28 Jul To Your Health: Reference Guide to Missing Nutrient in America: Fiber, Microbiome & More!
Welcome back to the series which offers a deep dive into the world of health, wellness, and disease prevention with Telluride local Dr. Alan Safdi.
Dr. Alan is a board-certified physician in Internal Medicine and Gastroenterology, a Fellow of the American College of Gastroenterology, and a respected leader in healthcare. His blogs have featured and will continue to showcase the most current information in his fields: health, wellness and longevity.
In this installment of “To Your Heath,” Dr. Alan does a deep dive into “the missing nutrient” in our diets and why it matters. The full title of the following story includes “How Fiber, Exercise, Diet, and Our Environment Shape the Microbiome—and Possibly Our Future Health.”
Scroll down to read the salient details. Consider this post a guide to how to maintain the health of the microbiome and much more.
And click here for more health and wellness podcasts from Dr. Alan.
The Missing Nutrient in America: How Fiber, Exercise, Diet, and Our Environment Shape the Microbiome—and Possibly Our Future Health
For most of modern medical history, dietary fiber was treated primarily as a treatment for constipation. Patients were told to eat bran, drink more water, and take a fiber supplement if they were having difficulty moving their bowels.
We now know this view was far too narrow.
Fiber is not merely roughage that helps food move through the intestine. It is the principal food source for many of the trillions of microorganisms living within the colon. Those microorganisms and the substances they manufacture may influence digestive health, immune regulation, cardiovascular disease, metabolism, inflammation, brain health, and perhaps even how successfully we age.
Fiber could well be one of the most important and most neglected nutrients in the American diet.
The microbiome: an overlooked organ system
The human microbiome consists of the enormous community of bacteria, viruses, fungi, and other tiny organisms that live in and on the body. The greatest concentration is found in the gastrointestinal tract, particularly in the colon.
These organisms are not simply passengers. They help break down components of food that our own digestive enzymes cannot process.
They manufacture biologically active compounds, interact with the immune system, help maintain the intestinal barrier, influence bile-acid metabolism, and communicate with distant organs.
The gastrointestinal tract has sometimes been called the “second brain” because it contains an extensive enteric nervous system and communicates continuously with the central nervous system. I would extend that concept to include the microbiome. The microbiome is not literally another brain, but its metabolic and signaling capacity is so extensive that it may influence nearly every major system in the body.
It may affect the health of our actual brain, our heart and blood vessels, our metabolism, and our immune system. Alterations in the microbiome are being studied in relation to inflammatory bowel disease, rheumatoid arthritis, multiple sclerosis, allergies, diabetes, obesity, cardiovascular disease, Parkinson’s disease, cognitive decline, depression, and numerous other conditions.
These relationships do not prove that an altered microbiome causes each disease. In some cases, the disease itself, its medications, or the associated dietary changes may alter the microbiome. Nevertheless, the accumulating evidence suggests that the microbiome is an important participant in human health rather than an incidental observer.
Medicine largely overlooked this ecosystem for decades. We became exceptionally skilled at identifying and killing pathogenic organisms, but paid far less attention to the beneficial communities that had coevolved with humans. Even today, the microbiome remains underemphasized in routine medical education and preventive care.
America’s fiber deficiency
The average American consumes only about 15–17 grams of fiber per day. Many adults consume even less. General recommendations are approximately 25 grams daily for women and 30–38 grams for men, with some variation by age and energy intake.
That means a large proportion of Americans fail to reach even the minimum recommended intake.
That deficiency is not difficult to understand. The modern American diet is dominated by refined grains, fast food, animal products, snack foods, sweetened beverages, and ultra-processed products. Processing often removes the naturally occurring fiber found in plants. Meat, poultry, fish, eggs, and dairy products contain essentially no dietary fiber.
Fiber is found almost exclusively in plant foods:
• beans and lentils
• peas and other legumes
• fruits and vegetables
• intact whole grains
• nuts and seeds
• herbs and spices
A person may consume adequate calories, protein, vitamins, and minerals while still starving many beneficial organisms in the colon.
Why bacteria need fiber
Human digestive enzymes cannot completely break down many dietary fibers. Those fibers travel through the small intestine and reach the colon, where bacteria ferment them.
This fermentation produces short-chain fatty acids, principally:
‘• butyrate
• acetate
• propionate
These compounds are not simply digestive waste products. Butyrate is an important energy source for the cells lining the colon. Short-chain fatty acids also help maintain the intestinal barrier, regulate immune activity, influence glucose and lipid metabolism, and may reduce certain inflammatory signals.
They may also participate in communication between the intestine and the brain through immune, metabolic, hormonal, and neural pathways.
That is why we often explain that fiber is not only feeding the individual eating it. Fiber is feeding an entire microbial ecosystem, and that ecosystem then produces substances that may affect the rest of the body.
Different organisms prefer different types of fiber. Therefore, variety may matter as much as quantity. Eating 30 or more different plant foods over the course of a week has become a useful practical goal. That number is not a rigid medical threshold, but it encourages the diversity of fruits, vegetables, beans, grains, nuts, seeds, herbs, and spices likely to provide a broader range of fermentable substrates and polyphenols.
Soluble and insoluble fiber
Fiber is often divided into soluble and insoluble categories, although many foods contain a mixture.
Soluble fiber dissolves or forms a gel in water and is often readily fermented by gut bacteria. Important sources include:
• oats and barley
• beans, lentils, and peas
• chia and flax seeds
• apples and citrus fruit
• berries
• psyllium
Soluble fiber can help lower LDL cholesterol, slow carbohydrate absorption, moderate post-meal glucose elevations, and feed organisms that produce short-chain fatty acids.
Insoluble fiber adds bulk to stool and can help promote regular bowel movements. It is found in foods such as wheat bran, many vegetables, nuts, seeds, and the skins of fruits.
Both types are valuable. It would be a mistake to conclude from one study that insoluble fiber is unimportant simply because soluble fiber showed a stronger statistical association with a particular outcome.
Fiber and dementia: an important new study
A 2026 analysis from the Rush Memory and Aging Project brought renewed attention to the possible relationship between fiber intake and cognitive health.
The study followed 1,508 older adults who did not have dementia at baseline. Their average age was approximately 80. They were followed for an average of 7.6 years. During that period, 430 participants developed dementia.
Participants in the highest category of total fiber intake which was only approximately 27 grams per day, had a reported 46% lower risk of developing dementia than those consuming the least fiber. The association was especially notable for soluble fiber, with the highest intake associated with a reported 44% lower risk.
Even participants consuming approximately 19–22 grams daily appeared to have a lower risk than those with the lowest intake.
These findings are exciting, but they must be interpreted correctly. This was an observational study, not a randomized clinical trial. Dietary intake was self-reported, and people who consume more fiber may differ from low-fiber consumers in many other ways.
They may exercise more, smoke less, have better access to healthcare, eat fewer ultra-processed foods, and consume more vitamins, minerals, and polyphenols.
Researchers attempted to adjust statistically for many of these factors, but no observational study can eliminate every potential confounder.
Therefore, we cannot say that eating 27 grams of fiber will reduce an individual’s dementia risk by exactly 46%. What we can say is that this prospective study adds to a growing body of evidence associating plant-forward, fiber-rich dietary patterns with healthier cognitive aging.
There are several biologically plausible explanations. Fermentation of soluble fiber produces short-chain fatty acids that may influence inflammatory and immune signaling. Higher-fiber diets can also improve insulin sensitivity, blood pressure, lipid profiles, body weight, and vascular health, all of which can affect the brain.
In other words, a diet that protects the microbiome may also protect the blood vessels and metabolic systems upon which the brain depends.
A possible microbiome signal before dementia develops
Another recent study examined blood and stool samples from 150 adults aged 50 and older, including cognitively healthy adults and people with subjective or mild cognitive impairment.
Researchers identified gut bacteria and molecules related to diet and microbial metabolism. Using combinations of six metabolites and a machine-learning model, they reported more than 80% accuracy in separating cognitively healthy adults from those with mild cognitive impairment.
That does not mean that a clinically validated microbiome blood test for dementia is now available. The study was small, and predictive algorithms can appear highly accurate in the population in which they were developed, but perform less well when tested in larger, independent groups.
Nevertheless, the study is conceptually important. It suggests that alterations in microbial metabolism might be detectable in the bloodstream during relatively early stages of cognitive change.
Future research may determine whether these metabolites are simply markers of disease or active participants in its development.
Either way, they could eventually contribute to earlier detection, risk assessment, or personalized dietary interventions.
Fiber and cardiovascular health
The cardiovascular benefits of fiber are supported by several complementary mechanisms.
Lower LDL cholesterol
Viscous soluble fibers can bind bile acids in the intestine and increase their excretion. The liver must then use cholesterol to manufacture replacement bile acids, which can contribute to lower circulating LDL cholesterol.
Improved glucose control
Fiber slows digestion and carbohydrate absorption, reducing rapid post-meal glucose elevations. Better glucose regulation may lower the risk of insulin resistance and type 2 diabetes, both major cardiovascular risk factors.
Greater satiety and weight regulation
High-fiber foods are generally more filling and less calorie-dense than highly processed foods. This may help with long-term weight control without requiring extreme caloric restriction.
Reduced inflammation
Short-chain fatty acids produced by bacterial fermentation may help maintain the gut barrier and regulate inflammatory signaling. A healthier barrier may reduce the passage of certain bacterial products into the circulation.
Effects on microbial metabolites
Gut organisms participate in the production of compounds such as trimethylamine N-oxide, or TMAO, which has been associated with cardiovascular risk. The relationship is complicated and remains under investigation, but dietary pattern and microbiome composition may influence how these compounds are produced.
The cardiovascular benefit of a high-fiber diet does not depend on one single pathway. Fiber is part of a larger dietary pattern that improves cholesterol, blood pressure, glucose metabolism, inflammation, body weight, and microbial function simultaneously.
Fiber and digestive health
Fiber remains essential for digestive health, even though its role extends far beyond constipation.
Adequate fiber can:
• increase stool bulk
• improve stool consistency
• promote regularity
• reduce straining
• support microbial diversity
• provide fuel for colon cells
• help reduce the risk of diverticular disease
• contribute to a dietary pattern associated with lower colorectal cancer risk
Fiber is not a universal treatment for every gastrointestinal symptom. Some people with severe bloating, active inflammatory disease, strictures, gastroparesis, or particular forms of irritable bowel syndrome may require individualized advice.
For most people, however, increasing fiber gradually and consuming adequate fluids is a safe and practical strategy.
My own fiber intake
I have followed a vegetarian, predominantly plant-based diet for approximately 40 years. On most days, I consume approximately 80–110 grams of dietary fiber, and occasionally more.
That is much higher than standard recommendations and should not necessarily be viewed as an appropriate immediate target for everyone. My gastrointestinal tract and microbiome have adapted to this intake over many years.
A typical day might include:
• oatmeal with chia seeds, flax seeds, cinnamon, and blueberries
• lentil and spinach soup, sometimes with farro
• peanuts, pistachios, or other nuts
• tofu or other soy foods
• whole-wheat or rye bread
• edamame
• lentil pasta with tomato sauce and vegetables
• a large mixed-green salad
• steamed broccoli and other vegetables with a variety of mushrooms
• beans, lentils, or other legumes
• a bowl of mixed fruit
• 2 pieces of dark chocolate
Depending on portion size, a day containing this combination can exceed 100 grams of fiber.
The important lesson is not that everyone must eat precisely as I do or consume 100 grams of fiber. The lesson is that it is possible to obtain very large amounts of fiber from satisfying, nutrient-dense foods without relying on processed fiber bars or multiple supplements.
Anyone currently eating 10–15 grams daily should not suddenly jump to 70 grams. That can produce substantial gas, bloating, discomfort, or diarrhea. Fiber should generally be increased over several weeks, allowing the microbiome and gastrointestinal tract time to adapt.
A realistic initial goal is to add one fiber-rich food at each meal: oatmeal or berries at breakfast, beans or lentils at lunch, and vegetables, intact grains, or legumes at dinner.
Overall dietary pattern matters
Fiber should not be viewed as a magic ingredient that can simply be added to an unhealthy diet.
A highly processed cereal or snack bar may contain added isolated fiber while also containing refined starch, sugar, sodium, and numerous additives. That is not metabolically equivalent to eating beans, oats, berries, nuts, seeds, and vegetables.
Whole plant foods provide packages of nutrients that include:
• multiple types of fiber
• vitamins and minerals
• polyphenols
• antioxidants
• unsaturated fats
• slowly digested carbohydrates
• thousands of phyto-chemicals that may interact with the microbiome
Healthy dietary patterns such as Mediterranean, MIND, and appropriately planned plant-based diets appear beneficial because of the total combination—not because of a single nutrient.
Foods particularly helpful in feeding the microbiome include:
• beans, lentils, chickpeas, and peas
• oats, barley, farro, and other intact whole grains
• onions, garlic, leeks, and asparagus
• leafy green and cruciferous vegetables
• apples, citrus fruit, and berries
• chia, flax, and other seeds
• walnuts, pistachios, almonds, and peanuts
• herbs, spices, tea, coffee, cocoa, and extra-virgin olive oil
• fermented foods, when they are not excessively high in salt or added sugar
Rotating these foods is likely more valuable than repeatedly eating one food marketed as a “superfood.”
Exercise also affects the microbiome
Diet is not the only lifestyle factor that influences microbial communities.
Studies comparing active and inactive individuals frequently find differences in microbial diversity and in organisms involved in short-chain fatty-acid metabolism. Exercise may improve intestinal transit, insulin sensitivity, immune regulation, and systemic inflammation, all of which can influence the intestinal environment.
However, exercise studies are difficult to interpret because physically active people often eat differently, have less body fat, take fewer medications, and follow other healthy behaviors.
Intervention studies suggest that exercise itself can modify the microbiome, but responses vary and some changes diminish when training stops. The type, intensity, duration, and consistency of exercise may all matter.
I exercise every day, combining aerobic activity and resistance training. I do not believe exercise substitutes for a high-quality diet, nor does a good diet eliminate the need for exercise. The strongest health strategy combines both.
Exercise may support the microbiome, and the microbiome may in turn influence metabolism, inflammation, and possibly exercise recovery. This is an emerging two-way relationship rather than a settled prescription for specific bacterial changes.
Sweeteners and the microbiome
Commercial sweeteners are now present in a substantial proportion of processed foods. Those include artificial sweeteners such as aspartame, saccharin, and sucralose; stevia-derived compounds and monk-fruit sweeteners; and sugar alcohols such as erythritol, xylitol, sorbitol, and maltitol.
A recent laboratory study exposed 25 gut bacterial species to 39 commercially used sweeteners. Approximately three-quarters of the tested compounds altered the growth of at least one bacterial species.
The researchers also examined combinations of sweeteners with compounds such as caffeine and vanillin and with several medications. They identified more than 100 interactions. Particularly notable were interactions between stevia-related compounds and duloxetine, an antidepressant, which suppressed the growth of two bacterial species under laboratory conditions.
That is an intriguing finding, but it should not be over-interpreted. The experiment was performed on isolated bacterial cultures, not in living humans. The intestine contains hundreds of interacting species, food components, mucus, immune cells, digestive secretions, and constantly changing chemical conditions.
We cannot conclude that a person taking duloxetine must avoid stevia, nor can we say that all noncaloric sweeteners are harmful. Human trials will be necessary.
The study does, however, reinforce an important principle: food additives and medications do not enter the body in isolation. They may interact with each other and with the microbiome in ways that traditional research has not routinely evaluated.
My practical advice is not to panic about an occasional sweetener. It is to reduce dependence on highly processed foods and beverages whose sweetness has been engineered to maintain cravings. Water, unsweetened tea, coffee, whole fruit, and minimally processed foods should form the foundation.
Pesticides and organic foods
Agricultural chemicals are another area of concern. Some pesticides and herbicides have antimicrobial properties or may affect microbial metabolism in experimental systems. Chronic exposure could theoretically alter microbial balance, intestinal permeability, or immune signaling.
Human evidence connecting ordinary dietary pesticide exposure to specific microbiome-mediated diseases is still incomplete. We should avoid claiming that eating one conventionally grown apple damages the microbiome or causes dementia.
At the same time, it is reasonable to minimize unnecessary exposure when practical.
I purchase organic versions of many foods, particularly produce that is eaten frequently or tends to carry greater pesticide residues. However, I would never advise someone to stop eating fruits and vegetables because organic options are unavailable or unaffordable.
The established health benefits of consuming fruits, vegetables, legumes, and whole grains are far stronger than the evidence that conventionally grown produce is more harmful than eating no produce.
Practical measures include:
• eating a wide variety of produce
• washing fruits and vegetables thoroughly
• peeling selected foods when appropriate
• purchasing organic versions of frequently eaten higher-residue foods when affordable
• not allowing concern about pesticides to reduce overall plant intake
Organic food is one potential method of reducing exposure, not a substitute for dietary quality.
Microplastics, nanoplastics, and the gut
Microplastics and nanoplastics are increasingly being detected in food, water, air, and human tissues. Microplastics are small plastic fragments; nanoplastics are smaller particles that may interact differently with cells and tissues.
Experimental studies suggest that plastic particles may affect:
• oxidative stress
• inflammatory signaling
• intestinal barrier function
• immune responses
• microbial composition and metabolism
The human evidence remains early. Detection of plastic particles in the body does not by itself prove that they cause a particular disease. Accurately measuring exposure is difficult, and people are exposed to mixtures of different polymers, particle sizes, and associated chemicals.
Nevertheless, the ubiquity of exposure and the biologic findings justify concern and further research.
Reasonable steps to reduce exposure include:
• avoiding heating food in plastic
• using glass or stainless steel for hot foods and liquids
• replacing heavily scratched plastic containers
• reducing dependence on bottled water when safe tap or filtered water is available
• limiting heavily packaged and ultra-processed foods
• ventilating indoor spaces and reducing accumulated dust
• washing hands before eating
These steps are practical and low risk, but they should not be presented as proven methods of preventing dementia, cancer, or cardiovascular disease.
PFAS and the microbiome
Per- and polyfluoroalkyl substances, commonly called PFAS, are persistent chemicals used in water-resistant, grease-resistant, and nonstick products. They are often described as “forever chemicals” because many remain in the environment and human body for prolonged periods.
Research is examining whether PFAS exposure alters bile-acid metabolism, immune function, liver metabolism, intestinal barrier integrity, or the microbiome. Much of the microbiome-specific evidence is still experimental or observational.
The broader health concern surrounding PFAS is sufficient to support sensible exposure reduction, particularly in communities with contaminated drinking water.
Potential steps include:
• checking local water-quality reports
• using an appropriately certified filter when contamination is present
• reducing use of grease-resistant food wrappers and packaging
• replacing damaged nonstick cookware
• avoiding products marketed primarily for stain or water resistance when unnecessary
Again, we should distinguish plausible concern from proven clinical outcomes. We do not yet know whether changing PFAS exposure will produce measurable improvements in an individual’s microbiome or cognitive health.
The microbiome responds to the whole environment
The microbiome is shaped by more than diet alone. Factors may include:
• birth and early-life feeding
• geography and household environment
• medications, especially antibiotics
• physical activity
• sleep and circadian rhythms
• psychological stress
• tobacco and alcohol exposure
• air pollution
• pesticides and industrial chemicals
• infections and chronic disease
• age
That complexity explains why simply swallowing a probiotic containing a few bacterial strains is unlikely to reverse the consequences of a low-fiber diet, inactivity, poor sleep, and chronic exposure to ultra-processed foods.
The goal should not be to identify one perfect bacterial species. It should be to create an intestinal environment in which a diverse, resilient, and metabolically beneficial ecosystem can thrive.
What should people do now?
The microbiome field is advancing rapidly, but the most reliable recommendations are surprisingly familiar:
1. Increase fiber gradually. Most adults should first work toward at least the generally recommended 25–38 grams per day.
2. Eat a wide variety of plants. Include legumes, fruits, vegetables, whole grains, nuts, seeds, herbs, and spices.
3. Prioritize whole foods over isolated fiber additives. A fiber-enriched processed snack is not equivalent to beans or oats.
4. Exercise consistently. Combine aerobic exercise, resistance training, and regular daily movement.
5. Limit ultra-processed foods. These products are commonly low in natural fiber and high in refined starches, sugars, sodium, and additives.
6. Use sweeteners thoughtfully. Current evidence does not justify declaring every approved sweetener dangerous, but habitual consumption of intensely sweet processed products is unnecessary.
7. Use antibiotics when medically appropriate, not indiscriminately. They can be lifesaving, but they also alter microbial communities.
8. Reduce unnecessary plastic and chemical exposure. Avoid heating plastic and take sensible measures to limit PFAS and pesticide exposure without becoming fearful of food.
9. Do not allow the pursuit of perfection to undermine good nutrition. Conventionally grown vegetables are better than no vegetables.
10. Think in terms of lifestyle patterns. Diet, exercise, sleep, environment, and medical care work together.
A missing nutrient—and a missing medical opportunity
Fiber is inexpensive, widely available, and associated with benefits throughout the body. Yet it remains absent from most American meals and receives far less attention than protein, carbohydrates, supplements, or weight-loss medications.
The recent dementia findings are not proof that fiber prevents Alzheimer’s disease. The metabolite study is not yet a clinical blood test. The sweetener findings came from isolated laboratory cultures. Research involving plastics, pesticides, PFAS, and the microbiome remains incomplete.
But the overall direction of the evidence is difficult to ignore.
A diverse, fiber-rich, minimally processed diet supports the organisms that help maintain the intestinal barrier and manufacture compounds with effects far beyond the colon. Regular exercise appears to reinforce many of the same metabolic and anti-inflammatory pathways. Reducing unnecessary environmental exposures is a reasonable precaution while the science continues to develop.
We should not promise that nourishing the microbiome will prevent every chronic disease. We should recognize that it may represent one of the most important connections between the way we live and the way we age.
The microbiome has been neglected for decades. Fiber has been treated as little more than a remedy for constipation. Both deserve a much larger place in preventive medicine.
What we feed our microbes may ultimately help shape the health of our intestines, our immune system, our heart, perhaps even our brain.
Dr. Alan, more:
Dr. Alan Safdi is board-certified in Internal Medicine and in Gastroenterology and a Fellow of the American College of Gastroenterology. A proven leader in the healthcare arena, he has been featured on the national program, “Medical Crossfire” and authored or co-authored numerous medical articles and abstracts.
Dr. Alan is a long-time Telluride local, has been involved in grant-based and clinical research for four decades. He is passionate about disease prevention and wellness, not just fixing what has gone wrong.
He is also Chief Medical Officer Quadrant Health, partnered with and co-owned by Stanford University and Mayo Clinic.
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