If you open the program with a fast, sixty hours is the target. This is what happens over those three days, why sixty is the number, what you can drink, and who should leave the fast out entirely.

Why sixty hours

At forty-eight hours you have done most of the work but stopped just before the point where your body fully turns to Fat is a macronutrient that provides a concentrated source of energy at 9 calories per gram.Full definition →. At seventy-two you have added a third night without food for very little extra benefit, and that third night is genuinely unpleasant. Sixty hours, three mornings, gives you the result with a level of difficulty you can manage.

What happens inside, day by day

Your body stores sugar as Glycogen is stored carbohydrate found in muscles and liver that the body uses for energy.Full definition →, in the muscles and liver, and it stores water alongside it. For roughly the first day to a day and a half it burns through that glycogen. As the glycogen empties, the water stored with it leaves too, which is why the scale can drop several kilos in the first two days. That early drop is water rather than fat, but it still means something: it is the sign your body is changing over. Once the glycogen is gone, your body turns to fat for fuel. You will know it is happening from a few signs: a metallic taste in the mouth, a dry mouth when you wake, a faint ringing in the ears, and a light, empty feeling in the stomach. None of these is dangerous. The hardest stretch for most is the second evening and night, when the early energy has gone and morning still seems far off. It passes.

What you can drink

This is a water fast, but it does not have to be only water, and a few additions make it far easier without changing the result: water still or sparkling, black coffee, plain or herbal tea, a slice of lemon or lime. Keep anything with calories out of it. The point is an empty fuel tank, and these keep the tank empty while making the days easier to sit through.

Who should skip it

A 60-hour fast is a strong start for the people it suits, and the wrong choice for some others. If you take medication, are pregnant or breastfeeding, have a history of disordered eating, or have a medical condition that food and blood sugar affect, speak to a doctor before fasting, or leave the fast out. The rest of the program, eating under your number and walking, works without it.