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Why Your Brain Hates Junk Food

By Scott C. Anderson


The health of your brain, science is making clear, has a lot to do with the health of your gut, which, of course, has much to do with what you feed it.

02/01/24 - Pschology Today

The health of your brain, science is making clear, has a lot to do with the health of your gut, which, of course, has much to do with what you feed it. That raises a whole new set of concerns about junk food and offers yet more evidence that modern diets are failing us. Many highly processed foods are potentially dangerous, in part because they disregard the fate of the billions of bugs that live in the gut and carry out a surprising array of bodily functions. But it is pure folly to ignore these bacteria, especially since they are so crucial to physical and mental health.
How important? A new study from Tufts University contends that better microbe-oriented diets “could avert approximately 1.6 million hospitalizations and result in an estimated net savings of $13.6 billion in health care costs in the first year alone.”
The microbes in your gut differ from those in mine, and they vary daily. The diversity of gut microbes allows for an astonishing number of genes, outnumbering our own genes by a factor of 100. Courtesy of those genes, good bacteria produce nourishing substances that feed and heal the cells lining your gut. If you don’t support those good bacteria, your gut cells may become hungry and disease-prone. Your gut may become leaky enough to allow bacteria and toxins through.
Once bacteria breach the gut lining, the heart pumps them to every organ in the body, including the brain. The upshot: depression, anxiety, paranoia, psychosis, cognitive difficulties, and dementia.
The term processed as applied to food can be confusing. It defines a food that has undergone any change from its natural state. Many processed foods are perfectly fine. Shelled nuts, for instance, are processed to remove an inedible shell. Grains are processed by milling to turn them into flour. Other foods are so highly processed that it’s difficult to identify the source material. Think cheese puffs, Twinkies, vegan burgers.
One of the first steps in processing foods is removal of fiber. After all, fiber is indigestible and makes products brown. Take the fiber out and you have beautiful white foods that are easy to color any way you wish.
But fiber is an important macronutrient, and it is meant especially for your gut microbes, not you. That single alteration—eliminating fiber—may be the worst thing that has happened to the American diet over the last 70 years. As a result, the microbial composition of our guts is changing, and some species are even becoming extinct.
There’s more: Processed foods often contain emulsifiers, which smooth texture, extend shelf life, and keep ingredients mixed. Some, like carboxymethylcellulose and polysorbate 80, can significantly affect intestinal microbes, killing some species, boosting others, but generally destroying diversity, the single best measure of gut health, and promoting gut inflammation, often a precursor to systemic problems.
There are thousands of nutrients in food that are good for you, but they can be classified into four broad categories: fiber, fat, protein, and carbs. Let’s imagine a food called EquiStuff, made with an equal amount of each macronutrient.
Now take out the fiber to improve taste and texture, and you’re left with one-third fat, one-third protein and one-third carbs. If we then take the fat out of EquiStuff, we’ve got a totality of half protein and half carbs.
We didn’t set out to do this, but the carbs in EquiStuff have gone from 25 percent to 50 percent. By taking out two macronutrients, we doubled the carbs. You don’t even need to add carbs like sugar to make something sweeter, although that is done often enough. Remove the fat and fiber and the job is done for you. Sadly, sugar is not good for a balanced gut microbiome.
Fructose and glucose are the main sugars in our diet, but they are processed differently by the body. Glucose is the primary energy source, beloved by brain cells, ushered directly into cells by insulin to provide energy to muscles and other tissues. Fructose, on the other hand, is processed by the liver.

Because fructose is sweeter, easier to use, and cheaper than cane sugar (thanks to government subsidies), food manufacturers add extra amounts to products in the form of high-fructose corn syrup, the most popular version of which has a fructose level of 55 percent, although it can go as high as 90 percent.
A fructose-rich diet can alter the microbiome, leading to gut problems, metabolic disorders, and brain dysfunction. The liver converts excess fructose to fat. In the process, cellular energy levels are miscalibrated by the liver, triggering hunger. It’s an ominous sign when eating doesn’t sate you but makes you hungrier.
Over time, a fructose overload can lead to obesity and liver disease. A continued surplus of fructose may increase the risk of nonalcoholic fatty liver disease and proceed further to nonalcoholic steatohepatitis, two diseases affecting an increasingly large percentage of the population. They are disproportionately affecting children, who are already in the throes of an obesity epidemic. Almost 40 percent of obese children will go on to develop liver disease.
Some studies have found that extra fructose in the diet—even temporarily—can disturb normal nerve growth in the brain. A week of fructose consumption, far less than needed to affect liver function, is sufficient to reduce plasticity of the brain’s hippocampus, a structure particularly vulnerable to nutritional stressors. It also compromises energy production by the mitochondria and weakens defenses against oxidative stress. The effects on neural function may be setting the stage for neurodegenerative diseases.
The liver’s impact on the brain is profound. Depression, anger, and irritability have long been known as fellow travelers with liver disease. Hippocrates knew 2,500 years ago that liver patients exhibited troubled behavior: “Those whose madness arises from [the liver] shout, play tricks, and will not keep still, but are always up to some mischief.” Hippocrates also said, “All disease begins in the gut.”

Bacteria in your gut are significant producers of neurotransmitters, including dopamine and serotonin, among many bioactive substances. The gut-produced agents, which can communicate with your brain via the vagus nerve, are the same neurotransmitters targeted by psychoactive drugs, such as SSRIs.

Pumped up by excess sugar, pathogenic bacteria in the microbiome can alter the lining of the gut and lead to leaky gut, opening the door to systemic inflammation. Over time, inflammation can undermine cognition and mood.

For the sake of brain health, cutting back on processed food and replacing it with fiber-filled veggies, like onions, broccoli, artichokes, and beans, is a smart strategy. Good fiber and beneficial bacteria themselves can be found in fermented foods like sauerkraut, kimchee, kefir, and yogurt.
If you can’t flip the dietary script, try probiotic or prebiotic fiber supplements for a concentrated dose of the good stuff. This isn’t an all-or-nothing life change. But every step you take toward increasing fiber in your diet is a step toward rejuvenating your gut bacteria and protecting your brain health.

Junk Food, Junk Brain

  • Junk food reduces both neurogenesis and neuroplasticity.
  • Junk food is designed to ignite tastes that encourage you to demand more of what undermines you.
  • A diet loaded with junk food rapidly and lastingly changes brain receptors that respond to food cues, impeding self-control while motivating you to eat more.
  • High levels of sugar and fat in junk food tilt the brain toward inflammation, notably in the hippocampus, seat of memory.