Weight Watchers — now rebranded as WW — has introduced an app called Kurbo, for children 8 to 17 years old. As a registered dietitian who specializes in helping people recover from disordered eating, I strongly recommend that parents keep this new tool — and any weight-loss program — away from their children.

Our society is unfair and cruel to people who are in larger bodies, so I can empathize with parents who might believe their child needs to lose weight, and with any child who wants to. Unfortunately, attempts to shrink a child’s body are likely to be both ineffective and harmful to physical and mental health.

Over the last 60 years, numerous studies have shown that among people who lose weight, more than 90 percent gain it back over the long run. For example, a 2000 study of adults 20 to 45 found that less than 5 percent lost weight and kept it off long term. And a 2015 study of more than 176,000 higher-weight people age 20 and older found that 95 percent to 98 percent of those who lost weight gained back all of it (or more) within five years. A 2007 review of the scientific evidence found that most people likely gained back more.

There’s not as much data that specifically looks at the effectiveness of weight loss in kids, perhaps because weight loss wasn’t widely recommended for minors until fairly recently; in school to become a dietitian nearly a decade ago, I learned that it was considered harmful to put children on weight-loss diets. But the evidence we do have in this population suggests that long-term weight loss is just as unlikely for children as it is for adults.

A 2017 systematic review of studies of weight-loss programs for kids ages 6 to 11 found that those interventions produced only small, short-term reductions in weight, and that weight-loss studies in kids were flawed at best. “The overall quality of the evidence was low or very low, and 62 trials had a high risk of bias,” the reviewers wrote. That review included numerous studies examining the family-based protocol that Kurbo cites as its scientific basis.

Many people — children and adults — are unable to lose substantial amounts of weight in the first place, let alone keep it off long term. WW admits as much in the fine print for Kurbo: “Results not typical. Weight loss and/or BMI reduction will vary by age, weight and height.”

More important, trying to shrink children’s (or anyone’s) bodies can put their health at risk.

That’s because intentional weight loss is likely to result in weight cycling — the repeated cycles of weight loss and regain commonly known as yo-yo dieting — and weight cycling carries its own risks, independent of body mass index.

People who have weight cycled are at higher risk of health problems including high blood pressure, chronic inflammation, and overall mortality. While these conditions typically get blamed on weight itself, in reality weight cycling accounts for at least some of the additional risk that comes with being at a high B.M.I. In fact, weight cycling may explain all of the excess mortality risk seen in people at the high ends of the B.M.I. spectrum.

The scientific name for anti-fat bias is weight stigma (a.k.a. fat stigma or fatphobia), and experiencing it has been linked to increases in risks of type 2 diabetes, hypertension, cardiovascular disease, and mortality, as well as depression, anxiety and low self-esteem. In fact, a 2017 study published in the Annals of Behavioral Medicine found that weight stigma posed a greater overall health risk than what people ate, and about an equal risk as physical inactivity. Given that weight stigma tends to lead both adults and kids to avoid physical activity, weight stigma can take a double toll on people’s health.

Moreover, weight stigma isn’t just limited to overt nastiness and bullying. For example, several studies have found that merely reading news reports about the supposed health risks of larger body size leads to greater anti-fat prejudice. (Those results held true even when the reports included an explicit anti-weight-stigma message.) A 2017 study in adult women found that those whose parents had commented on their weight in childhood reported greater body dissatisfaction years later, regardless of their actual B.M.I. And a 2014 study found that young women who were told in an experiment that their weights “weren’t ideal” experienced higher levels of cortisol (the stress hormone), regardless of their actual size.

Should you think that the solution to weight stigma is simply to lose weight, remember the data we just discussed on the ineffectiveness of weight-loss efforts. We simply don’t know of a way for more than a tiny fraction of people to lose weight and keep it off long term — and that’s just as true for WW’s new app as it is for any other diet or lifestyle change.

Another common harmful effect of dieting is disordered eating, for people of all ages. It’s especially common for kids to interpret even the most seemingly benign messages about “healthy eating” in strict ways; indeed, most of the clients I’ve treated for eating disorders or subclinical disordered eating had their disorders triggered or exacerbated by messages they learned about food and weight in childhood. More than 55 percent of high school girls and 30 percent of boys report engaging in harmful practices including fasting, taking diet pills, vomiting and abusing laxatives to lose weight. As many as 60 percent of 6- to 12-year-old girls are worried about their weight.

WW has described the app as a “holistic tool,” not a diet, but the way it’s been branded doesn’t change the effect it could have on its users. Programs like this are fertile ground for disordered eating, encouraging kids to track what they eat using a “traffic light” system that divides foods into red, yellow and green categories, implicitly coding certain foods as “good” and others as “bad.”

Efforts to lose weight are likely to be ineffective and can be harmful to people of all ages, but they are especially troubling when imposed on children who aren’t in a position to make their own choices and who haven’t had the opportunity to develop a peaceful relationship with food and their bodies.

Sophia Carter-Kahn was one of those kids who was put on a diet. Now in her late 20s, Ms. Carter-Kahn started going to Weight Watchers around age 10, after a doctor told her she needed to lose weight. As she told me on my podcast, Food Psych, in 2018, all the rules and numbers she learned at Weight Watchers led her to obsess about food in a way that was incredibly disordered. Those patterns took years to overcome.

She told me she only recently started to ask herself questions about how eating particular foods made her body feel. “I’d never thought about that before,” Ms. Carter-Kahn said. Dieting had kept her focused on points, calories and pounds, rather than on what her own body was telling her.

Numerous studies have found that intuitive eating — getting back in touch with your body’s innate cues about what, when and how much to eat, instead of following what some diet guru or app tells you to eat — is linked to better health outcomes than even the most “flexible” diet.

If we truly want to help children be the healthiest and happiest people they can be, we need to stop putting them on diets of any kind, which are likely to worsen their overall well-being. Instead, we need to start teaching them to trust their own inner wisdom about food. And we need to help them make peace with their bodies, at any size.

Christy Harrison is a registered dietitian nutritionist and the author of the forthcoming book “Anti-Diet: Reclaim Your Time, Money, Well-Being and Happiness Through Intuitive Eating.”

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