Why Drinking Water Is the Most Important Rule of Fasting

Of all the instructions Upton Sinclair gave in his 1911 guide The Fasting Cure, one stood above the rest in emphasis and repetition: drink water. A great deal of water. Not occasionally, not when you think to — but throughout the day, consistently, from the first hour of the fast to the last. Sinclair identified inadequate water intake as the single most common cause of fasting failures, more than any food choice, mindset issue, or timing mistake. More than a century later, modern physiology explains exactly why he was right.

Direct Answer

Water is the most important rule of fasting because the body relies entirely on fluid intake — not food — to flush the waste products produced during fat burning, cellular repair, and metabolic adaptation. Without adequate water, toxins and metabolic byproducts accumulate, producing headaches, dizziness, nausea, fatigue, and a miserable fasting experience. Many people who feel terrible when fasting are not fasting incorrectly — they are simply dehydrated.

What Sinclair Observed in 1911

In Upton Sinclair's 1911 book The Fasting Cure, water was not a minor footnote. It was the operational backbone of the entire method. His instructions were direct: drink large quantities of water throughout the fast. Not flavoured water, not broth, not juice — plain water, in volume, continuously.

From his own experience and from the 277 cases collected from readers who had tried fasting, Sinclair traced a significant portion of reported failures back to a single cause: people had not drunk enough water. They fasted, felt unwell, concluded that fasting was dangerous or unsuitable for them, and stopped — when the actual problem was dehydration.

He went further than plain water. Sinclair recommended hot water between meals, crediting this practice to Dr. J.H. Salisbury — a Victorian physician who advocated hot water as a digestive cleanser and a way to maintain internal warmth and circulation during periods without food. Sinclair found that hot water between meals provided comfort, reduced hunger sensations, and seemed to support the body's elimination of waste.

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

Why the Body Needs Water During Fasting

Fat Burning Produces Waste

When the body begins burning stored fat — typically within 12–24 hours of fasting — it converts triglycerides into fatty acids and glycerol, which are then metabolised into energy. This process produces carbon dioxide (exhaled from the lungs), water (used internally or excreted in urine and breath), and ketone bodies (used as fuel or excreted in urine and breath). All of these require adequate fluid levels to process and eliminate efficiently.

If fluid intake is low, ketone excretion slows, urine becomes more concentrated with metabolic waste, and the symptoms associated with "feeling bad" while fasting intensify — headaches, bad breath, nausea, and fatigue become more pronounced than they need to be.

Insulin Drops and Sodium Follows

One of the first effects of fasting is a drop in insulin. This is the desired result — insulin suppression is what enables fat burning and cellular repair. But insulin also signals the kidneys to retain sodium. When insulin falls, the kidneys begin releasing sodium into the urine at a higher rate. Sodium takes water with it. The result: increased urine output and a faster route to dehydration and electrolyte imbalance if fluid intake does not compensate.

This is why people who begin fasting often notice they urinate more frequently in the first 24–48 hours. The body is clearing the glycogen-bound water (glycogen stores water at a ratio of roughly 3g water per gram of glycogen) and then releasing sodium as insulin drops. Both processes demand increased fluid intake.

The Headache Connection

Fasting headaches are among the most commonly reported early symptoms. Most are not caused by the absence of food. They are caused by mild dehydration combined with electrolyte loss — primarily sodium, potassium, and magnesium. These minerals are carried out of the body in urine as fasting continues. Without adequate water to maintain circulation and adequate electrolytes to support nerve function, the blood vessels in the head dilate and the characteristic fasting headache begins.

Sinclair's repeated emphasis on water was observationally accurate: he noticed that people who drank freely during their fasts fared dramatically better than those who did not. The mechanism is now well understood.

How Much Water to Drink

Sinclair did not specify a precise volume. Modern guidance generally points toward 2–3 litres of water per day during fasting, with more needed in hot weather or during exercise. A practical rule: if your urine is pale yellow to clear, you are adequately hydrated. If it is dark yellow or amber, you need more water.

Adding a small pinch of sea salt to water — or ensuring electrolyte intake through food during the eating window — helps compensate for the sodium and mineral losses that accompany fasting-related insulin suppression. This was not something Sinclair described in biochemical terms, but his anecdotal observation that well-hydrated fasters experienced far fewer symptoms points to exactly this mechanism.

Hot Water: The Overlooked Tool

Sinclair's specific recommendation for hot water between meals deserves attention. Modern fasting practitioners often overlook this entirely, focusing on cold water or sparkling water instead. Hot water has a distinct effect: it warms the digestive tract, encourages peristaltic movement, and — as Sinclair and Dr. Salisbury both observed — seems to reduce the discomfort of an empty stomach by maintaining warmth and gentle internal movement.

Plain herbal tea (no sweeteners, no milk) achieves the same effect and remains consistent with a clean fast. The principle behind both is identical: warm fluid supports the fasting body in ways that cold water does not fully replicate.

Connecting 1911 to Modern Science

What Sinclair observed anecdotally, modern physiology can now explain in detail. The mechanisms he identified — body waste products requiring elimination, internal cleansing requiring a fluid medium, the dangerous role of dehydration in worsening fasting symptoms — are all borne out by what we now understand about ketosis, renal sodium handling, electrolyte balance, and metabolic waste production.

Research on fasting consistently supports the same conclusion: hydration is not optional. A study by Cahill (2006, Annual Review of Nutrition) describing the metabolic adaptations of fasting confirms that the kidneys carry a significant elimination burden during prolonged fasting and require adequate fluid to do so. Longo and Mattson's 2014 review in Cell Metabolism notes that cellular autophagy — the clean-up process triggered by fasting — requires a functioning cellular fluid environment to operate.

The principle Sinclair emphasised in 1911 out of practical observation maps precisely onto what science has since confirmed through measurement.

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Frequently Asked Questions

Does drinking water break a fast? No. Plain water has no calories, no insulin response, and does not interrupt any of the metabolic processes that define fasting. Drink freely throughout.

Can you drink too much water while fasting? Yes — drinking extreme quantities of plain water without any electrolyte intake can dilute sodium in the blood (hyponatraemia). This is rare but possible. The solution is to include a small amount of sea salt or mineral-rich water alongside plain water, particularly during fasts longer than 24 hours.

What about sparkling water? Sinclair did not have sparkling water, but plain carbonated water (no flavourings, no sweeteners) is compatible with fasting. It does not spike insulin or interrupt fat burning.

Does the temperature of the water matter? Not for hydration itself, but warm or hot water may reduce stomach discomfort and support the digestive tract during a fast — the reason Sinclair specifically recommended it.

What are the signs I am not drinking enough during a fast? Dark yellow urine, persistent headaches, dizziness when standing, dry mouth, and a general sense of fatigue that worsens through the day rather than improving are the clearest indicators.

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This article draws on historical research from 1911 and is for informational purposes only — not medical advice.

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