Why 20-Mile Walks Were Reported During Extended Fasts (And What That Tells Us)

It sounds impossible: a person who hasn't eaten in three or four weeks lacing up their shoes and walking 20 miles in a single day. Yet this is exactly what Upton Sinclair documented in his 1911 book The Fasting Cure — and it wasn't an isolated fluke. Understanding why the body can do this reveals something important about how fasting actually works.

The Historical Account

In The Fasting Cure, Sinclair recorded the case of a woman who fasted for 33 days while continuing to work at a sanatorium. On day 24 of her fast — more than three weeks without food — she walked 20 miles. Sinclair treated this as remarkable but not inexplicable; he saw it as evidence that the body, once past the difficult early days of a fast, redirects freed-up energy toward sustained physical activity rather than shutting down.

A second case reinforces the pattern. A man who had been reduced from 186 pounds to 119 pounds after a serious railway accident fasted for 6 days, then went on to regain 27 pounds, resumed playing tennis, and — in a detail Sinclair clearly found remarkable — walked 442 miles over 11 days during his recovery period.

Sinclair's own experience pointed the same direction. During his first 12-day fast, the opening days brought real weakness and dizziness on rising. But by his second 12-day fast, after his body had adapted, there was no weakness at all — he walked 4 miles every morning and did light gym work throughout, all while eating nothing.

Sinclair's Explanation

Sinclair's theory, framed in the language of 1911, was that digestion consumes an enormous share of the body's daily energy budget. Once the digestive and assimilative systems "go out of business" during a fast, he argued, that energy doesn't disappear — it becomes available for other uses, including movement. He also believed that the early days of a fast (the first 2–3 days) were the hardest specifically because the body hadn't yet completed this shift; once hunger disappeared and the body settled into the fasted state, physical capacity — in his observation — often improved rather than declined.

He was careful to distinguish this from heavy labor, which he consistently warned against during fasting. Walking, by contrast, he considered compatible with — even beneficial to — an extended fast.

What Modern Exercise Science Confirms

A century of physiology research has filled in the mechanism Sinclair could only observe from the outside.

Fat becomes the primary fuel. After roughly 24–36 hours without food, liver and muscle glycogen stores are substantially depleted, and the body shifts toward burning fat for fuel — both directly as fatty acids and, in the brain, as ketones. Fat stores, even in a lean person, represent tens of thousands of stored calories. For low-to-moderate intensity activity like walking, this is an enormous, slow-burning fuel reserve compared to the roughly 2,000 calories held in glycogen.

Walking is the ideal fasted activity. Research on fasted exercise consistently shows that low-intensity, steady-state movement — walking being the clearest example — is well tolerated even during extended fasts, because it relies heavily on fat oxidation rather than the fast-access glucose that high-intensity effort demands. This lines up precisely with what Sinclair observed: fasters could walk substantial distances, but heavy physical labor was a different story.

Norepinephrine rises during fasting. Studies on prolonged fasting (72 hours or more) have found increases in circulating norepinephrine, a stress hormone that helps mobilize fat stores and can support alertness and physical drive — a plausible modern explanation for why some fasters report feeling more energetic, not less, once they're several days in.

Adaptation matters. Sinclair's own account — weak and dizzy in his first fast, strong and mobile in his second — mirrors what exercise physiologists now call "fat adaptation." The first exposure to extended fasting is the hardest because the body's fuel-switching machinery isn't yet primed. With repeated exposure, that transition becomes faster and less disruptive.

What This Means for Fasted Walking Today

None of this means every faster should aim for 20-mile days. Sinclair's cases were extreme outliers within his own dataset of 277 reported fasting episodes, not the typical experience. But the underlying principle holds up well: gentle, sustained movement like walking is one of the most fasting-compatible forms of exercise available, at any fast length, once the initial adjustment period has passed.

For people practicing shorter daily fasting windows (16:8, 18:6, OMAD), a fasted walk — first thing in the morning, or during the final hours of the fasting window — is widely reported as one of the easiest ways to build activity into a fasting routine without disrupting the fast or causing excessive fatigue.

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Frequently Asked Questions

Is it really safe to walk long distances while fasting for multiple days?

Sinclair's cases suggest it can be tolerated by some people once the body has adapted to fasting, but these were self-reported, unsupervised historical accounts, not controlled studies. Anyone considering an extended fast alongside significant physical activity should do so under medical supervision.

Why does walking feel easier than other exercise during a fast?

Walking is a low-intensity activity that relies mainly on fat oxidation, which remains abundant even deep into a fast. Higher-intensity exercise draws more heavily on glycogen, which becomes scarce after roughly a day without food — making sprinting or heavy lifting feel much harder than walking during an extended fast.

Does fasted walking burn more fat than walking after eating?

Fasted walking shifts the body toward fat oxidation for fuel more readily than walking after a meal, when insulin is elevated and fat burning is suppressed. Total fat loss over time depends on overall calorie balance, not any single walk, but many people find fasted walks a practical way to reinforce a fat-burning state.

Why did Sinclair's second fast feel so much easier than his first?

He attributed it to his body having already adapted to fasting once before. Modern science supports this: repeated exposure to fasting improves the efficiency of the metabolic switch from glucose to fat and ketones, which is why experienced fasters often report far less fatigue than beginners.

Should I try heavy exercise during an extended fast because Sinclair's cases walked so far?

No — Sinclair himself distinguished walking from heavy physical labor and consistently recommended against strenuous work during fasting. Long-distance walking at a comfortable pace is very different from intense exertion, which places far greater demands on glycogen and recovery.

Related Articles

• Can you exercise during a fast? What historical cases and modern research say
• Walking while fasting: why low-intensity movement helps
• The case for light exercise during a fast (not rest)

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.