Fasting and Respiratory Illness: Historical Evidence from the 1910s

When you come down with a cold or flu, your appetite typically drops. That loss of hunger is usually treated as a problem to overcome — something to push through with broth, crackers, and extra eating. But for over a century, some researchers and practitioners have argued it is actually your body's intelligence at work.

Upton Sinclair's 1911 book The Fasting Cure offers one of the earliest systematic collections of fasting accounts related to illness — including respiratory conditions. What those cases showed, and how they connect to what we now understand about immunity and fasting, is worth examining.

Historical Context: Fasting in the 1910s

Sinclair was a journalist, not a physician. He discovered fasting through personal health necessity — chronic headaches, persistent nervousness, and approximately $15,000 spent on doctors over eight years without lasting benefit. After his first successful fast resolved problems he had failed to fix through medicine, he published his experience in Cosmopolitan magazine.

The response was overwhelming. He received 600–800 letters from readers who tried fasting themselves. These accounts — 277 documented fasting episodes from 109 individuals — became the basis of The Fasting Cure.

Among the conditions reported were cases of "grippe" (influenza), chronic catarrh (persistent inflammation of the mucous membranes in the nose and throat), and respiratory weakness. All content in this article reflects those historical accounts from 1911 and should be read as such — not as clinical guidance.

What Sinclair's Cases Showed

Among the conditions reported as helped by fasting, Sinclair included:

• Grippe (influenza) — multiple accounts described shorter, less severe illness
• Catarrh — chronic nasal and throat inflammation that respondents said improved significantly after extended fasts
• Chronic cold susceptibility — recurrent vulnerability reportedly reduced after fasting

Sinclair's theoretical explanation was consistent with his broader view of disease. He believed the body becomes ill when a toxic, overburdened system allows bacteria and infection to take hold. He wrote that "lowered vitality invites infection" — meaning the same internal fermentation and overfeeding that caused headaches and rheumatism also made the body more susceptible to respiratory illness.

His argument: when fasting begins and initial hunger passes, the digestive system "goes out of business," freeing the body's energy and resources for cleansing and repair — and, in his framing, for fighting off invaders.

The Animal Observation That Stuck

One of Sinclair's most memorable points was the observation about sick animals. He noted that dogs, cats, and other animals instinctively refuse food when ill — and typically recover. He wrote: "Even dogs fast when they are ill. I look forward to the time when human beings may be as wise as dogs."

This was not simply poetic. It raised a genuine question: if every other animal reduces food intake when sick, why do humans insist on eating through illness?

This question now has a partial scientific answer. Inflammatory signals from infection genuinely suppress appetite through cytokine pathways — this is a feature, not a bug. The body is redirecting resources from digestion to immune response and repair. Sinclair observed this in 1911; modern immunology has confirmed the mechanism.

Sinclair's Caution: Tuberculosis

One important exception in Sinclair's otherwise enthusiastic record was tuberculosis. He was specifically cautious about TB, noting that TB patients had often already lost significant weight and were nutritionally depleted. Prolonged fasting for them was not something he recommended.

This caution matters. The historical accounts of respiratory benefit related primarily to acute conditions (colds, influenza, grippe) and chronic but manageable conditions (catarrh). Serious wasting illness was explicitly treated as a different category.

He also identified the other major risk in his records: breaking the fast incorrectly. Half of the cases where fasting failed to produce lasting benefit were attributed to resuming food too quickly or with the wrong foods — a separate issue from whether fasting helped during the illness itself.

What Modern Science Adds

Sinclair worked without the tools to explain why fasting might help the body manage illness. Modern research offers useful context.

Autophagy and immune function: Fasting activates autophagy — the process by which cells break down and recycle damaged components, including intracellular pathogens. Some viruses replicate inside cells, and autophagy is one mechanism the immune system uses to clear them. This was entirely unknown in 1911.

Inflammatory regulation: Acute fasting reduces circulating inflammatory markers, including CRP and certain interleukins. Excessive inflammation during respiratory illness — rather than the virus itself — drives much of the symptom burden. Some research suggests reducing the inflammatory load during illness may ease severity.

Immune cell renewal: Extended fasting of 72 hours or more has been shown in human research to trigger significant immune system regeneration, including the production of new white blood cells. This is far beyond what most people would do during a typical illness, but it demonstrates that the fasting-immune connection is physiologically real.

Appetite suppression is not a malfunction: The loss of appetite during illness is now understood to be driven by pro-inflammatory cytokines (including IL-1β and TNF-α) acting on the hypothalamus. The body is conserving energy for immune work. Overriding this signal by forcing eating during acute illness may not serve recovery.

What the Historical Cases Cannot Tell Us

The 277 cases in Sinclair's survey were self-reported, unblinded, and collected via magazine responses — what would now be considered a very low-quality evidence base. Sinclair acknowledged this openly. He was not claiming clinical proof; he was arguing for investigation.

We cannot determine from these accounts whether fasting caused the improvement, whether the illnesses would have resolved anyway, or whether reporting bias inflated positive results. What the records do provide is a rich historical document showing that interest in fasting as a response to illness is more than a century old, and that it produced enough apparent benefit to sustain serious attention.

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

Connection to Modern Practice

The principle that appetite loss during illness should sometimes be respected rather than overridden aligns with current thinking. What has changed is specificity: we now distinguish between different types of infection, different stages of illness, and different fasting durations — distinctions Sinclair could not make.

For healthy adults experiencing a mild cold or flu, reducing eating for a day or two rather than forcing meals is unlikely to be harmful and may support the immune response. For serious respiratory illness, compromised immunity, nutritional depletion, or anyone on medication, professional guidance is essential before changing eating patterns.

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Frequently Asked Questions

What did Upton Sinclair say about fasting during respiratory illness?

Sinclair argued that most illness, including respiratory conditions, is caused by overfeeding and internal fermentation that lowers vitality and invites infection. He collected cases where fasting appeared to help with grippe, catarrh, and chronic susceptibility to colds — always framing them as personal reports, not clinical evidence.

Did Sinclair recommend fasting for tuberculosis?

No — he was specifically cautious about TB. He noted that TB patients were often already nutritionally depleted from weight loss, making prolonged fasting inappropriate. His positive accounts related to acute illness and chronic manageable conditions, not wasting disease.

Why do animals stop eating when they are sick?

It is an evolutionary adaptation. When ill, pro-inflammatory cytokines suppress appetite through brain signalling — this redirects energy from digestion to immune response and repair. Sinclair observed this in 1911; modern immunology has confirmed the mechanism.

Is it safe to fast when you have a respiratory illness?

This depends on the person, the severity of the illness, and their overall health. For healthy adults with mild illness, a day of reduced eating is generally tolerated. Anyone with serious illness, nutritional deficiency, or on medication should consult a healthcare professional before fasting.

How does autophagy relate to respiratory illness?

Autophagy — the cellular self-cleaning process activated by fasting — can help clear intracellular pathogens. Some viruses replicate inside cells, and autophagy is one mechanism the immune system uses to manage them. This does not mean fasting is a cure for respiratory illness, but it provides a biologically plausible connection that Sinclair could not have known about in 1911.

Related Articles

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• The real reason you feel better after fasting: Sinclair's fermentation theory explained
• How intermittent fasting promotes autophagy

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.