We often think of diet in terms of weight or physical health—but what if what you eat can change your brain function in just five days?

A recent study uncovered something both fascinating and a little alarming: even a short-term diet high in ultra-processed snack foods—think cookies, chips/crisps, sugary treats—can significantly affect how your brain responds to insulin, and these effects may linger long after you return to your regular eating habits.

What they Did

In this study, researchers split healthy young men into two groups:

One group continued eating their usual diet.
The other group added 1,200–1,500 extra daily calories of ultra-processed snacks for just five days.

What the Study Found

Despite no changes in weight, cravings, or body composition, the snack-heavy group showed:

Decreased brain insulin sensitivity
Lower activity in cognitive-related brain regions
Signs of reduced brain integrity, even after resuming their normal diet

The researchers concluded:

“A short-term diet rich in sugar and saturated fat has prolonged effects on the brain that outlast the time-frame of its consumption.”

Why This Matters for Mental Health

Your brain needs glucose for fuel—and insulin is the hormone that helps brain cells access that glucose. When the brain becomes insulin resistant, it’s like locking the fuel out of the engine. This can lead to:

Mental fatigue
Brain fog
Increased inflammation
And possibly even long-term cognitive decline

In other words, your highly processed snacking may be doing more than just messing with your blood sugar—it could be affecting your mood, memory, focus, and overall brain health.

What You Can Do

Limit ultra-processed snacks where possible
Focus on whole, unprocessed foods like fruits, vegetables, legumes, and whole grains
Stay informed - knowldge is power

Want to learn more about ultra-processed foods? Tune into the Ultra-Processed Foods - Part 1: A Beginner's Guide podcast episode, or download my free guide Understanding Ultra-Processed Foods. And if you want to deep dive into what our modern diets are doing to our brain and mental health, check out my book, Unprocessed*.

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