Fiber, a nutrient we often think of as mere filler, is essential to our health and wholeness—particularly in the gut and brain. Cate Montana reports
Dr Denis Burkitt worked in Uganda as a surgeon for 20 years. He arrived back in England in 1966, and three years later, he published an article titled “Related Disease—Related Cause?” It was based on his observations that adults aged 40–60 years living in Uganda had a far lower incidence of colon cancer, diverticulitis, appendicitis, hernias, varicose veins, diabetes, atherosclerosis and asthma than adults of a similar age in the UK.
Burkitt attributed the radical health differences to the amount of dietary fiber consumed via natural foods available in Africa versus the highly processed foods regularly consumed in wealthier Western nations. Over the years, he continued to build what became known as “the fiber hypothesis” along with three other physicians of his day, Peter Cleave, G. D. Campbell and Hugh Trowell.
Since he garnered worldwide fame as “The Fiber Man,” Burkitt’s hypothesis regarding the connection between fiber deficiency and disease has been verified and vastly extended. It now includes the increasing risk of liver, colon and breast cancers as well as death from cancer, cardiovascular disease, respiratory diseases, diabetes and infectious diseases.1
Back in the day, it seemed strange that a substance defined as “the carbohydrate parts of plant foods that the body can’t digest or absorb” could be a key to good health. Nonetheless, dietary fiber rapidly began to be hyped in ads for whole-grain products in the 1970s, especially cereals.
Consumers generally viewed said cereals, like Post’s Bran & Prune Flakes and Grape-Nuts (which contain neither grapes nor nuts) as about as palatable as the cardboard boxes they came in. And the main advertised health benefit was equally unpalatable as far as public good taste was concerned, inspiring numerous euphemisms for healthy bowel movements, such as “regularity” or “digestive comfort.”
Dietary fiber itself is composed of a number of carbohydrate types:
Each kind of dietary fiber has a different structure and physiological effects. This is why the current and long-standing classification of dietary fibers as either soluble or insoluble (dissolvable in water or not; see below) is currently being revised.
In the cutting-edge world of fiber, these two simple classifications are being expanded to include traits like water-holding capacity, structural charge, fiber matrix and fermentation rate—all of which have slightly different impacts on human physiology. Even the old universal definition of fiber itself is being debated.
In Japan, for example, fiber is now defined as “the whole components of foods that resist digestion by human digestive enzymes (including those of animal origin as well as plant origin).”2 But the real frontier in the world of dietary fiber is characterized by discoveries regarding its powerful impact on the gut microbiome and the knock-on impact on brain function and cognitive health.
Fiber in food slows the absorption of sugar into the bloodstream so that glucose and insulin levels don’t spike. Insulin helps cells communicate to regulate glucose transport and metabolism, and any glucose dysregulation can cause oxidative stress, inflammation and mitochondrial dysfunction. These conditions ultimately lead to impaired neuronal and cognitive function.3
Maintaining a healthy weight is vital for brain health, and fiber also makes you feel full so you don’t overeat. Best of all, it helps maintain microbial diversity in the gut, aids in lipid and glucose metabolism, and reduces neuroinflammation by modulating short-chain fatty acids and the immune system. Recent studies show that “mixed-fiber supplementation can restore metabolic stability and cognitive function.”4
“When people think about brain health, they often focus on omega-3 fats, antioxidants, exercise, sleep or cognitive training,” says David Perlmutter, MD (drperlmutter.com), neurologist and New York Times best-selling author of Grain Brain (Little, Brown Spark, 2013). “All of these matter.
“But today we understand that dietary fiber may influence one of the most important players in brain health—the microglial cell, which makes up the brain’s innate immune system. These cells constantly survey the brain’s environment, removing debris, clearing damaged proteins, supporting repair and helping maintain healthy neural function.
“When functioning appropriately, microglia are among the brain’s greatest defenders. But when they become chronically activated, they can drive inflammation, neuronal dysfunction and degeneration.”
Studies now show that microglial activation is a central mechanism in the development and possibly the treatment of Alzheimer’s disease.5 Hyperactivation triggers inflammation by making too many pro-inflammatory proteins and small signaling proteins, leading to the neuronal degeneration found in Parkinson’s disease.6
As well, microglia play an important role in the development of amyotrophic lateral sclerosis (ALS)7 and other neurodegenerative conditions, such as frontotemporal dementia, progressive supranuclear palsy, corticobasal degeneration, dementia with Lewy bodies and Huntington’s disease.8
So, what is causing the body’s microglia to dysregulate? Lack of fiber and a concomitant dysregulation of the gut microbiome. A high-fiber diet has been consistently shown to increase both the amount and the diversity of gut bacteria, which supports their production of bioactive short-chain fatty acids.9
Fiber fermentation in the colon produces these fatty acids, including acetate, butyrate and propionate. In turn, they signal the gut-brain axis, influencing metabolism, neurotransmission, immune function and even gene expression.
Dietary fiber helps to correct gut microbiota dysbiosis caused by high bile acids linked to chronic metabolic diseases such as diabetes and hyperlipidemia (high blood fats).10 People diagnosed and treated for depression have been found to consume significantly less fiber than healthy controls.11 And finally, studies of mice show that increasing dietary fiber may prevent or reverse microglial dysregulation.12
“When the microbiome is healthy and well nourished, the gut lining remains strong and selective,” says Perlmutter. “But when fiber intake is low, microbial diversity often declines. This imbalance, known as dysbiosis, can contribute to increased intestinal permeability, sometimes referred to as leaky gut.
“As the gut barrier becomes compromised, inflammatory compounds may gain greater access to the circulation, triggering activation of the immune system. The result is chronic low-grade inflammation that quietly affects virtually every organ system in the body, including the brain.”
Among other things, ongoing low-grade inflammation caused by gut dysbiosis ends up negatively affecting the microglia in the brain. This in turn damages and eliminates synapses, the communication points between neurons. The result is impaired cell communication and a buildup of toxic proteins in the brain, leading to the development of neurological disorders such as ALS, Parkinson’s and Alzheimer’s and even autism.13
The many successes from Dr Perlmutter’s case files help illustrate the connection between mental health, the microbiome and a shift in diet.
Martina. Although she’d been depressed since her 20s, Martina wasn’t prescribed an antidepressant until midlife. She was also taking a non-steroidal anti-inflammatory (NSAID) drug for chronic pain in her extremities following a diagnosis of fibromyalgia.
She’d been born vaginally but not breastfed, which meant she had started life with a deficiency of microbiota and other biologically active components available in breast milk that aid a child’s immune system development. She had taken multiple courses of antibiotics as a child for throat infections, plus the antibiotic tetracycline for 18 months in her teen years because of acne.
Unsurprisingly, gut issues, including constipation and diarrhea, had plagued her all her life. Tests showed she was gluten-sensitive, and her lipopolysaccharide level—a marker of gut permeability and inflammation—was through the roof.
The main focus was restoring gut health. Dr Perlmutter placed Martina on a gluten-free diet and an aggressive oral probiotic program along with prebiotic foods and vitamin D supplementation. He also recommended regular aerobic exercise and more sleep. Within two weeks, completely on her own, she quit her antidepressant medication.
At her six-week follow-up appointment, she was off all medications, and her chronic anxiety had vanished. For the first time in decades, she was having regular bowel movements and told Perlmutter, “I feel like the fog has finally lifted.” As for the chronic fibromyalgia pain, she said she’d forgotten all about it.
Jason. Twelve-year-old Jason was on the autism spectrum. He had been born vaginally, but his mother had taken antibiotics daily throughout her third trimester for persistent bladder infections.
Shortly after Jason’s birth, he also took multiple courses of antibiotics for persistent ear infections. During his first year, he was on antibiotics “more often than not.” He began showing developmental issues at about 14 months of age.
By age 12, he displayed many behaviors characteristic of obsessive-compulsive disorder and chronic anxiety. He lacked social skills and would hardly interact with others.
After hearing Jason’s full history, Perlmutter ordered a stool analysis and discovered his gut had virtually no Lactobacillus species whatsoever, an indication of serious trauma to the microbiome.
He had Jason start an aggressive oral probiotics and vitamin D protocol that showed immediate positive results in diminished anxiety. Jason was also able to tie his own shoes for the first time in his life.
Encouraged by the “gut approach,” his mother inquired about a fecal microbial transplant, the most aggressive therapy available to reset and recolonize a very sick microbiome. It entails transplanting good bacteria from a healthy person’s colon into the colon of the patient.
Perlmutter approved the transplant, which was done at another clinic using a friend’s healthy daughter as a donor.
A month later, he received a video of Jason jumping joyfully up and down on a trampoline while talking engagingly with his mother. A report from his teacher stated that Jason was now “present and very conversational.”
His mother was ecstatic, saying, “He now initiates conversation himself. There is no more handwringing, and he doesn’t talk to himself anymore. He is so calm and interactive.
“The other day, he sat in a chair for 40 minutes talking to me while I was getting my hair done. For the first time ever, he’s singing hymns in church, and we feel very blessed. Thank you for helping to heal my son.”
Back in the 1970s, the Fiber Man himself, Dr Burkitt, recommended no less than 40 g of dietary fiber intake per day for all adults. But today dietary fiber recommendations are all over the map, varying from source to source.
In the UK, government fiber recommendations are set at 30 g per day minimum for adults. The US Department of Agriculture recommends 25 g of fiber intake per day for women aged 31–50 and 31 g for men. Those in their 20s need more fiber. Children and adults over 50 need less.
In developed nations, only one dietary fiber statistic is steady and reliable across the board: Where diets are predominantly high in sugar and processed foods, nobody is getting enough fiber. And the proof is in the ever-increasing number of people in these nations suffering from neurological and cognitive disorders partially if not entirely caused by a deficient microbiome and gut dysbiosis.
In the UK, 96 percent of adults do not meet the dietary fiber recommendations. According to the Dietary Guidelines for Americans 2020–2025, more than 90 percent of women and 97 percent of men in the US do not meet recommended intakes for dietary fiber. In fact, the average daily dietary fiber intake for American adults is only 16 g per day.
“I think most people still look upon fiber as this extra stuff, this mass, this inert material that is part of food that you eat that really has no flavor and no nutritional value whatsoever,” says Perlmutter. “Once upon a time, I thought that way myself.
“Today? Today I think it’s so vital, I wish fiber could be considered the fourth macronutrient along with protein, carbohydrates and fat.”
Of course, as Perlmutter points out, many other factors contribute to gut health issues nowadays. Antibiotics wreak havoc on gut bacteria,14 and steroids such as prednisone also have negative effects.15
Nonsteroidal anti-inflammatory drugs (NSAIDs) destabilize the gut microbiome.16 Medications like proton pump inhibitors (PPIs) take a negative toll.17 Anti-acid drugs and histamine-2 blockers upset the health of the microbiome as well.18
And then there are all the environmental pollutants, like phthalates, heavy metals and Bisphenol A altering the microbiota–gut–brain axis and contributing to neurological and mental health disorders.19
Avoiding medications like these and using as many environmentally friendly products as possible definitely helps. But eating a wide variety of whole foods—organic if possible—that provide sufficient dietary fiber on a daily basis is key to gut and brain health.
Perlmutter illustrates this by talking about a study published in the Journal of Prevention of Alzheimer’s that followed a group of 1,325 people over the span of almost 13 years. Every participant kept a strict journal logging what they ate every day during the study.
“Interestingly, it found that people consuming even one serving of ultra-processed foods per day—one handful of chips or one flavored yogurt or whatever—had a 13 percent increased risk of developing Alzheimer’s,” says Perlmutter. “Having 10 or more servings a day, which is not uncommon, was associated with a 270 percent increased risk of Alzheimer’s disease.”
Which means if you want a healthy gut microbiome and a healthy brain to go with it, the thing to do is to eliminate your exposure to the bad highly processed foods and replace them with good whole or minimally processed foods. And at the very top of the good-things list is dietary fiber in any and all of its forms.
Soluble fiber dissolves in water and helps to reduce cholesterol and blood sugar levels. It’s found in many foods, including apples, bananas, oats, peas and beans of all kinds, brussels sprouts, broccoli, sweet potatoes and avocados.
Insoluble fiber is basically roughage—in other words, the skin and structural components of fruits, vegetables and grains that make these foods crunchy. It attracts water into the intestines, creating softer stools, promoting regular bowel movements, and helping to prevent constipation and hemorrhoids.
The amount of fiber in a food typically doesn’t change much when you cook it, but there are some exceptions. Boiling can cause some fiber to be released into the water, so draining the water means losing fiber—steaming is often better. And a lot of fiber is in fruit and vegetable peels, so you should leave them on when you can.
There’s also the question of measurement. Since veggies break down and lose water content during cooking, the same amount has less volume after cooking. That means cooked produce frequently contains more fiber per cup than raw produce.
Even if the amount of fiber doesn’t change with cooking, the type of fiber can. For instance, when starchy foods like potatoes and rice are cooked and then cooled, their starch turns to resistant starch and stays that way even if it’s reheated.
Resistant starch is a slowly digested fiber, which prevents blood sugar spikes and feeds beneficial gut bacteria, leading to more short-chain fatty acid production that promotes gut health and fights inflammation. So it may be worthwhile to eat your starches left over instead of freshly cooked.
What kind of diet features enough dietary fiber? A diet rich in whole plant foods. Choosing brown rice over white, and whole-grain breads and pastas over highly refined products, is obviously best.
Here’s a list of high-fiber foods to add to your plate this week.
Whole grains (per cup)
Fruits (per cup)
Dried fruits (per ¼ cup)
Vegetables (cooked, per cup)
Beans and legumes (per cup)
Nuts and seeds (per ounce)
To keep the gut and brain healthy, consume these:
Brain Maker Foods
Vegetables: Leafy greens, cabbage, artichoke, alfalfa sprouts, asparagus, broccoli, brussels sprouts, cauliflower, celery, fennel, garlic, ginger, green beans, jicama, leeks, mushrooms, onions, parsley, radishes, scallions, shallots, turnip, water chestnuts, bell peppers, cucumber, eggplant (aubergine), pumpkin and other squash, tomato, zucchini (courgette).
Low-sugar fruits: Avocado, lemon, lime.
Fermented foods: Yogurt and kefir; kimchi and sauerkraut; kombucha; fruits and vegetables pickled in brine (not vinegar); fermented meat, fish and eggs. Be wary of commercial yogurts with sugar and artificial ingredients.
Healthy fats: Extra-virgin olive oil, coconut oil, sesame oil, ghee, grass-fed tallow and organic or pasture-fed butter, cheese (except bleu cheese), avocado, coconut, nuts and nut butter, olives, seeds.
Protein: Wild fish (salmon, cod, mahi mahi, trout, herring, sardines), shellfish and mollusks (clams, crab, lobster, shrimp, mussels, oysters), grass-fed meats and poultry (beef, lamb, liver, bison, chicken, turkey, duck, ostrich), wild game.
Non-gluten grains: Amaranth, buckwheat, rice (brown, wild), millet, quinoa.
In moderation: Cow’s milk and cream, carrots, parsnips, legumes (except chickpeas), coffee, tea.
Different strains of probiotics—live microorganisms that are often referred to as “good bacteria”—provide different health benefits. These are the ones that Dr Perlmutter recommends for fortifying the intestinal lining, regulating the immune system, supporting brain health, and reducing gut permeability and inflammation:
And these are some supplements that Dr Perlmutter recommends for their anti-inflammatory and antioxidant effects, which boost brain health:
Microbiome test
The Cyrex Array 2 Intestinal Antigenic Permeability Screen is a simple blood test that determines gut permeability and detects antibodies associated with leaky gut. It’s available through healthcare providers that work with Cyrex Laboratories (cyrexlabs.com).
Brain biomarker tests
All three of these tests are available as a single panel called BrainScan, offered by Apollo Health online (apollohealthco.com/brainscan). No doctor’s prescription is required.
Mitochondria test
Mescreen (mescreen.com) assesses the efficiency of mitochondrial function and structure.
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