The Flawed Construct of “Ideal Body Weight”

“There’s really no such thing as an ideal body weight,” says Rebecca Boswell, PhD, Director of Princeton Center for Eating Disorders and Administrative Director of Psychiatric Services at Penn Medicine Princeton Medical Center. “These calculations are based on mathematical equations and averages, but not on individuals. They’re one part of a broader picture, but they don’t take into account the specific needs of patients with eating disorders.”

“Ideal body weight” equations were developed using biased datasets and unrepresentative samples. Historically, white European male height/weight averages were used to develop a ratio (the Quetelet Index) that was a precursor to body mass index (BMI). Insurance companies began using height and weight to estimate mortality risk for men and women in 1912. A seminal dataset (Met Life Tables) collected by insurance companies factored in one-inch heels when calculating heights, used self-reported weights, and included primarily young white policyholders. In turn, BMI was created with data from white men rather than a representative sample of people.

Even today, BMI standards are relatively arbitrary, according to Dr. Boswell. She notes that “ideal body weight” doesn’t tell providers much more, since it’s highly correlated with average BMI.

Dr. Boswell and Lead Dietitian Jenna Deinzer, RD recently conducted a literature review and examined commonly used methods to determine ideal body weight. Among their findings for adults, they noted that:

• Supporting data was often incomplete, with little to no information on whether samples included variation in race, ethnicity, or age.
• Many methods were created by pharmacists to determine dosing regimens for medications.
• Most formulas produce a single body weight rather than a range.
• None of the methods studied addressed or measured body composition or overall medical and nutritional status. 

Providers are faced with a challenge: while they can rely on growth charts in children to provide a more complete picture of expected individual weight trajectories, they lack updated data and similar context to establish better weight goals for adults with eating disorders.

When numbers fall short

Because ideal body weight doesn’t incorporate body diversity, age, weight suppression, psychological wellness, metabolism, and individual experience, the resulting calculation may fall short of being optimal for the patient.

“Ideal body weight calculations often result in lower target weight goals than many of our patients need for recovery,” says Dr. Boswell. “Our hypothesis is that relying solely on these formulas may lead to an increased relapse rate in adults with eating disorders.”

Deinzer provides a hypothetical example: A young female patient who was thriving at 140 pounds develops an eating disorder, and her weight drops to 120 pounds. The ideal body weight calculation is also 120 pounds, but the patient’s labs, vitals, and overall health demonstrate that she is struggling. The calculation also sends a mixed message to the patient, who needs a higher weight restoration to recover from the eating disorder, be medically stable, and achieve positive long-term outcomes.

“Our deep dive into the research shines a light on the need to better individualize care so that patients can lead a fuller life,” says Deinzer. “With this knowledge, providers can shift into more flexibility on what defines a sustainable recovery.” 

The Princeton Center for Eating Disorders team continues to examine existing practices and patient data with the goal of collaborating with other experts in the field to explore new models of care.

“The world has changed, and older formulas no longer fit,” says Dr. Boswell. “Ideal body weight can be part of our toolkit rather than an absolute recommendation of where someone’s body needs to be to achieve recovery. When we look at the big picture, we can do much better.”