By Sumathi Reddy

Before the pandemic, Basma O’Neill’s 15-year-old daughter was lean but healthy, with a robust appetite.

But after the coronavirus pandemic hit and schools closed in Graham, Wash., her daughter spent most of her time alone in her room. She wore pajamas and sweats, and ate at different times than her family.

She was diagnosed with anorexia nervosa in January; by February her weight was 86 pounds, down from around 118 pounds.

"I 100% believe the social isolation from the pandemic triggered the eating disorder," Ms. O’Neill said.

Experts across the country who treat eating disorders in adolescents and young adults say they are seeing unprecedented demand for treatment that arose during the pandemic. Inpatient units have doubled or tripled capacity, wait lists for residential programs and outpatient services are months long, and the patients coming in are sicker than ever.

Experts say they have seen the biggest increase during the past year in anorexia nervosa, an eating disorder where people deprive themselves of food. Other disorders being seen include bulimia nervosa, where people binge on food and then try to get rid of it with laxatives or vomiting, and binge-eating disorder, where people consume excessive amounts of food in a short period.

Tracy Richmond, director of the eating-disorder program at Boston Children’s Hospital, recently finished a study showing hospitalization rates of eating-disorder patients at Boston Children’s more than tripled in the pandemic, with the inpatient numbers rising from three or four to more than 10 and as many as 16 at a time. Demand for outpatient treatment also has risen sharply, from an average of six case reviews a week to as many as 23.

"Patients who come in are just really sick," Dr. Richmond said. Some have lost as much as 50% of their body weight.

"We sort of look at it like a second pandemic, the mental-health needs in adolescents," she said, "and eating disorders is part of that."

Dr. Richmond said preliminary data she has collected from 14 eating-disorder treatment centers nationwide indicate hospitalizations at least doubled during the pandemic.

A common scenario, she said, is a teen who comes in saying she decided to try to become healthier during the pandemic, often guided by TikTok videos and other social-media posts from fitness influencers. Social isolation, boredom and fear of gaining weight during quarantine also led to unhealthy behaviors.

“Kids who relied on things like sports and theater to give them a place lost all that,” Dr. Richmond said.

Eating disorders have most commonly been diagnosed in young, white girls but increasingly in boys and minority groups. Dr. Richmond said she is seeing patients as young as 10 or 12 years old.

Eating Recovery Center, a private network of 30 centers in seven states, says it received about 2,000 more new-patient calls in the first two months of 2021, a 90% increase over the same period in 2020.

“Everything changed in these kids’ lives,” said Anne Marie O’Melia, chief medical and clinical officer of the Eating Recovery Center. “Their lives were full of uncertainty, fear and isolation. These are the things that are very difficult in a developing brain.”

Eating disorders often bring a sense of control during a period of stress, she said. Diagnoses of depression and anxiety, common with eating disorders, also have been higher in the pandemic.

There was no wait list for outpatient treatment at the Eating Disorders Center at Children’s Mercy Kansas City before the pandemic. Now the wait is about six months.

Michaela Voss, the center’s medical director, said many of the adolescents they see had body-image issues or disordered eating before the pandemic. “And then lockdown happened, and they didn’t have anything else to do but to continue with those thoughts, and that’s when it got serious,” she said.

Other patients replaced social activities they had relied on for mental well-being with unhealthy coping mechanisms, such as excessive exercising or restricted food intake.

Jason Nagata, assistant professor of pediatrics at the University of California, San Francisco who specializes in eating disorders, said the rise in Zoom calls and social-media usage has caused some young people to focus more on body image.

Dr. Nagata said he also has seen high-school and college athletes develop eating disorders after training routines were disrupted. They “turned to excessively exercising on their own to compensate for loss of control, or to try to get back into their routine,” he said, including some who began exercising five or more hours a day.

Although anorexia nervosa is an eating disorder, excessive exercise can be a similarly compensatory behavior, Dr. Nagata said. “Even if you don’t restrict caloric intake, if you’re trying to lose weight by incredible amounts of exercise it can have the same effects physiologically,” he said.

A red flag for parents is when a child becomes preoccupied with their weight, appearance, or food and exercise in a way that affects their quality of life, such as withdrawing from a social activity or feeling anxiety, Dr. Nagata said.

Katherine Hill, medical director at Equip, a startup telehealth company that aims to address inequality in access to treatment for eating disorders, said many medical providers incorrectly assume patients have to be underweight. “You can be diagnosed with an eating disorder at any weight,” Dr. Hill said.

Patients with bigger bodies can lose a large percentage of their body weight rapidly yet often don’t get noticed until they are very sick, she noted.

Nicole Nesbit’s 16-year-old daughter was 5-foot-8 and normally around 155 pounds, her mother said. When the pandemic hit, she began going on very long walks and bike rides from their home in Ware, Mass. She stopped drinking and eating some of her favorite treats, like Arizona ice tea and muffins.

Her daughter was wearing baggy clothes, and it wasn’t until December that Ms. Nesbit noticed the weight loss. When her daughter went to see her pediatrician, she had lost 20 pounds but was still at a normal weight for her height. The doctor told the mother not to worry.

In February, her daughter passed out—probably from low blood pressure, a common health complication in anorexic patients—and went to the emergency room. She started seeing an eating-disorder specialist and began family-based therapy, where the patient’s meals and snacks are supervised, but it wasn’t enough. “She was getting up in the middle of the night to exercise, throwing up into bottles,” her mom said. “She would switch out her supplement drinks, or roll food down her sleeve.”

By April her daughter weighed 110 pounds. After she tried to self-harm, she was referred to an inpatient center at Bay State Children’s Hospital in Springfield, Mass., and diagnosed with major depression, followed by a residential treatment program; she recently completed a five-week virtual program.

Her blood pressure has stabilized and she has gained back much of the weight she had lost. But psychologically, she hasn’t recovered, her mother said.

“I have to watch her every move,” Ms. Nesbit said. “She sleeps in our bedroom now. She uses the bathroom with the door open. I go in when she showers…She’s not eating on her own. There are still battles.”

Eating disorders don’t go away by themselves. They require intervention, Neville H. Golden, chief of adolescent medicine at Stanford Medicine, noted. “It’s important to get help,” he said. “The earlier an intervention is initiated, the better the outcome.”