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Healthy Weight

This Everyday Food Choice Could Be Undermining Your Fitness Goals

Author: Ava Durgin

April 15, 2026

Assistant Health Editor

By Ava Durgin

Assistant Health Editor

Ava Durgin is the former Assistant Health Editor at mindbodygreen. She holds a B.A. in Global Health and Psychology from Duke University.

Image by Santi Nuñez / Stocksy

April 15, 2026

Birthday cake protein bars. Low-calorie barbecue chips. Protein soda. Walk through any grocery store today, and you'll find entire aisles dedicated to foods that have mastered the art of sounding healthy. Bold claims splash across every package: "packed with protein," "only 100 calories," "guilt-free." What those labels conveniently leave out is just how ultra-processed these products actually are—synthesized ingredients, artificial flavors, chemical preservatives, and all.

And a lot of us are eating these foods because we're trying to be healthier. We're reaching for the protein bar instead of the candy bar, the “healthy” soda instead of the real one. But new research suggests that the ultra-processed nature of these foods is working against us in a way that calorie counts and macros simply don't capture, showing up not just in our overall health, but in the actual quality of our muscle.

MRI scans, ultra-processed food, & your muscles

Researchers at the University of California, San Francisco, wanted to understand what a diet high in ultra-processed foods actually does to muscle tissue, not in theory, but visibly, on imaging. They analyzed data from 615 adults with an average age of 60.

To measure muscle quality, the team used standard MRI scans of the thigh. They then cross-referenced those scans with detailed dietary data, tracking what participants had eaten over the prior year, specifically what percentage of their diet came from ultra-processed foods. On average, that number was around 41%.

Ultra-processed foods in this context aren't just junk food. The category includes breakfast cereals, packaged bread, flavored chips, frozen meals, soft drinks, energy drinks, hot dogs, and yes, many of the protein-forward "health" products lining store shelves today.

RELATED READ: Confused About What To Eat For Muscle Health? Follow This Guide

Time to rethink your grocery cart

The more ultra-processed food someone consumed, the more intramuscular fat they had stored in their thigh muscles. On MRI, this shows up as fatty degeneration, streaks of fat literally replacing muscle fiber.

What makes this finding particularly striking is what it wasn't explained by. The association held regardless of total calorie intake, total fat intake, physical activity level, BMI, or sociodemographic factors.

In other words, two people could be eating the same number of calories, exercising the same amount, and carrying similar body weight, but the one eating more ultra-processed food would likely have fattier, lower-quality muscle tissue.

It challenges the way most of us think about food and body composition. We’re so used to focusing on calories and protein targets, but this research suggests there’s another layer to the story. The quality of your food seems to matter in its own way, something standard nutrition labels don’t fully capture.

RELATED READ: Working Towards A Healthy Weight? Ditch These “Clean” Foods ASAP

The takeaway

It's easy to get caught up in calories, protein grams, and hitting your macros, but your body is paying attention to more than numbers. It's responding to the actual ingredients.

That doesn't mean one frozen meal is going to wreck your muscle health. But zoom out, and the pattern becomes harder to ignore. What you eat consistently shows up in your body in ways you can’t always see right away.

We tend to think of muscle as something you build in the gym. This research is a reminder that you’re also shaping it in the kitchen.