How to Fast Safely If You've Never Done It Before
Thousands of people decide to fast for the first time every day. Most of them go in with very little information — and that, according to Upton Sinclair, is the single biggest mistake you can make.
Writing in his 1911 book The Fasting Cure, Sinclair was emphatic: "No one should begin to fast until he has read up on the subject and convinced himself that it is the thing to do." Over a century later, that guidance holds up completely. Knowledge is what separates a safe, productive fast from an experience that leaves you dizzy, frightened, and convinced fasting isn't for you.
What This Means for a First-Timer
If you've never fasted before, your most important task is preparation — not willpower. The physical experience of fasting is manageable. The mental experience, if you're unprepared, can be alarming. Sinclair devoted an entire chapter to what he called "the fear of fasting," arguing that nervous terror during a fast is not just uncomfortable but genuinely harmful.
The good news: once you understand what to expect, the fear loses its grip.
Historical Context: Sinclair's Framework for Safe Fasting
When Sinclair conducted his first major fast — 12 days — he noted several things that surprised him. The first four days brought intense physical lassitude and dizziness when standing up. Yet his mind, he wrote, was unusually clear throughout. He read, wrote, and thought with a sharpness he hadn't experienced in years.
What he and the hundreds of correspondents who wrote to him after his magazine article was published all observed was the same pattern:
- Days 1–3: hardest. True hunger, possible weakness, some dizziness.
- Day 3 onward: hunger fades dramatically. Energy and mental clarity often improve.
- Beyond day 5: many people report feeling better than before they started.
The first-timers who struggled were almost always those who had read nothing, prepared nothing, and panicked at the first sign of discomfort.
The Three Most Common First-Time Fears (and the Facts)
Fear 1: "I'll be too weak to function"
Sinclair's survey of 277 fasting cases found that most people continued their daily work throughout — clerks, writers, and even one woman who worked at a sanatorium while fasting for 33 days. Intellectual work is typically unaffected from day 2–3 onward; heavy physical labour is more challenging and not recommended for long fasts.
Modern research supports this. Ketones — the fuel the body produces during fasting — are a highly efficient energy source that many people find supports better focus and steadier energy than glucose.
Fear 2: "I'll damage my heart or organs"
Sinclair addressed this directly: the body does not consume healthy tissue while there is anything else available. Stored fat, waste matter, and morbid tissue are metabolised first. The body has evolved specifically to survive extended periods without food.
The modern equivalent: fasting triggers autophagy — the body's cellular clean-up process — which recycles damaged proteins and malfunctioning cells. This is a repair mechanism, not a destructive one.
Fear 3: "Hunger will become unbearable"
After the initial phase, it doesn't. Sinclair noted that once the first 2–3 days pass, genuine hunger largely disappears. This is not willpower — it's biochemistry. As insulin falls and ketones rise, the hormonal signals that drive hunger quieten down. Many fasters report that the appetite suppression during a proper fast is more complete than anything they've experienced from dieting.
Practical Steps for a Safe First Fast
Step 1: Prepare your food environment before you start
Sinclair was clear that diet matters enormously before and after a fast. If you've been eating high amounts of sugar and starch, your insulin levels are elevated even when you stop eating — which makes hunger worse and the first days harder. Before attempting a fast of any length, spend a few days reducing sugar, bread, pasta, and packaged food. Replace them with protein, healthy fats, and vegetables.
This single step makes the first-time fast dramatically easier.
Step 2: Choose an appropriate first length
A first-time faster should not begin with a 7-day water fast. Start with a 14–16 hour fast: finish eating at 8pm, skip breakfast, and eat your first meal at 10am or noon the next day. This is long enough to experience the metabolic shift — but short enough that any discomfort is well within your ability to manage.
Extend gradually. Most people reach 16–18 hours within a week or two with very little difficulty.
Step 3: Water is not optional
Sinclair repeatedly emphasised this point more than almost any other. Drink large amounts of water throughout any fast. Hot water between meals was especially recommended (credit to Dr. Salisbury, whose work Sinclair frequently referenced). Failing to drink enough water was identified as the root cause of many reported fasting failures — headaches, weakness, and nausea that people attributed to the fast itself were often simply dehydration.
Practically: drink water, herbal teas, and black coffee during your fasting window. These don't break your fast and will smooth out most of the rough edges of early fasting.
Step 4: Manage your electrolytes
Modern fasting practitioners have added an important layer to Sinclair's water advice: electrolytes. When insulin drops during fasting, the kidneys excrete more sodium, which pulls potassium and magnesium out with it. This is the most common cause of headaches, dizziness, and muscle cramps during early fasting.
Add a pinch of sea salt to water during your fast. Eat foods rich in potassium and magnesium when you break your fast — avocados, leafy greens, and nuts are good sources.
Step 5: Break the fast correctly
Sinclair called the moment of breaking a fast "the most dangerous moment" — not because fasting itself is dangerous, but because eating too much too quickly after fasting stresses a digestive system that has been at rest. He recommended starting with small amounts of orange juice or grape juice, building slowly over 2–3 days for longer fasts.
For everyday 14–18 hour fasts, this isn't a significant concern. Break your fast with something light — a small salad, some eggs, a protein dish — rather than immediately sitting down to your largest meal of the day. Wait 20–30 minutes, then eat normally.
Step 6: Rest, but don't be inactive
Sinclair and many of his correspondents found that light movement — walking, gentle exercise — actually helped during fasting, particularly once the first couple of days passed. Long rest in a horizontal position can worsen the dizziness and lethargy of early fasting. Fresh air and movement keep blood circulating and reduce the sedentary inertia that makes the first days harder.
Step 7: Tell someone, or fast with company
Sinclair strongly recommended fasting with the guidance of someone who had done it before — not because fasting is medically dangerous for most people, but because an experienced, calm presence dramatically reduces the fear response that can otherwise spiral into genuine physical distress. If you can find a friend or family member who fasts, tell them what you're doing. If not, at minimum, have someone aware of what you're attempting.
Who Should Consult a Doctor First
Sinclair's enthusiasm for fasting was broad, but he was explicit about exceptions. Before beginning any fast, consult a healthcare professional if you:
- Are pregnant or breastfeeding
- Have type 1 or type 2 diabetes, especially if on insulin or blood sugar medication
- Have a history of eating disorders
- Have a serious heart condition
- Are significantly underweight
- Are on regular prescription medication
The Connection to Modern Science
What Sinclair described from personal experience and anecdote in 1911 has been confirmed in multiple areas by modern research. The disappearance of hunger after 2–3 days corresponds to the hormonal transition to ketosis. His observation that mental clarity improves during fasting aligns with research on BDNF (brain-derived neurotrophic factor), which rises during fasting. His insistence on water corresponds directly to what we now understand about electrolyte management.
He got a remarkable amount right — even if the mechanisms he proposed (his "fermentation theory") were not the actual biological explanation.
Book Callout
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Frequently Asked Questions
How long should my first fast be?
Start with 14–16 hours. This means finishing dinner by 8pm and not eating until 10am–12pm the next day. It's long enough to feel the metabolic shift into fat-burning without the challenges of longer fasts.
Will I feel terrible during my first fast?
You may feel some hunger and low energy in the first 2–3 days of adopting a fasting lifestyle. This usually passes. The severity is strongly linked to how much sugar and starch you were eating before starting — the cleaner your prior diet, the easier the transition.
Can I exercise during my first fast?
Light exercise — walking, gentle movement — is fine and often helpful. Avoid high-intensity training during your first 7–10 days of fasting. Once your body has adapted to burning fat for fuel, most people find fasted exercise works well.
What is the single most common mistake first-time fasters make?
Breaking the fast with too much food, too fast. After a fast, the digestive system is in a rested state. Starting with a light meal and eating slowly protects you from discomfort and teaches your body to respond well to food again.
Can I drink black coffee during my fast?
Yes. Black coffee — no milk, no sugar — does not meaningfully break a fast and can actually extend it by suppressing appetite. Sinclair would not have known about coffee's insulin-neutral properties, but modern evidence supports it. The same applies to herbal teas and plain sparkling water.
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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.
Cite as: Sinclair, U. (1911). The Fasting Cure. Mitchell Kennerley.