After more than 25 years in oncology rehabilitation, clinical education, and survivorship care, I’ve learned to be cautious about “good” numbers from common tools like BMI and bathroom scales that don’t match what is actually happening in the body. A recent study confirms it: When used alone, BMI has been found to misclassify up to 34 percent of the general population as obese.

This year, it became more personal when I considered taking a GLP-1 after major surgery. As a clinician, I’ve measured body composition for patients taking GLP-1s, and their results were, quite frankly, staggeringly bad. Tracking my own numbers only reinforced what I tell patients and what emerging data now confirms: BMI can appear reassuring or alarming while masking a very different internal reality. That is why we need to look beyond BMI and measure full body composition.

BMI and the scale miss the individual context that matters

BMI and scale weight do have their uses. Even the Centers for Disease Control rely on them to track obesity trends at the population level because they are inexpensive, easy to access, and simple to collect across large groups. The problem is that, somewhere along the way, these tools, and the expectations attached to them, started to be used to assess the health of individuals.

Two people can have the same body weight yet very different BMIs, and even the same BMI can reflect entirely different bodies. A classic example is the athlete who would be labeled overweight or obese by BMI despite having low body fat and high muscle mass.

The more concerning scenario is the patient whose internal physiology is shifting in ways that demand critical attention but goes unnoticed because their weight appears stable and BMI falls within the “normal” range. What BMI and classic weight measurements can’t detect is a loss of muscle mass and intracellular water with a relative rise in extracellular fluid. While the number on the scale may not change in a way that raises alarms, the body is losing its structural and metabolic foundation. Essentially, it is deteriorating right under BMI’s nose.

Healthy weight is built on three internal components

For decades, the public narrative around healthy weight has focused almost entirely on exercising more, eating less, managing stress, and getting better sleep. However, over the past several years, we’ve learned that achieving and maintaining a healthy weight is far more individualized than that. Importantly, it is also dependent upon the preservation of three core components of the body that support resilience and long-term health:

• Muscle mass: Muscle isn’t just about strength or appearance; it plays a central role in metabolism, glucose management, stability, and everyday function.
• Intracellular water: Because muscle tissue is largely composed of water, changes in muscle mass are closely tied to shifts in intracellular water. It reflects overall cellular health and metabolic activity.
• Balanced extracellular water: Elevated extracellular water levels can mask muscle losses and indicate physiological stress.

A recent study found that, among older adults, higher levels of intracellular water are associated with greater strength, improved mobility, and lower frailty. These factors not only impact healthy weight but a healthy life. Looking at these three components together provides a far clearer picture of healthy weight and long-term health than a scale and BMI never could.

What rapid GLP-1-driven weight loss does to the body

GLP-1 medications have brought renewed attention to weight loss because they can drive rapid changes in body weight. A recent study found that people with obesity can lose 15 and 25 percent of their body weight within the first year of treatment. However, GLP-1s also carry risks and side effects that can disrupt body composition in ways that may lead to long-term damage if not properly monitored and managed.

For example, GLP-1s work primarily by suppressing appetite. In many cases, they are so effective that patients reduce their caloric intake excessively and struggle to consume adequate protein and fluids without intentional interventions such as food journaling. A 2025 study found that up to 38 percent of patients taking GLP-1s showed signs of malnutrition.

Over time, this degree of sustained caloric restriction contributes to muscle loss and shifts in fluid balance often before the scale or BMI indicate a problem. While the number on the scale is dropping and clothes may fit more loosely, underlying health can be quietly deteriorating.

Critically, these internal changes can begin quickly, and once the cascade starts, it can be difficult to reverse. Early on in rapid weight loss, intracellular water and muscle mass often decline while extracellular water increases, even as total body weight falls. Without insight into what is happening inside the body, these warning signs are easy to miss, and significant muscle loss can be mistaken for a sign of progress. To make matters worse, as muscle mass decreases, metabolic rate slows, making further weight loss harder, causing patients to take extreme measures to reach their goals.

Why weight management must look inside the body

A more precise approach to measuring healthy weight and weight loss combines body composition management with clinical context. Functional assessments such as balance, walking capacity, and grip strength offer valuable insight into muscle mass and physical resilience. Noninvasive bioimpedance spectroscopy (BIS) assessments can further quantify fluid distribution and help clinicians track individual trends in muscle and fat mass over time.

This level of assessment does more than generate numbers; it provides clinicians with actionable insight, enabling earlier intervention when problems emerge, more informed adjustments to nutrition or activity, and support for weight loss strategies that protect the body’s foundation rather than erode it.

When clinicians look beyond weight alone and monitor these internal components, weight management becomes safer, more precise, and more meaningful for long-term health. Healthy weight isn’t just about losing pounds. It is about maintaining muscle, preserving intracellular water, and keeping extracellular fluid in balance so the body can function well long after the scale changes.

Maureen McBeth is a physical therapist and certified lymphedema specialist.