and Beyond.

At the Renfrew Center of Philadelphia, the country's first residential facility for treatment of eating disorders, 23.5 percent of women treated are over age 35. A local Renfrew support group for midlife women became so oversubscribed in the last five years that there are now three such groups, serving a total of 30 women.

National statistics mirror those trends. Between 1999 and 2009, inpatient hospital admissions for eating disorders showed the greatest increase among older patients; one-fourth of those admissions were of women over 45.

Women whose eating disorders spike in midlife have likely been suffering for years - not only from the physical toll of bingeing, purging or restricting food, but from decades of shame and isolation. Older patients often say, "I must be the only woman my age who deals with this," says Beth Rosenbaum, the social worker who leads Renfrew's Philadelphia-area support groups for midlife women.

When Greenstein entered residential treatment at Renfrew, she was not the oldest woman there. And when she left, after nearly three months of individual and group therapy, sessions in mindfulness and education about nutrition, she was determined to stay healthy.

"I definitely struggled for the first couple of weeks; I started purging again. But as we started to talk about babies more, as I started to socialize, [the eating disorder] got further and further into the background."

Maine describes eating disorders in midlife as the outcome of a "perfect bio–psychosocial storm." An underlying fixation on weight and body image, stirred with midlife losses (children leaving for college; divorce; the death of parents), and fears of aging can erupt into a cycle of overeating and purging, food restriction or obsessive exercise.

Marty Lerner, a psychologist and executive director of Milestones in Recovery, an eating disorders treatment center based in Cooper City, Fla., says eating disorders are diseases, similar to alcoholism, with periods of remission and relapse. Their cause is a cocktail of "genetics, neurological reward circuits, hormones, processed foods, stress and cortisol levels," he says. And a barrage of images depicting slender women of all ages - now, thanks to social media, streamed steadily into the palm of one's hand - doesn't help.

As Gayle Hodgins grew older, she came to despise what she saw in the mirror: the map of lines under her eyes, the loose skin on her upper arms and the weight that seemed to pile on after the birth of her second daughter when she was 42.

One night, celebrating her husband's birthday at the Capital Grille, Hodgins realized that her appetite - for filet mignon, potatoes mashed with butter and cream and a candlelit dessert - was bigger than her stomach.

"I'd always joked with girlfriends about how I wished I could just throw up," she says. That night, she excused herself to the ladies' room and tried it. Afterward, she returned to the table for a second helping. "I felt fine. I didn't have this moment of 'Oh, my God.' Little did I know it was going to change my whole life."

That night launched a cycle of bingeing and purging that continued for years. Hodgins, now 56, would indulge in her husband's lavish homemade dinners - lasagna, ciabatta with garlic and herbs, strawberry/chocolate shaving ice cream - then go to the bathroom and vomit.

If she snacked on a Hershey bar late at night, she would picture the chocolate as a "bad seed" that needed to be purged by morning. And though she spent a week in 2010 at the eating disorders clinic at Belmont Behavioral Health Center in Philadelphia, she continues to wrestle with food and self-image.

"It's recover and relapse, recover, and relapse. I eat a mozzarella stick and an apple during the day; on weekends, I try to fast until dinner. If I eat too much, I purge. It's part of who I am at this point."

Physicians and psychologists say it can be difficult for women with eating disorders to recover in midlife, because they must unspool decades of behavior and thinking. On the other hand, they are often more motivated than teenagers forced into treatment by their parents.

Maine said some older patients seek treatment because their eating disorders have taken a cumulative toll: digestive problems from years of vomiting or bingeing; acid reflux; erosion of dental enamel; exacerbated menopause symptoms due to fat depletion. Others want to break the cycle and protect their daughters from suffering as they did.

For women in midlife or older, therapeutic approaches that recognize their strengths - many women with eating disorders are highly accomplished in their careers - and developmental stages are the most effective, Maine says. "We want to think of getting better as a journey, not a destination. And social support - being in group therapy with other women - can be a huge part of that journey."

On a recent Tuesday, three women gather for the group they call TSAB - Thirty-Something and Beyond - at the Renfrew Center in Radnor. They include a 40-year-old mother of two who sought treatment when an anorexia relapse left her too weak to push her baby's stroller; a 50-year-old who came to Renfrew seeking "relief from self-hatred"; and a 64-year-old who traces her binge eating back to losses she felt as a teen.

None want her name published, but they all talk freely about the ongoing struggle to free themselves from the grip of disordered eating. The 40-year-old tells the group how lost she felt after the birth of her second daughter. The stress of parenting, she says, is both her biggest trigger and her most powerful motivator to stay well. She'd been treated for anorexia in college, and her relapse made her wonder, "When are you going to grow up?"

But two years of regular attendance at the TSAB group, surrounded by women her own age and older, has drummed home an essential message: There's no such thing as "too old" for an eating disorder. There's no such thing as "too late" to get better.