Why Fermentation in the Gut Causes Disease (and How Fasting Stops It)

Long before anyone had heard of the gut microbiome, one writer was convinced that the food rotting in our intestines was the hidden cause of almost every chronic illness. It sounds crude by today's standards, but the underlying idea — that an overloaded, poorly emptied gut breeds disease — turns out to have more in common with modern gut science than you might expect.

Historical Context: Sinclair's 1911 Theory

In his 1911 book The Fasting Cure, Upton Sinclair — the journalist famous for The Jungle — laid out a theory he'd developed after years of chronic illness and failed treatments. Having spent roughly $15,000 (a staggering sum at the time) on doctors, surgeons, and sanatoriums with little lasting benefit, Sinclair eventually discovered fasting and became its most passionate evangelist.

Central to his thinking was what he called "autointoxication." Sinclair argued that when people eat more than their bodies can properly digest and eliminate, the surplus food doesn't just pass through harmlessly. Instead, it lingers in the digestive tract and begins to ferment, producing toxins faster than the body's elimination organs can clear them out. He believed this fermentation-driven poisoning was the root cause behind headaches, rheumatism, skin eruptions, and a long list of other ailments his era struggled to explain.

Sinclair's Core Argument

Sinclair's reasoning ran roughly like this:

Overfeeding creates surplus material the body cannot fully process at the pace it's delivered.
That surplus ferments in the gut, producing byproducts he described as poisons.
Elimination organs get overwhelmed, allowing those byproducts to accumulate rather than clear.
The accumulated toxins clog the system, showing up as chronic conditions — rheumatism, headaches, skin trouble, and lowered resistance to infection.
Fasting interrupts the cycle — with no new food coming in, digestion "goes out of business," freeing the body to catch up on cleansing and repair.

He pointed to the coated tongue often seen during a fast as visible evidence: a thick coating, in his view, meant the body was actively expelling stored waste, and a tongue that cleared signaled the process was finishing.

It's worth being clear-eyed about this: "autointoxication" as Sinclair described it was a popular but scientifically unproven theory of his era, and modern medicine doesn't use that framework. But the broader instinct — that constant digestive load can contribute to chronic low-grade dysfunction, and that giving the gut a break has measurable benefits — has aged far better than the specific mechanism Sinclair proposed.

What Sinclair Observed in His Readers

Sinclair's book drew on 277 fasting cases reported by 109 readers who wrote to him after his original magazine article ran in Cosmopolitan. Of those, 100 reported some benefit and 17 reported none. Conditions people said improved included rheumatism, chronic headaches, asthma, catarrh, and various stomach and intestinal complaints — precisely the kind of conditions Sinclair attributed to fermentation and toxic buildup.

He also noted a pattern in the cases that didn't hold: roughly half the failures, by his own accounting, traced back to how the fast was broken rather than the fast itself — people reintroducing food too quickly and undoing whatever benefit the digestive rest had provided.

The Modern Science Connection

Today's research doesn't validate "autointoxication" as a literal mechanism, but it does support several of the pieces Sinclair was circling:

Gut microbiome imbalance is now well established as a contributor to inflammation and metabolic disease — not unlike Sinclair's idea of harmful byproducts building up in the digestive tract.
Fasting periods trigger autophagy, the cellular cleanup process where the body breaks down and recycles damaged components — a modern, mechanistically grounded cousin of what Sinclair called the body "redirecting energy to cleansing."
Time-restricted eating and gut rest have been shown in current research to support gut barrier integrity and shift microbiome composition in ways associated with reduced inflammation.
Overeating and excess simple carbohydrates are linked in current nutrition science to unfavorable shifts in gut bacteria — echoing Sinclair's warning about starch and sugar feeding fermentation.

In other words, Sinclair had the wrong biochemistry but was pointing, however crudely, at something real: what and how much we eat shapes gut health, and periods without food give the digestive system room to reset.

Sinclair's Practical Takeaway

For Sinclair, the antidote to fermentation-driven disease wasn't a specific pill or procedure — it was subtraction. Stop adding fuel to the fire, let the gut empty and rest, and the body's own systems would handle the rest. He was equally emphatic that how you resumed eating mattered as much as the fast itself: rushing back into starchy, sugary food after a fast, in his view, simply restarted the fermentation cycle that caused the trouble in the first place.

For the complete guide, get Intermittent Fasting in Practice on Amazon → and claim 3 months free on our fasting app at fastinginpractice.com/redeem.

Frequently Asked Questions

What did Upton Sinclair mean by "autointoxication"?

He used the term to describe self-poisoning from undigested or improperly eliminated food fermenting in the gut. It was a popular theory in his era but isn't a recognized diagnosis in modern medicine — though the general idea that digestive load affects health has some modern parallels.

Is fermentation in the gut actually bad for you?

Some fermentation is normal and even beneficial — it's how gut bacteria produce short-chain fatty acids from fiber. The issue Sinclair was gesturing at is closer to what we'd now call dysbiosis: an imbalance where fermentation byproducts and bacterial overgrowth contribute to bloating, inflammation, and other symptoms.

Does fasting really give your gut a "rest"?

In a practical sense, yes — periods without food reduce the digestive workload and allow processes like the migrating motor complex (the gut's "housekeeping" muscle contractions) to run uninterrupted, which can help clear residual material between meals.

How long did Sinclair think a fast needed to be to help?

His personal fasts were both 12 days, but the reader cases in his book ranged widely, averaging around 6 days. He believed even shorter fasts of a few days could produce noticeable benefit.

Is starch and sugar really worse for gut fermentation?

Sinclair certainly thought so, and modern research on high-sugar diets and gut dysbiosis offers some support — excess refined carbohydrate intake is associated with unfavorable microbiome shifts, though the relationship is more nuanced than his "yeast-pot" description suggested.

This article draws on historical research from 1911 and is for informational purposes only — not medical advice. Always consult a qualified healthcare professional before making any dietary changes.

Sinclair, U. (1911). The Fasting Cure. Mitchell Kennerley.

Why Fermentation in the Gut Causes Disease (and How Fasting Stops It)