The Fiber Deficiency Crisis: Why Most People Are Missing This Critical Nutrient | Dr. Karan Rajan

Learning from Our Ancestors

Dr. Rajan begins by placing fiber consumption in an evolutionary context, referencing modern-day hunter-gatherer societies like the Hadza tribe in Tanzania. These communities consume extraordinarily high amounts of fiber—sometimes exceeding 100 to 120 grams per day—a stark contrast to the average Western diet that often falls drastically short. This fiber-rich diet correlates with impressively low rates of chronic diseases and metabolic disorders, a pattern attributed not only to their active lifestyles and strong communities but also heavily influenced by their high fiber intake.

This comparison highlights an important truth: modern industrialized diets, characterized by processed and refined foods, lack the diversity and volume of fiber essential for optimal health. The gut microbiome of these hunter-gatherer populations thrives on the fiber-rich substrates they consume, leading to improved digestion, immune function, and metabolic health.

The Physiological Role of Fiber

While fiber is indigestible to humans in the traditional enzymatic sense, Dr. Rajan explains that its true value lies in its role as a substrate for the gut microbiome—specifically the bacteria residing in the colon. Through fermentation, these bacteria produce short-chain fatty acids (SCFAs), which serve as vital metabolic byproducts benefiting the host. These SCFAs stimulate enteroendocrine cells (notably L cells) lining the colon, triggering the release of several pivotal hormones like GLP-1, GIP, oxyntomodulin, and cholecystokinin. These hormones regulate appetite, improve insulin sensitivity, and contribute to multiple aspects of metabolic health.

Fiber does more than act as a mere bulking agent in stool; it actively modulates hormonal and neurological pathways that influence satiety, glucose metabolism, and systemic inflammation. Additionally, a fiber-rich diet supports a diverse and resilient gut microbiome that can enhance nutrient and mineral absorption and produce essential vitamins such as vitamin K and some B vitamins, creating a feedback loop essential for optimal health.

Consequences of Long-Term Fiber Deficiency

Dr. Rajan notes that chronic low fiber intake, often seen in modern diets, can lead to a multitude of health problems. On a day-to-day level, insufficient fiber often results in bloating, constipation, and sluggishness—a familiar experience for many who have shifted to low-fiber, restrictive diets like ketogenic or carnivore plans. Fascinatingly, research shows that gut microbiomes can start adapting to higher fiber intake within 24 to 48 hours, allowing for early beneficial shifts even if subjective improvements take longer to manifest.

Over longer periods, inadequate fiber intake is linked with serious risks such as increased incidence of colorectal cancer, heart disease, and metabolic dysfunction. A low-fiber environment forces gut bacteria to switch from fermenting carbohydrate substrates to degrading the gut's own mucosal lining, releasing harmful byproducts like ammonia and phenols that further damage gut barrier integrity. This "self-cannibalization" contributes to leaky gut and systemic inflammation, often underpinning chronic illnesses.

Understanding the Diversity of Fiber

One key topic discussed was the complexity and diversity of dietary fiber. Dr. Rajan explains that fiber is an umbrella term encompassing soluble fibers, insoluble fibers, and resistant starches. Soluble fibers (found in oats, legumes, and many fruits) dissolve in water and often act as prebiotic fibers that nurture beneficial gut microbes. Insoluble fibers (commonly in the skins of fruits and vegetables, seeds, and nuts) provide bulk and promote regularity but do not dissolve. Resistant starch, present in cooled cooked starchy foods and certain green fruits, is a "pseudo-fiber" that resists digestion in the small intestine and feeds gut bacteria in the colon similarly to prebiotics.

For individuals unaccustomed to fiber, Dr. Rajan emphasizes the importance of ramping up intake gradually—around 5 grams per week—to allow the colon and microbiome to adapt and to minimize side effects like bloating or gas. Moreover, individual tolerance can vary widely, shaped by genetics, early life microbiome exposure, and habitual diet. For example, some individuals, including Dr. Rajan himself, have difficulty digesting inulin-rich fibers, which ferment rapidly and can cause discomfort. Others may find legumes particularly challenging after periods without them, due to the loss of specialized gut bacteria. This underscores the need for personalized approaches and patient reintroduction strategies.

Practical, Fiber-Rich Foods for Modern Diets

When it comes to integrating fiber in accessible and enjoyable ways, Dr. Rajan presents several specific foods from his personal pantry staples. Fruits like pears and kiwis stand out because besides providing soluble fiber, they contain unique compounds like sorbitol and actinidin, which naturally promote gut motility and regularity. Frozen berries, which maintain their fiber and polyphenol content year-round, are praised for their antioxidant properties and their status as prebiotic fiber sources benefiting gut and brain health alike.

Dr. Rajan also advocates for frozen vegetables such as peas and edamame, which are not only high in fiber but provide quality plant protein. These versatile foods can be readily added to meals or consumed as snacks, making them practical options for diverse diets. Chia seeds also feature prominently; their mucilaginous soluble fiber expands in water, creating a gel-like substance that supports satiety and microbiome health. Dr. Rajan notes the common misconception that chia seeds must be soaked to be safe or effective—he clarifies that consuming them with adequate water is sufficient to gain their benefits.

Fiber and Chronic Disease

Beyond gut-specific effects, Dr. Rajan highlights fiber's role in cardiometabolic health, particularly its ability to lower cholesterol levels and improve liver function. Fiber interferes with the enterohepatic circulation by binding to bile acids, which carry cholesterol, in the gut, thereby reducing cholesterol reabsorption and promoting its excretion. This has important implications for individuals with genetically elevated cholesterol absorption, a trait more common in people of South Asian descent.

Dr. Rajan shares his personal experience with mild fatty liver disease to illustrate how dietary fiber, alongside other lifestyle changes, supports liver health. He discusses the impact of high fructose intake from fruit juices and processed carbohydrates in promoting fatty liver disease, underscoring that moderation and quality of carbohydrates are crucial. Additionally, he warns against excessive use of certain supplements like green tea extracts or turmeric in high doses that can inadvertently cause liver injury, emphasizing the importance of moderation and physician consultation.

Reevaluating Physical Activity

Shifting into broader lifestyle discussion, Dr. Rajan challenges the long-held fitness goal of walking 10,000 steps per day, citing newer meta-analyses suggesting that 7,000 steps can confer nearly the same cardiovascular and metabolic benefits with diminishing returns beyond that. He promotes sustainability and incremental gains over perfection, echoing a central theme throughout the conversation.

In tandem with this, he stresses the importance of building "patterns over perfection," advocating for achievable small goals that lead to lasting health behavior changes rather than getting discouraged by rigid targets. This mindset applies not only to physical activity but also to nutrition, sleep, and general wellness.

Sleep and Supplementation

Dr. Rajan candidly shares his struggles with sleep during demanding medical training years and offers practical advice such as optimizing bedroom temperature to aid sleep onset and avoiding intense evening exercise that elevates heart rate and body temperature. He also notes the impact of environmental and partner-related disturbances on sleep quality and gently suggests that separate sleeping arrangements can be a beneficial choice for some couples.

Regarding supplementation, Dr. Rajan takes a balanced but pragmatic stance. He embraces targeted supplements like protein powders to meet daily requirements, creatine for strength and recovery, vitamin D with vitamin K2 to compensate for deficiencies commonly found in higher latitudes, algal omega-3 oil as a sustainable alternative to fish, and prebiotic fiber for microbiome support. He also mentions occasional magnesium supplementation for sleep, though he acknowledges variable evidence for its efficacy. Crucially, supplementation is framed as a complement—not a substitute—for a well-rounded, fiber-rich, nutrient-dense diet and active lifestyle.