In 2012, British journalist Michael Mosley sparked a global trend with his BBC documentary, “Eat, Fast and Live Longer.” Seeking a simple way to get healthy without growing his to-do list, Mosley discovered fasting. “What I discovered was truly surprising — it involves no pills, no injections and no hidden costs. It’s all a matter of what you eat. Or rather, what you don’t eat,” Mosley says in the introduction, over a video of meat searing on a Korean-style grill.
His 5:2 diet — two days of limited eating (600 calories or less) and five unrestricted days — became widely popular.
By 2017, the 5:2 diet was still trending when Natalie Lister encountered adolescents with obesity at her Sydney clinic. Some of her patients at the Children’s Hospital at Westmead Clinical School were already trying intermittent fasting. Others were asking her if they could. “But nothing had really been tested in young people,” she said.
Dieting is often counterproductive, leading to harmful fluctuations in weight and risk of disordered eating. And there is no definitive evidence that intermittent fasting is helpful across the board, or leads to lasting health gains. But given her patients’ independent forays into diet culture, Lister wanted to fill the knowledge gap and understand what benefits could come from a diet well-managed by health professionals.
To do so, Lister and her colleagues developed a pilot study, and then a randomized clinical trial of intermittent fasting and other diets in teenagers with obesity-related health issues.
Results published in JAMA Pediatrics last month suggest that supervised diets, including intermittent fasting, could help some adolescents with weight-related health problems. They also found that well-managed diets can lower the risk of eating disorders and mental health issues. The findings could inform clinicians’ decision-making as they face the challenge of balancing obesity treatment with the risk of triggering eating disorders and other negative outcomes.
A new generation of obesity drugs, called GLP-1s, offers obesity specialists a powerful tool for helping patients lose weight and possibly protect against a slate of diseases. One in five U.S. children has obesity. Still, other clinicians are reluctant to resort to injectable drugs in children and adolescents, especially since patients may need to continue the medications for a long time to keep the weight off. Also, obesity medications aren’t readily available to young people in Australia (neither is bariatric surgery). About 8% of children in Australia have obesity. That’s part of why Lister and her collaborator, Hiba Jebeile, wanted to see how safe diets could be for children already at higher risk of eating disorders, depression and other health problems.
The data also comes as Washington scrutinizes the nation’s high rates of chronic disease, especially among children, and what to do about it. In a hearing this week, lawmakers spent hours spitballing ways to use food to prevent and treat metabolic diseases like obesity.
Starting with 800 calories a day
For the first month, 141 teenagers from children’s hospitals in Australia followed a “very-low energy” diet of around 800 calories per day, using meal replacements like sweet shakes and soups. Afterward, half followed an intermittent fasting diet, with three restricted days (600-700 calories) and four unrestricted days. The other half followed a “continuous energy-restricted” diet, with daily calorie limits between 1,400-1,700. This phase lasted nearly a year.
Due to pandemic lockdowns, the study shifted to telehealth, which, along with frequent check-ins, ensured participants had support from pediatricians, a dietitian and a psychologist. Adolescents with diagnosed eating disorders were excluded, but some with disordered eating tendencies were eligible. In all, about 20% of the children had some binge eating symptoms, and about half had symptoms of depression. The study also wanted to see whether adolescents with mental health issues and obesity would benefit from a diet.
Results and cautions
The study’s primary goal was to assess changes in BMI z-scores, a measure of how far above or below the average BMI they are for their age and gender. That’s standard practice for a clinical trial on weight loss. But in practice, “this was not communicated to the young people at all,” Lister said. The adolescents set their own weight goals (which weren’t allowed to be lower than a calculated “healthy body weight”) and discussions focused on their health and quality of life.
Weight loss mostly occurred during the first, most restrictive stage. Once divided into two groups, weight loss slowed but was mostly maintained. In the intermittent fasting group, BMI z scores reduced from a mean of 2.34 at baseline to 2.06 at 52 weeks (meaning the group’s average BMI got a little closer to the greater average for their ages and genders). BMI in that group went from 34.83 to 33.21. In the continuous restriction group, BMI z scores dropped from a mean of 2.45 at baseline to 2.17 at the one-year mark. BMI went from 35.95 to 34.42.
Out of 141 participants, five in the intermittent fasting group and four in the continuous restriction group achieved their goal weight. No significant differences in weight loss were observed between the two diets.
Both groups saw minor improvements in liver function, and the continuous energy-restricted group had reduced insulin resistance. There were no major changes in body composition or cardiometabolic health in either group. One participant developed gallstones and needed their gallbladder removed, an adverse event the authors said was “possibly related” to the diet.
The small improvements seen in the study can be helpful for children with obesity-related health issues, Lister said. But she also emphasized that these intensive interventions are designed for children with serious health conditions — not for general weight loss. More study is needed to understand which children would benefit most from a similar diet, both researchers said.
“It’s not about telling all the kids in the school to go on an intermittent energy-restricted diet because it’s good for their health,” Lister said. “We don’t want young people left to go it alone, because that’s where the risks can be.”
Mental health benefits
Jebeile, who studies how weight loss affects mental health, has found that structured weight management programs often provide “overall benefit” to children’s mental health. This latest trial affirmed that finding. During the intervention, the teenagers showed less concern about shape and weight and had less disordered eating, possibly due to the consistent guidance and support from health care professionals. “Dietary restraint,” another red flag for eating disorder risk, increased during the first four weeks of intensive dieting, but normalized by the end of the trial.
Previous studies have found preliminary evidence that improvements in diet quality can improve mental health. In this trial, many of the improvements in mental health came after the most restrictive period, which could point to a general confidence-boost from weight loss early in the study. But the participants were also seeing a dietician every week, and able to chat by phone and text. ”We think it’s to do with having the support,” Jebeile said.
The researchers acknowledged that the pandemic lockdowns may have influenced participants’ symptoms, too. Reports of depression and disordered eating increased among Australian youth during Covid lockdowns, according to several studies. The team plans further analysis on dietary data and long-term effects.
Looking for disordered eating
Despite the structured approach, some say that these diets — especially the restrictive 800 calorie per day phase — look a lot like the diets of people with eating disorders. “I see kids that we end up diagnosing with full threshold anorexia nervosa who are eating 800 calories a day,” said Christine Peat, a psychologist and clinical associate professor at UNC’s Center of Excellence for Eating Disorders.
Eating disorders are about more than the number of calories a person consumes, Peat acknowledged. But young people with obesity are already vulnerable to eating disorders.
In the Australian study, about 75% of participants had dieted before entering the weight management program. About one-fifth of participants showed up with some disordered eating. Because of those risks, all of the adolescents transitioned to a healthy eating plan in line with dietary guidelines at the end of the 52 weeks. The researchers also linked the children to primary care providers or dieticians, and checked in six weeks after the end of the study period, and again at the two-year mark. That data will be published in a separate study.
While the study found a reduction in certain markers of disordered eating, the long-term risks remain uncertain. Research suggests eating disorders can take several years to emerge, but not many studies follow participants for that long (usually, they run out of funding).
Most participants reported at least one side effect, such as hunger, fatigue, or irritability. Nearly one-third dropped out, primarily due to the diet’s difficulty, though researchers said the attrition rate was normal for the type of intervention. One participant who withdrew was flagged as at-risk for an eating disorder, and then diagnosed with anorexia. The researchers said that was proof their screening and monitoring protocols work. Jebeile said there were no mental health differences between those who completed the study and those who didn’t.
The need for supervision
Lister and Jebeile emphasize that these diets are safe only under strict medical supervision and are not meant for widespread use among adolescents.
But consistent hand-holding from dietitians and pediatricians and obesity specialists is hard to access both in the U.S. and Australia. Clinical guidelines for pediatric obesity, released by the American Academy of Pediatrics last year, were criticized for recommending a level of well-rounded care that doesn’t exist in many communities.
That’s part of why more research is needed, Jebeile said — to advocate for more programs devoted to that care. In future research, the team aims to tailor interventions to those who would benefit most.
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