Losing Weight With a GLP-1? Exercise May Be the Difference Between Losing Fat—and Losing Bone

Weight loss is only part of the goal. Protecting the body you’re left with matters too.

GLP-1 medications have changed weight management dramatically. Drugs such as semaglutide and liraglutide—and newer incretin-based medications such as tirzepatide—can produce levels of weight loss that were previously difficult to achieve without bariatric surgery.

For many patients we see in our office, that weight loss can have substantial health benefits.

But there is an important question that deserves more attention:

When you lose a significant amount of weight, what exactly are you losing?

Ideally, we would like most of that weight to come from body fat while preserving skeletal muscle and bone. Research suggests that this does not happen automatically.

Since the explosion of the GLP-1 medications for weight loss, I’ve consistently seen patients developing bone and joint problems that seem to come out of the blue after using these medications for some time.

The good news is the research is now showing us how we can reduce some of the detrimental side effects of these medications.

Weight Loss Can Include Significant Lean-Mass Loss

There’s substantial research in this area that is helping patients make the most of the weight loss benefits of GLP-1 medications, while also preserving vital skeletal muscle and bone that support long term health and vitality.

In the STEP-1 body-composition study, people treated with semaglutide experienced a large reduction in body fat, but total lean body mass also decreased by approximately 9.7%.

More recently, a DXA substudy of the SURMOUNT-1 trial examined body composition in 160 participants. People receiving tirzepatide lost an average of 21.3% of their body weight, while fat mass decreased 33.9% and lean mass decreased 10.9%.

Approximately three-quarters of the weight lost was fat and one-quarter was lean mass.

That doesn’t mean GLP-1 medications uniquely “destroy muscle.” Significant weight loss from caloric restriction commonly includes some loss of lean tissue.

But it does mean that simply watching the number on the scale may not tell the whole story.

Losing 40 pounds while maintaining muscle is physiologically different from losing 40 pounds while losing a substantial amount of muscle along with the fat.

What About Bone?

We know that bones support our bodies structurally, and that preserving bone density prevents debilitating fractures as we age.

However, since our bones are also giant mineral reservoirs, they play a vital role daily role in healthy metabolic function. Maintaining strong bone mineral density is critical for long term health and normal energy metabolism.

This is where a fascinating 2024 randomized clinical trial published in JAMA Network Open becomes especially relevant.

Researchers studied 195 adults with obesity. After an initial eight-week low-calorie diet, participants were assigned to one of four groups:

• Exercise
• Liraglutide
• Liraglutide plus exercise
• Placebo

After the entire study period, the exercise and liraglutide groups achieved fairly similar amounts of weight loss: approximately 11.2 kg with exercise versus 13.7 kg with liraglutide.

But their bones responded very differently.

At the hip:

Exercise: −0.013 g/cm² BMD
Liraglutide: −0.026 g/cm² BMD

That’s approximately twice the BMD loss in the liraglutide group.

At the lumbar spine:

Exercise: −0.003 g/cm² BMD
Liraglutide: −0.020 g/cm² BMD

Numerically, that’s roughly 6.7 times the BMD loss.

The differences between liraglutide and exercise were statistically significant at both the hip and lumbar spine.

These numbers should not be interpreted as evidence that GLP-1 medications directly damage bone. Bone loss was a secondary outcome of the study, and weight loss itself can reduce bone mineral density.

But the next finding provides an important clue about what patients can potentially do about it.

The Group That Lost the Most Weight Protected Its Bones

The combination group—liraglutide plus exercise—lost approximately 16.9 kg, more weight than any other group.

Yet despite losing the most weight, adding exercise to liraglutide preserved site-specific bone mineral density relative to placebo.

Think about the significance of that finding.

The people who lost the most weight weren’t necessarily destined to lose the most bone.

Exercise changed the equation.

The researchers concluded that combining exercise with GLP-1 therapy allowed substantial weight loss while helping preserve bone health.

Why Would Exercise Make Such a Difference?

Bone isn’t simply an inert framework holding the body together.

It is living, metabolically active tissue that continually responds to its mechanical environment.

Every time muscles contract against resistance, every time we walk, climb stairs or lift something heavy, mechanical forces are transmitted through the skeleton.

Bone cells—particularly osteocytes—sense those mechanical forces.

This process is called mechanotransduction: mechanical information is converted into biological signaling.

Those signals help tell bone:

“This structure is being used. Maintain it.”

Now consider what can happen during rapid weight loss.

A person who loses 40 pounds has removed 40 pounds of habitual mechanical loading from the skeleton.

If some of that weight loss is skeletal muscle, another source of mechanical stimulation is potentially reduced: muscle-generated force on bone.

At the same time, dramatically reduced food intake can make adequate protein and nutrient intake more difficult.

The physiology can therefore begin moving in the wrong direction:

Rapid weight loss → reduced mechanical loading → possible muscle loss → less muscular force on bone → increased vulnerability to bone loss.

Exercise—particularly appropriately prescribed resistance and weight-bearing exercise—provides a mechanical signal in the opposite direction.

Muscle May Be Part of the Bone-Protection Story

The same JAMA study provides another intriguing clue.

During the one-year intervention, the exercise group increased lean mass.

The liraglutide-plus-exercise group preserved lean mass despite having the greatest reduction in fat mass.

The researchers specifically noted that lean mass is more strongly associated with bone mass than total body weight.

That makes physiological sense.

Muscle and bone operate as a functional unit. Stronger muscles don’t merely move bones—they place forces through them.

Preserving muscle during weight loss may therefore accomplish two things simultaneously:

maintaining physical strength and metabolic tissue while continuing to mechanically stimulate the skeleton.

If You’re Taking a GLP-1, Think Beyond the Scale

The message from this research isn’t that patients should avoid GLP-1 medications.

It’s that successful weight loss should ideally mean losing excess fat while protecting muscle, bone and physical function.

For someone undergoing substantial GLP-1–assisted weight loss, several issues deserve consideration:

Resistance training. Muscle needs a reason to remain. Progressive resistance exercise provides that stimulus while simultaneously loading bone.

Weight-bearing activity. Walking is valuable, but bone responds particularly strongly to adequate mechanical loading. Depending on age, health and orthopedic status, resistance and appropriate impact-loading exercises may provide a stronger skeletal stimulus.

Adequate protein. When appetite falls dramatically, total protein intake can fall with it. Maintaining adequate protein becomes particularly important during a caloric deficit.

Rate and magnitude of weight loss. Larger and faster reductions in body weight mean larger changes in mechanical loading and nutritional intake.

Bone-health risk. Postmenopausal women, older adults, people with osteopenia or osteoporosis, and individuals with previous fragility fractures deserve particular attention. In appropriate patients, clinicians may consider assessment of vitamin D status, calcium intake and bone mineral density.

Measure more than body weight. Waist circumference, strength, physical performance and, when appropriate, body-composition or bone-density measurements can provide information that a bathroom scale cannot.

The Goal Isn’t Just Weight Loss

GLP-1 medications can be extraordinarily effective at helping people lose weight.

But perhaps we should redefine what constitutes a successful outcome.

The goal isn’t simply:

“How much weight did you lose?”

A better question may be:

“What did you lose—and what did you preserve?”

If 30, 40 or 50 pounds are coming off, we want as much of that reduction as reasonably possible to represent excess adipose tissue—not the muscle that allows you to remain strong or the bone that supports you for the rest of your life.

The encouraging finding from the research is that these outcomes may be modifiable.

In one randomized trial, adding exercise to GLP-1 therapy produced the greatest overall weight loss while simultaneously protecting bone mineral density and preserving lean mass.

That changes the conversation.

For patients using GLP-1 medications, exercise shouldn’t simply be viewed as another way to burn calories.

It may be one of the signals that tells the body what it needs to keep while the weight comes off.

The bottom line is damage control: if you’re successfully using GLP-1 medications to lose weight, adding in exercise as part of your weight loss can be critical for maintaining health in the long term.

References

Jensen SBK, Sørensen V, Sandsdal RM, et al. Bone Health After Exercise Alone, GLP-1 Receptor Agonist Treatment, or Combination Treatment: A Secondary Analysis of a Randomized Clinical Trial. JAMA Network Open. 2024;7(6):e2416775.

Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. New England Journal of Medicine. 2021;384:989-1002. STEP-1 body-composition substudy data were subsequently reported.

Look M, Dunn JP, Kushner RF, et al. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes, Obesity and Metabolism. 2025.

Villareal DT, Fontana L, Weiss EP, et al. Bone Mineral Density Response to Caloric Restriction-Induced Weight Loss or Exercise-Induced Weight Loss: A Randomized Controlled Trial. Archives of Internal Medicine. 2006;166:2502-2510.