Why Body Composition Tracking Should Be Non-Negotiable During GLP-1–Induced Weight Loss

If you’re prescribing GLP-1 medications or managing patients who are using them (whether through your clinic or self-directed online programs), there’s one metric you should be tracking that far outweighs BMI in clinical value: body fat percentage (BF%).

The Problem with BMI

Body mass index has long been used as a proxy for health risk. It’s quick, it’s cheap, and it’s universally recognized, but it’s also deeply flawed. A recently published study in the Annals of Family Medicine makes this abundantly clear. Researchers followed a nationally representative sample of U.S. adults for 15 years and found that:

High BF% increases all-cause mortality risk by 78%, even after adjusting for confounders like age, race, and socioeconomic status.
Normal-weight obesity is common and dangerous. Up to 40% of individuals with a normal BMI had excess body fat, and their mortality risk was significantly higher.
BF%, not BMI, was independently associated with cardiovascular mortality. In fact, BMI lost predictive power entirely after adjustments.

In other words, BMI might still be a useful epidemiologic tool – but in the clinic? It’s imprecise.

And using BMI to measure the effectiveness of obesity treatment is incredibly flawed.

The Hidden Risk:

Let’s talk about what happens when someone loses weight rapidly with GLP-1s – especially in the absence of a comprehensive care model.

Weight loss without monitoring body composition is like flying blind. GLP-1s are incredibly effective at reducing appetite and promoting caloric restriction, but unless the body is actively working against it, the body is going to lose muscle alongside fat.

That loss isn’t just an aesthetic issue – it’s metabolic and functional. Muscle mass protects against insulin resistance, frailty, and osteoporosis.

When muscle is lost, and then weight is inevitably regained (as it is in most patients after stopping the medication), the composition of that regained weight is disproportionately fat. The result? A worse body composition than where they started.

Why Midlife Women Are at Highest Risk

In our clinic, we routinely see women in their 40s and 50s with body fat percentages >50%, even in those with BMIs in the normal or preobesity range. Most of these women are chronic dieters – survivors of decades of weight cycling. These are not women “failing” weight loss. These are women whose physiology has adapted – often tragically – to a lifetime of restriction and misinformation.

This group is particularly vulnerable to GLP-1–induced muscle loss for three reasons:

Hormonal shifts in perimenopause make muscle preservation harder.
Dietary intake often becomes overly restrictive once appetite is suppressed.
Exercise may decline due to fatigue, time pressures, or musculoskeletal limitations.

The consequence? Rapid weight loss that looks good on the scale – but leaves them metabolically and functionally worse off.

The goal of obesity treatment is the reduction of adiposity. Thus, monitoring and adjusting therapy based on adiposity is not only logical, it’s clinical due diligence. You wouldn’t treat blood pressure without measuring blood pressure, right?

What We’re Doing Differently

At Heartland Weight Loss, we monitor body composition monthly in all patients undergoing active weight loss. We use an InBody machine – not because it’s perfect, but because it’s accessible, fast, and highly correlated with DEXA (the gold standard). We also recently started measuring grip strength to track not just muscle mass but muscle power and functional reserve.

And we act on the data. If a patient is losing muscle disproportionately, we adjust the treatment plan – adding protein, resistance training, reducing (or even discontinuing) pharmacotherapy.

If you’re prescribing GLP-1s or managing patients using them, this isn’t optional. And in the future, it may be medicolegal protection.

What You Can Do:

Body composition scales from InBody or SECA can integrate easily into your practice flow. They are preprogrammed to print one-page summaries with all the data you need – along with explanations for patients.
Tracking BF% monthly gives a much better picture of progress than watching the scale alone.
Educate your patients: Emphasize protein intake and resistance movement. If you’re not the one doing this counseling, refer them to someone who will. These interventions can often minimize loss of muscle – but not always.

And finally, stay humble and curious. Obesity medicine is evolving fast. What we once thought was “better than nothing” may turn out to be harmful. Now that we know better, we need to do better.

If you’d like to read the full study mentioned here, click here to access the article.