How Upton Sinclair Discovered Fasting and Transformed His Health

Upton Sinclair is best remembered for The Jungle, his 1906 exposé of the American meatpacking industry that shocked the nation and helped reshape food safety law. But Sinclair's other major health intervention — fasting — receives far less attention, despite generating what he described as the most dramatic improvement in his personal wellbeing of his entire life.

In 1911, Sinclair published The Fasting Cure, a record of his personal fasting experiments and the accounts of 277 other people who had tried it. It is a remarkable document: passionate, firsthand, occasionally exaggerated, and — viewed from the distance of a century of science — surprisingly accurate about several things that modern research has since confirmed.

The Health Problems That Led Him There

Before discovering fasting, Sinclair suffered for years from a cluster of debilitating symptoms: chronic nervousness, persistent insomnia, and headaches so frequent he later wrote that he was "never more than fifteen minutes ahead of a headache." He was, by his own account, a physical wreck who could not do sustained intellectual work without paying for it with days of prostration afterward.

What made this particularly frustrating for a man of his intellect and drive was that he had tried everything available. He estimated he had spent approximately $15,000 — an enormous sum in 1911, roughly equivalent to a quarter million dollars today — on physicians, surgeons, druggists, and sanatoriums. He had experimented with vegetarianism, raw food diets, and meat-only diets. Some brought brief improvement; none produced lasting change.

"I had spent over five hundred dollars trying to get well on medicines," he wrote, recounting a letter from one of his readers. "It cost me only thirty cents to use your method, and for that thirty cents I obtained relief a million-fold more beneficial."

The Discovery

Sinclair encountered fasting through the physical culture movement that was gaining traction in early twentieth-century America. Bernarr Macfadden, a fitness entrepreneur who ran sanatoriums and published health magazines, was among its prominent advocates. The idea that voluntary food abstention could heal chronic illness struck Sinclair as either quackery or the most important health discovery he had encountered. He decided to test it on himself.

His first major fast lasted twelve days.

The First Twelve Days

Sinclair's account of his first extended fast is vivid and specific. The first four days were difficult — he experienced intense physical lassitude, dizziness when he stood up, and a persistent weakness that made him doubt he could continue. He lost fifteen pounds in those four days, which he later recognised as a sign of "extremely poor tissue state" — a large amount of water and metabolic waste that his body rapidly shed when the digestive burden was removed.

After the first few days, something unexpected happened. The physical weakness remained, but his mind became unusually clear. He wrote that he "read and wrote more than I had dared to do for years before." He reported sleeping well throughout the fast. From day four onward, he lost only two more pounds across the remaining eight days — the body's economy kicking in, conserving essential tissue while metabolising waste.

After twelve days, he broke the fast carefully — starting with small amounts of orange juice and grape juice, transitioning to warm milk, building back gradually over several weeks. He gained thirty-two pounds in twenty-four days on a milk-based recovery diet, and described the entire recovery period as one of "extraordinary peace, mental activity, and desire for physical movement" unlike anything he had experienced in years.

This pattern — initial difficulty, then clarity, then profound recovery — would recur in his second fast and in dozens of the cases he later collected.

The Second Fast

Sinclair fasted again, also for twelve days. The contrast with his first experience was striking. This time there was no weakness. He walked four miles every morning and did light gymnasium work throughout the fast. He lost nine pounds over eight days. He described his mind as so active that he "read and wrote incessantly." When the fast ended, he ate oranges and figs for a week and recovered his weight.

His conclusion from these two experiences became the foundation of his book: fasting was not dangerous, was not starvation, and was not even particularly difficult after the first two or three days. It was, he believed, the mechanism by which the body repaired damage that accumulated from years of overfeeding and the resulting fermentation of excess nutrients in the digestive tract.

His Theory of Why It Worked

Sinclair's explanatory framework for fasting was rooted in the Victorian medical concept of "autointoxication" — the idea that over-fermented food in the digestive tract produced toxins that poisoned the blood and tissues, causing everything from headaches to rheumatism to nervous exhaustion. This theory was scientifically contested even in 1911 and has not survived in its original form.

But the core observation — that the digestive system, when given a complete rest, could redirect energy to repair processes elsewhere in the body — aligns in important ways with what modern science has confirmed. What Sinclair called "cleansing" corresponds in contemporary biology to autophagy: the cellular process by which the body breaks down and recycles damaged proteins and organelles. For a modern look at what happens in your body during a fast, see What happens to your body during intermittent fasting.

Where Sinclair's theory was accurate:

• The body does preferentially metabolise damaged and non-essential tissue before healthy tissue during extended fasting — this aligns with what we now understand as selective autophagy
• Digestive rest does allow repair processes to proceed that are suppressed by the constant demands of digestion
• Hunger does largely disappear after the first two to three days of a fast, as the body transitions to fat metabolism — this is now well-documented in the physiology literature

Where his theory overreached:

• The "autointoxication" mechanism was not validated by subsequent microbiology
• The claims about tuberculosis, serious infections, and organ disease being cured by fasting alone were not substantiated
• The 90-day fasts reported at Macfadden's institution, if accurate, were undertaken under conditions we cannot evaluate and would not be recommended today

The 277 Letters

After his magazine article on fasting appeared in Cosmopolitan, Sinclair received between 600 and 800 letters from readers who had tried fasting on the basis of his account. He tabulated 277 of these cases. Of 109 people reporting on these episodes, 100 reported benefit and only 17 reported no benefit. The conditions reportedly helped included rheumatism, chronic digestive disorders, asthma, insomnia, skin diseases, nervous exhaustion, and persistent headaches.

These were not clinical trials. They were self-reports from motivated readers with no control group and no medical verification. Sinclair acknowledged this. But the consistency of the accounts across diverse conditions and geographies was striking to him — and the pattern they described maps reasonably well onto what we now know about the anti-inflammatory and metabolic effects of fasting. For an overview of the evidence on fasting and inflammation, see Does intermittent fasting reduce inflammation?.

What Sinclair Got Right

More than a century later, some of Sinclair's most counterintuitive claims have aged remarkably well:

Hunger disappears. He insisted that the hardest days are the first two or three, after which hunger largely disappears. This is now understood through ghrelin physiology: ghrelin (the hunger hormone) spikes initially but adapts downward within 24–48 hours as the body transitions metabolic states.

Mental clarity improves. Sinclair consistently reported enhanced cognitive function from around day four or five. Contemporary research on ketones (the brain's alternate fuel during extended fasting) and BDNF (brain-derived neurotrophic factor, which fasting upregulates) provides a plausible mechanism for this observation.

Breaking the fast is the most dangerous moment. Sinclair wrote that breaking the fast incorrectly — with a heavy meal after an extended period without food — was the single greatest risk of fasting. This is now formalized as refeeding syndrome: the potentially dangerous electrolyte shifts that can follow a rapid return to high carbohydrate intake after extended fasting.

Water is essential. His most repeated practical instruction was to drink large amounts of water throughout. This remains central to safe fasting practice today.

What His Story Teaches Us

Sinclair's fasting story is not a medical case study. It is the account of one determined, intellectually rigorous person who tried a radical intervention on himself, documented it carefully, collected the experiences of hundreds of others, and published his findings at a time when fasting was widely dismissed by the medical establishment.

His courage in doing so — and the hostility he encountered from the medical press — mirrors the trajectory that fasting science would follow for the next hundred years: dismissed, then marginally accepted, then increasingly validated as the cellular mechanisms became understood. The 2016 Nobel Prize in Physiology for autophagy research was in some sense a vindication of the observation that fasting enables a profound biological repair mode — which is exactly what Sinclair had been saying, in cruder language, in 1911.

The lesson is not that Sinclair was right about everything. He was not. The lesson is that careful self-observation, combined with broad data collection, can identify real phenomena even before science has the tools to explain them.

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

Was Upton Sinclair a doctor or medical professional? No. Sinclair was a journalist and social reformer, best known for The Jungle. His fasting work was based entirely on personal experimentation and the collected testimonials of readers — not clinical practice. He was candid about this throughout the book.

How long did Sinclair's fasts last? His two main personal fasts were each twelve days. He never undertook what he called a "complete fast" — meaning one extended until genuine hunger returned, which for some subjects ran to many weeks. He considered his own twelve-day fasts beneficial but incomplete by that standard.

Is The Fasting Cure still relevant today? As a historical document, yes — it provides a detailed picture of how fasting was understood and practised before modern biochemistry. Several of Sinclair's core observations align with contemporary research on autophagy, hunger hormones, and metabolic switching. As medical guidance, it should be treated as a historical reference only, not a clinical protocol.

Did Sinclair's fasting cure his health problems permanently? By his own account, yes — he reported that the chronic headaches, insomnia, and nervous exhaustion that had plagued him for nearly a decade resolved after his fasting experiences and did not return when he maintained the dietary and fasting practices he had adopted. He attributed this to the combination of fasting, a diet based on broiled lean meat and hot water, and the avoidance of starchy fermented foods.

What is the modern scientific equivalent of what Sinclair called "cleansing"? The most direct modern parallel is autophagy — the cellular process by which the body breaks down and recycles damaged proteins, dysfunctional mitochondria, and cellular debris. Autophagy is upregulated during fasting, typically beginning meaningfully after 16–17 hours of food abstention. It is widely studied as a mechanism for cellular health maintenance and longevity.

This article references historical accounts from "The Fasting Cure" by Upton Sinclair (1911) and is for informational purposes only. It does not constitute medical advice. Always consult a healthcare professional before making changes to your diet.