03/12/2025
Processed Foods Change the Brain—in Just 5 Days
Ultra-processed foods now make up nearly 60% of the average American's diet—and just 5 days of eating them measurably impairs white matter integrity in brain regions crucial for decision-making and cognitive control, according to a new study.
Ultra-processed foods are linked to hypertension, obesity, cancer—including breast cancer—and increased all-cause mortality.
Research by leading obesity scientists has shown that when people eat highly processed foods, they consume up to 500 more calories per day.
And the effects happen fast. A new study found that just 5 days on a high-calorie ultra-processed diet led to:
A 63% increase in liver fat, despite no change in body weight.
Altered insulin activity in the brain, impairing metabolism.
Diminished reward sensitivity and reduced white matter integrity in regions controlling motivation, cognition, and impulse control.
The shocking part? These changes—resembling those of a brain from someone with obesity—persisted for up to a week after cutting out ultra-processed foods and returning to a regular diet.
Even in a group of healthy, normal-weight men, short-term overeating of calorie-dense but nutrient-poor snack foods profoundly changes brain behavior and metabolism. Imagine what years or decades of a poor diet can do.
Today's email breaks down this study and discusses its implications for your health.
Why ultra-processed foods are so harmful
Ultra-processed foods, also known as UPFs, are "formulations of mostly cheap industrial sources of dietary energy (calories) and nutrients plus additives [created] using a series of processes and containing minimal whole foods." Most of them are less food, more lab experiment.
What makes UPFs so unique isn't just that they're high in sugar, fat, and calories, they also contain a multitude of harmful "ingredients" (chemicals) that could be detrimental to human health, including colorings, sweeteners, binders, preservatives, etc. In the United States, there are more than 10,000 ingredients allowed in food. Some of these—like microplastics and BPA—are added directly and some—including heavy metals—are indirectly added during manufacturing. This includes:
Nitrates/nitrites—preservatives in uncured meats that can form carcinogenic nitrosamines.
Potassium bromate—added to package baked goods and linked to cancer.
Butylated hydroxyanisole (BHA)—a preservative in cured meats linked to cancer.
TBHQ—harms the immune system.
Titanium dioxide—a color additive shown to cause DNA damage.
Brominated vegetable oil—a stabilizer used in flavored drinks that can cause neurological harm.
Per- and polyfluoroalkyl substances (PFAS or forever chemicals)—used in food packaging like soups and are associated with cancer and reproductive system damage.
Artificial colors (red 40, blue 1, etc.)—cause behavioral difficulties in children and exacerbate ADHD symptoms.
Artificial sweeteners (e.g., aspartame)—linked to cancer.
UPFs are engineered to be extremely appetizing and less satiating—this causes people to eat more. A landmark study published in 2019 by obesity researcher Kevin Hall and colleagues found that consuming a diet high in ultra-processed foods caused participants to consume more than 500 calories per day more than when they ate a diet comprising unprocessed, whole foods. These calories came entirely from extra carbohydrate and fat intake, causing the participants to gain nearly 1 kilogram or 2 pounds in just 2 weeks.
The ways in which UPFs cause overeating aren't perfectly understood, but a new study sheds light on a potential mechanism that puts the brain—not the stomach—at the center of it all.
Quick pause! These weekly email deep dives are made possible by the generous contributions of FoundMyFitness members. I warmly invite you to become a Premium Member today. For just $15/month, you’ll gain access to everything we offer while helping us fulfill our mission of making science accessible to all! Learn more at the bottom of this email or by clicking the button below.
Become a Premium Member today!
The study included 29 men aged 19–27 who were assigned to one of two diets for 5 days: a high-calorie (ultra-processed) diet or a normal diet.
The high-calorie diet was, by design, higher in calories than the normal diet. Specifically, the participants consumed 1,500 calories more per day, provided in the form of highly processed, high-sugar, high-fat foods like candy bars, brownies, chips, and other snack foods.
The high-calorie diet didn't lead to weight gain, which isn't surprising given the short-term nature of the study. Body composition, insulin sensitivity, and inflammatory biomarkers like CRP and IL-6 were also unchanged.
However, over the course of just 5 days, liver fat increased by 63% in the participants eating the ultra-processed diet.
Things really get interesting when we look at what happened to the participants' brains.
After 5 days of a high-calorie diet, brain insulin activity increased, specifically in the right insular cortex, left rolandic operculum, and right midbrain/pons. After high-calorie foods were removed and the participants resumed a normal diet for 7 days, insulin activity was lower, specifically the right hippocampus and bilateral fusiform gyrus.
The high-calorie diet didn't just alter brain insulin action—it profoundly rewired how the participants experienced reward, reducing their reward sensitivity and increasing their punishment sensitivity, changes that persisted even after they resumed a normal diet. The high-calorie diet also impaired white matter integrity in brain regions responsible for reward and cognitive processing, which outlasted the timeframe of the high-calorie diet.
Final thoughts
These results are concerning for a few reasons, the first being that altered insulin signaling and activity of the brain's reward pathways happened quickly and in the absence of weight gain. This means they likely represent early warning signs that precede the development of obesity. UPFs fundamentally change the way the brain works before they change the body—wiring us to overeat and experience food and other rewarding things differently.
In fact, insulin signaling in the brain might be a crucial pathway governing metabolic and behavioral changes resulting from UPF consumption even in the absence of changes in peripheral (think muscle or fat tissue) insulin sensitivity. The participants who had an exaggerated brain response to insulin had the greatest increases in liver fat and the greatest decreases in reward sensitivity.
What's shocking is that 5 days of a high-calorie diet made the brains of these healthy, normal weight men structurally and functionally similar to people with obesity. Even after cutting the high-calorie foods out of their diet, these changes persisted for at least one week. I can only imagine the massive rewiring that is going on with years and years of exposure to an ultra-processed diet that, unfortunately, has become the norm in the United States and elsewhere around the world.
From a practical standpoint, this means that if you go through a short period of eating more processed foods than normal—perhaps around the holidays—you might experience changes in food cravings, satiety after meals, and even the ways that you enjoy food. These changes could persist even when you go back to eating healthy, whole foods. No, you haven't ruined your metabolism for good, but it's important to be aware of how what we eat affects us.
That being said, all hope isn't lost. At an individual level, we can take control of our health rather than letting foods take control of us. Eating a minimally processed diet is the best way to ensure your body is getting the nutrients and the information that it needs to regulate satiety, prevent overeating, and keep your mind healthy, mechanisms hijacked by UPFs.
At the population level, there are also widespread changes that we can make to better regulate the production, sale, and marketing of UPFs, especially to children. We can also be diligent about exercise promotion for ourselves and others. I outlined this strategy last month when I addressed the Senate Committee on Aging.
To hear more of my thoughts on UPFs and learn about their hidden dangers, check out these member Q&A episodes that we've hand-picked to accompany this email.
Q&A #50 with Dr. Rhonda Patrick (8/5/2023)
21:39 - How many chemicals does the US allow in our food?
24:50 - Carcinogens in processed meats—what should you avoid?
26:15 - Dangerous chemicals hiding in Pop-Tarts, candy, and drinks
26:51 - Do “forever chemicals” in packaging contaminate your food?
28:14 - Can artificial colors trigger ADHD symptoms?
29:02 - Are artificial sweeteners linked to cancer?
30:02 - How ultra-processed foods drive metabolic dysfunction and weight gain
32:33 - Why "BPA-free" packaging may still disrupt your hormones
34:45 - Hidden health risks of regularly eating processed food
Q&A #65 with Dr. Rhonda Patrick (12/7/2024)
14:12 - What are emulsifiers—and which common foods contain them?
16:53 - How 99% of food additives aren't reviewed for safety by the FDA
17:28 - What animal studies reveal about emulsifiers and gut health
18:52 - Do emulsifiers increase type 2 diabetes risk?
20:30 - Are emulsifiers harmful—or are processed food the real issue?
21:34 - The emulsifier most strongly linked to diabetes (and where it's hiding)
23:03 - Do emulsifiers contribute to cardiovascular disease?
