Why Is My Dog or Cat Losing Muscle? 5 Reasons You Shouldn’t Ignore

Have you ever looked at your pet and thought:

“Wait… were you always this bony?”

Maybe their spine feels easier to find. Their back legs look thinner. Your cat suddenly thinks the couch is way too high.

Or maybe your dog weighs almost exactly the same—but somehow looks less strong.

You could be noticing muscle loss.

And here’s the surprising part:

A pet can lose muscle without losing much weight.

Why Do Dogs and Cats Lose Muscle?

There isn't one single reason. Muscle is constantly being used, repaired, built, and broken down.

Here are five common reasons that balance can change.

1. They’re Getting Older

Just like us, dogs and cats can gradually lose muscle as they age. Age-related loss of muscle mass and function is called sarcopenia.

Research shows that aging dogs and cats can lose lean body mass even while carrying normal—or sometimes excess—body fat. Maintaining lean mass is especially important because muscle plays a major role in mobility and healthy aging. https://pmc.ncbi.nlm.nih.gov/articles/PMC11188961/ 

So the “bony senior pet” look isn't necessarily just old age.

It may be disappearing muscle.

2. They’re Moving Less

Muscles follow a pretty simple rule:

Use them or lose them.

Shorter walks, less running, fewer stairs, less jumping and more time snoozing mean the muscles aren't being challenged as often.

Regular movement and physical activity are important parts of maintaining lean mass as pets age. https://pmc.ncbi.nlm.nih.gov/articles/PMC12520850/

And yes, walking from the bed to the food bowl technically counts as movement.

We're just aiming a little higher.

3. They Aren’t Getting Enough Nutrition

Muscle needs building blocks—especially protein and amino acids.

Poor appetite, reduced food intake, an unbalanced diet, or difficulty digesting and absorbing nutrients can make maintaining lean tissue harder.

This can become particularly important in older pets. Research suggests that adequate dietary protein plays an important role in maintaining lean body mass in aging dogs and cats. https://pmc.ncbi.nlm.nih.gov/articles/PMC9942351/

4. They’re Losing Weight

Weight loss doesn't always mean fat loss.

Some of that lost weight can be muscle.

And the opposite can happen too: a pet can carry extra body fat while still losing lean muscle underneath it—a situation sometimes called sarcopenic obesity.

That's why the number on the scale doesn't tell the whole story.

5. Their Body Is Under Stress

Muscle loss can also happen alongside poor appetite, illness, pain, inflammation, digestive problems, or periods of reduced activity.

Muscle loss related to disease is different from normal age-related sarcopenia and may be described as cachexia. https://pmc.ncbi.nlm.nih.gov/articles/PMC12520854/

That’s why sudden or unexplained muscle loss deserves attention rather than simply being blamed on age.

How Can You Spot Muscle Loss?

Don't just look at the scale.

Gently run your hands along your pet's spine, shoulders, hips, and thighs.

Do they feel bonier than they used to? Are the back legs getting thinner? Is your dog slower getting up? Is your cat suddenly negotiating with themselves before jumping onto the counter?

Take photos from the side and above every few months too.

Gradual changes can be surprisingly difficult to notice when you see your pet every day.

Lean Mass+ Is About More Than Muscle

This is exactly why we created Lean Mass+.

It isn't designed only for a senior pet that's already losing muscle.

Lean Mass+ supports muscle, strength, recovery, gut health, healthy weight, nutrient absorption, and whole-body resilience. 

Muscle & Strength

Lean Mass+ supports lean muscle development and maintenance, making it useful for senior pets, active and working dogs, underweight pets, and pets that need additional body-condition support. 

Recovery & Tissue Health

Muscles aren't the only tissues that need support.

Lean Mass+ is formulated to support recovery and the health of muscles, bones, ligaments, tendons, and cartilage. Its colostrum also provides naturally occurring growth factors that support normal tissue growth, regeneration, and repair. 

That makes recovery an important part of the Lean Mass+ story—not just building muscle.

Gut Barrier & “Leaky Gut” Support

Here's where muscle health gets more interesting.

Your pet has to digest and absorb nutrients before their body can use them.

Lean Mass+ is formulated to support the intestinal lining and healthy gut-barrier function—the area often discussed when people use the term “leaky gut.” Its colostrum supports gut-lining integrity, while probiotics, prebiotics and postbiotics support the microbiome and digestive environment. 

So we're not only thinking about:

“How do we support muscle?”

We're also thinking:

“How do we support the body that has to nourish, maintain and recover that muscle?”

Healthy Weight & Nutrient Absorption

Lean Mass+ also supports gut health, nutrient absorption, healthy weight and body condition.

That's particularly relevant for picky eaters, underweight pets, senior pets, or animals rebuilding their strength after periods of stress, weakness, low appetite, or reduced activity. 

Whole-Body Resilience

The formula brings together Reishi, colostrum, 18 probiotic species, prebiotics, postbiotics, beta-glucans and other targeted nutrients to support digestion, the microbiome, immune function, recovery and everyday resilience.

In other words:

Muscle + Gut + Recovery + Resilience

That's a much bigger picture than simply trying to put weight on a skinny pet.

The Bottom Line

We spend a lot of time watching our pets' weight.

Maybe we should spend more time watching their muscle.

Because healthy aging isn't just about keeping the number on the scale steady.

It's about maintaining the strength to walk, run, climb, jump, play and recover.

Good nutrition matters. Movement matters. Gut health matters. Recovery matters.

And that's where Lean Mass+ fits in: supporting the body from the gut and tissues all the way to the muscles that keep your pet moving.

So next time you're giving your dog or cat a cuddle, feel along their shoulders, spine and back legs.

There may be a lot more hiding underneath all that fluff than the scale can tell you.