How to Prepare Your Body for a Fast Before You Start

Most people decide to fast on a Sunday night and start Monday morning with nothing but willpower. It usually doesn't go well. A little preparation in the days before you begin makes the difference between a fast that feels manageable and one that feels like a fight against your own body.

What Upton Sinclair Taught About Preparing to Fast

In The Fasting Cure (1911), Upton Sinclair was emphatic on one point above all others: "No one should begin to fast until he has read up on the subject and convinced himself that it is the thing to do." He wasn't talking about physical preparation so much as mental readiness — and he had good reason. Sinclair had watched fasters do well simply because they understood what was coming, and he had heard of others who panicked at the first pang of hunger and abandoned the process, or worse, worked themselves into genuine distress. His conclusion, drawn from the 277 cases he collected from readers, was that preparation is mostly about removing fear and ignorance before day one.

Prepare Your Mind First

Sinclair called fear "the first danger of fasting" — more dangerous, in his view, than hunger itself. He told the story of earthquake survivors who died of terror on the same timeline that calm fasters thrived on nothing but water. The lesson carries forward: before you fast, understand what a normal fast actually feels like. Real hunger typically fades by day two or three, not day one. Some fatigue, irritability, or a headache in the first 24–48 hours is common and expected, not a sign that something is wrong. Knowing this in advance stops you from panicking the first time your stomach growls.

Ease Into It With Food

Sinclair's fasters did best when they didn't go from a heavy, rich diet straight into zero food. If your typical week involves large meals, alcohol, or a lot of processed carbohydrate, spend two or three days beforehand eating lighter — more vegetables, moderate protein, less sugar and starch. This isn't about "detoxing" in some mystical sense; it simply means your blood sugar isn't swinging wildly when you stop eating, which makes the first day noticeably easier. Sinclair also noted that heavy pre-fast eating, thinking it would "store up" energy, tended to backfire — the body doesn't work that way.

Stock Up on Water

If Sinclair repeated one instruction more than any other, it was to drink generously. He identified inadequate water intake as the single biggest cause of failed or miserable fasts in the cases he studied. Before you start, get in the habit of drinking water regularly through the day, and plan to keep a bottle within reach once your fast begins. Hot water between meals was one of his specific recommendations, borrowed from Dr. Salisbury, and it remains a simple, practical habit today.

Set Up Your Environment

Decide in advance what you'll do during the hours you'd normally spend eating. Sinclair's fasters who struggled most were often the ones without a plan for their day — sitting around thinking about food. Clear tempting food out of sight, plan lighter activities for the first couple of days, and if possible, arrange a calm environment rather than a high-stress one. If you want a fuller walkthrough of setting up your physical space, see our guide on setting up your fasting environment for success.

Check In With Your Health

Sinclair's book predates modern medicine by over a century, and this is one area where current guidance should take priority over his. Before any extended fast, especially anything beyond 24 hours, it's worth reviewing your current medications, any chronic conditions, and whether you're pregnant, breastfeeding, underweight, or have a history of disordered eating — all of which call for medical supervision rather than a solo attempt. A short conversation with your doctor beforehand costs little and can prevent real problems.

Find Support If You Can

Sinclair repeatedly praised the value of fasting alongside someone experienced, or at least someone who wouldn't panic on your behalf. Worried friends and family can unintentionally undermine a fast just by their anxiety. If you can find a companion who has fasted before, or even just a supportive person who understands what you're doing, it tends to make the process calmer.

Where Modern Science Agrees

Contemporary research on time-restricted eating and intermittent fasting supports much of Sinclair's practical instinct, even if the underlying theory has changed. Studies on fasting adherence consistently find that people who understand what to expect — mild early fatigue, appetite changes that plateau rather than build, and the importance of hydration and electrolytes — report far less difficulty than those who go in blind. Preparation doesn't eliminate the adjustment period, but it does make it predictable, and predictable is much easier to tolerate than mysterious.

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FAQ

Do I need to eat a big meal the night before I fast? No. Sinclair's own cases suggest the opposite — a heavy final meal doesn't provide extra reserves and often just makes the first day feel heavier. A normal or slightly lighter meal is better preparation.

How many days of preparation do I actually need? Two to three days of lighter eating and consistent water intake is enough for a beginner's 16–24 hour fast. Longer fasts benefit from a longer, gentler run-up.

Is it normal to feel anxious before my first fast? Yes, and Sinclair considered this the main obstacle to address before starting. Reading about what to expect, as you're doing now, is itself part of the preparation.

Should I stop caffeine before I start fasting? Not necessarily, but if you rely on it heavily, tapering slightly beforehand can reduce the chance of a caffeine-withdrawal headache compounding early fasting fatigue.

What if I get a headache on day one? Mild headaches in the first 24 hours are common, often related to reduced sugar intake or mild dehydration. Increase water intake and consider a pinch of salt in it; it typically resolves as the fast continues.

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.

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

How to Prepare Your Body for a Fast Before You Start