How Processed Foods Sabotage Gains (and Your Knees!)

You’ve been hitting the gym, crushing leg day, and dialing in your macros (mostly). But if your diet’s packed with frozen pizzas, sugary cereals, and candy bars, you’re low-key sabotaging your muscle gains—and not just in the "I’m not shredded" way. A new study from the University of California, San Francisco, just dropped some bombshell science about how a diet high in ultra-processed foods (UPFs) mess with your muscle tissue.

Keep two things in mind as we continue:

1. It's not hopeless. You can eat some junk food. You can have a beer, a pizza, or desert. But if the majority of your food is artificial/highly processed....

2: ...the damage is way worse than you think. And we’ve got the images of the actual muscles (not too graphic, don’t worry) to prove it. Just knowing how bad the consequences can get does wonders for will power. So trust us, it's worth taking a peek at what happens if things go too far.

Rethink that “dirty bulk” – any muscle you might gain is flat out not the same, and we're just now beginning to understand how it can radically screw up your joints and knees.

Here’s the deal.

The Study That Blew Our Minds

This isn’t just some fluff science theory – this time they've got the pictures to prove it.

Radiologist Zehra Akkaya and her team studied over 600 healthy adults to figure out how lifestyle affects knee and joint health. They weren’t expecting to uncover a serious link between UPFs and your muscle tissue—but that’s exactly what they found. A high amount of processed foods in your diet significantly lowers the quality of muscle itself, changing it from the inside out.

This isn’t a conclusion reached from seeing some weird bio-marker in a blood test, or from a questionnaire asking people to estimate their own muscle quality or how strong they “feel”. The study was done by a radiologist, who does MRI imaging for a living.

The effects of ultra-processed foods can be seen in the cross-sectional images the MRI produces, allowing us to literally see what the inside of the muscles look like. It’s as if we could cut the legs right in half and see exactly what’s going on in those quads and hamstrings.     

So what did the study show? Those who ate more ultra-processed foods—think 65%+ of their calories from stuff like soda, pastries, and frozen dinners—had way more fat infiltrating their large muscles compared to those who kept UPFs to 35% or less of their diet. Imagine gears turning, doing their thing. Now imagine throwing a bunch of rubber hoses into those gears, interfering with how they mesh up. Not great for optimal performance.

Let's break that down: the largest muscles in their bodies are literally turning into fat tissue. This wasn’t just about being overweight or undertrained. Even when factoring in exercise levels, BMI, and calorie intake, the results held up. More UPFs = weaker, fattier muscles. And not fat on the muscles or around; we’re talking fat throughout and interrupting the muscle from the inside.

In Image A (above left), we see an actual MRI cross section of the upper legs of an individual eating a large percentage of processed foods, versus Image B (right), someone who kept processed foods to about a third of their diet.

To better understand what you’re looking at, imagine that old magic trick where they saw a person in half. But this time it’s done between the knee and the waist; And unlike that shady magician, the MRI lets you come right up on stage and look down inside.

The white exterior is skin and fat, the dark meat is muscle, and the neat light circle is the bone. Around the bone are the quads and hamstrings.

See how much more fat has wormed its way into the muscles in Image A, replacing and interfering with muscle? The huge increase in infiltrating fat (white) is especially clear when comparing the lower area of each image. Take another look below:

Why Does This Matter?

If you’re under 60 and thinking, “This doesn’t apply to me,” hold up. Fatty infiltration of muscles isn’t just an old guy problem. It’s the ultimate gains killer. Strong, lean muscles are crucial not just for aesthetics but for performance, injury prevention, and keeping your joints healthy—especially your knees.

Spoiler alert: muscles riddled with internal fat that shouldn’t be in there don’t work as well. They can’t contract as completely or with as much force. They are inefficient at generating power. They’re weak. They're less stable.

Worse, the study showed that weaker muscles accelerate wear and tear on your joints, particularly your knees. So, if you want to keep squatting, deadlifting, or even running without pain, you need to protect your muscle quality. That’s not happening if your diet looks like a vending machine exploded.

What Are Ultra-Processed Foods, Anyway?

Let’s get on the same page about what qualifies as ultra-processed foods. These are the heavily manufactured foods packed with additives, preservatives, and other junk. Examples include:

• Sugary breakfast cereals
• Soft drinks
• Packaged snacks (chips, candy, etc.)
• Frozen meals like pizza or mac ‘n’ cheese
• Instant noodles

These foods are engineered to taste amazing but offer almost zero nutritional value. Sure, they’re convenient, but they’re also the silent killer of your gains.

The Science Behind the Damage

So, how do UPFs wreck your muscles? Here’s the breakdown:

1. Chronic Inflammation: UPFs are loaded with refined carbs, unhealthy fats, and additives that cause chronic inflammation. This slows down muscle recovery and repair.
2. Nutrient Deficiency: Ultra-processed foods lack essential nutrients like protein, vitamins, and minerals that your muscles need to grow and function properly.
3. Fat Infiltration: Over time, the body replaces muscle tissue with fat cells in a process called intramuscular adipose tissue infiltration. This isn’t just cosmetic; it’s functional. Fatty muscles don’t perform well—period.
4. Hormonal Disruption: The additives in UPFs can mess with your hormones, including testosterone, which plays a massive role in building muscle and burning fat.

How to Clean Up Your Diet

Nobody’s saying you can never enjoy a slice of pizza again. But if you’re eating like a college freshman, it’s time for a reality check. Here’s how to clean up your diet without losing your mind:

• Focus on Whole Foods: Think lean proteins (chicken, fish, eggs), complex carbs (quinoa, sweet potatoes, oats), and healthy fats (avocado, nuts, olive oil). Need more ideas? Got you covered.
• Meal Prep Like a Boss: Take the time to cook a few big batches of food every week. It’ll save you from reaching for those frozen meals.
• Shop the Perimeter: Stick to the outer aisles of the grocery store where fresh produce, meat, and dairy live. Avoid the middle aisles packed with packaged junk.
• Hydrate Smart: Swap sugary sodas for water, green tea, or black coffee. Your muscles will thank you.
• 80/20 Rule: You don’t have to be perfect. Aim for 80% whole, nutrient-dense foods and 20% indulgences.

Final Thoughts

Listen, you can’t out-train a bad diet. If you want strong, functional muscles that don’t break down on you, it’s time to take your nutrition seriously. The science is clear: ultra-processed foods are a one-way ticket to weaker muscles, achy knees, and slower gains. Cut the junk, fuel your body right, and watch your performance—and your health—go next level.

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