How Illness Affects Muscle Mass:

From Short-Term Sickness to ICU Stay

It’s easy to overlook how quickly muscle mass can decline during an illness. You may think, “It’s only a few days—how much can really change?” But even in a short time, your muscles can begin to lose strength and size due to inactivity, reduced food intake, and the body’s stress response. In more extreme cases, like a stay in the ICU, muscle loss can be profound and have lasting effects on recovery.

My Personal Experience

This topic hit close to home for me recently. The day started like any other. I was working at the hospital in my role as an acute care physical therapist—my second job on top of my full-time work as an orthopedic and sports therapist. Everything felt normal until about 2 p.m., when I started feeling nauseous.

At first, I brushed it off, thinking it might just be something I ate. But as the day went on, things escalated quickly. By the end of my shift, I was dealing with some very unwelcome vomiting (sorry for the visual). Thankfully, my shift was wrapping up, and I made it home safely.

Once I was home, everything went downhill. The vomiting didn’t stop, and a fever of 102°F hit me hard. My body ached, I was shivering uncontrollably, and I felt absolutely miserable. But what really stood out to me was how weak I felt—like my strength was draining away. As someone who’s pretty in tune with my body, I couldn’t shake the feeling that I was losing muscle. Could that even happen in just a day or two?

Curious (and maybe a little concerned), I decided to see if what I was feeling had any truth behind it.

The Science of Muscle Loss During Illness

When you’re sick, your body shifts into survival mode to combat the illness. This survival response directly affects your muscles in a few key ways.

1. Inactivity

When you’re too tired, nauseous, or achy to move, your muscles don’t get the stimulation they need to maintain their size and strength. Without movement, the balance between muscle protein breakdown and synthesis shifts, resulting in muscle loss (Dirks et al., 2016).

2. Poor Nutrition

Illness often suppresses appetite, leading to reduced food intake. Whether you’re not hungry or can’t keep food down, your muscles are missing the protein and calories they need to rebuild and maintain themselves. When your body runs low on energy, it may start breaking down muscle for fuel—a process known as catabolism (Biolo et al., 1997).

3. Inflammation

When your immune system kicks into high gear to fight off illness, the inflammatory response can accelerate muscle protein breakdown. It’s like your body is borrowing resources from your muscles to eliminate the pathogen (Puthucheary et al., 2013).

Even with short illnesses like the flu or a stomach bug, this combination can leave you feeling noticeably weaker. In more severe cases, like ICU stays, the impact can be far greater.

ICU-Acquired Muscle Loss

Take the example of a patient in the ICU. Studies show that critically ill patients can lose up to 2% of their muscle mass per day due to immobility, inflammation, and inadequate nutrition (Puthucheary et al., 2013). Over a week, this can result in a staggering 20% muscle loss, making recovery a long and difficult process.

This is why early mobility in the hospital is so important. Acute care physical therapists play a key role in slowing the progression of muscle and strength loss, which can otherwise lead to significant debility.

So, What Can You Do About It?

While most of us won’t face ICU-level muscle loss, even a short illness can leave you feeling weak and frustrated. Here are three ways to protect your muscles and rebuild after you’ve recovered:

1. Stay as Active as You Can

Even light movement can help. Gentle stretching, short walks, or simply changing positions frequently can keep your muscles engaged and reduce the effects of inactivity.

2. Prioritize Protein

If your appetite is low, focus on small, protein-rich meals or shakes. These provide your muscles with the building blocks they need to repair and rebuild.

3. Rebuild with Resistance Training

Once you’re feeling better, ease back into exercise with a focus on resistance training. Start light and gradually increase intensity to regain your strength.

Key Takeaway

Whether it’s a stomach bug or a stay in the ICU, illness can take a toll on your muscles faster than you think. The good news? You can take steps to protect your body and bounce back stronger.

My recent experience was a tough reminder of how illness impacts not just how we feel but how we function. It’s another reason to approach recovery with intention—whether that means getting enough rest, prioritizing movement, or giving your body the nutrition it needs to heal.

Have you ever felt like you lost strength after being sick? Drop a comment below—I’d love to hear your thoughts or help with recovery strategies.

References

Biolo, G., Fleming, R. Y., Maggi, S. P., & Wolfe, R. R. (1997). Transmembrane transport and intracellular kinetics of amino acids in human skeletal muscle. American Journal of Physiology-Endocrinology and Metabolism, 273(5), E839-E849.

Dirks, M. L., Wall, B. T., Nilwik, R., Weerts, D. H., Verdijk, L. B., & van Loon, L. J. (2016). Skeletal muscle disuse atrophy is not attenuated by dietary protein supplementation in healthy older men. The Journal of Nutrition, 146(6), 1178-1186.

Phillips, S. M., & Winett, R. A. (2010). Uncomplicated resistance training and health-related outcomes: evidence for a public health mandate. Current Sports Medicine Reports, 9(4), 208-213.

Puthucheary, Z. A., Rawal, J., McPhail, M., et al. (2013). Acute skeletal muscle wasting in critical illness. JAMA, 310(15), 1591-1600.