Most people begin a fast the same way: they simply stop eating and hope for the best. Then they are surprised when the experience is brutal and the results are underwhelming. The truth is that how you enter a fast largely determines how it goes, and the setup deserves as much attention as the fast itself. The single most powerful lever is what you eat in the day or two before you begin, and it is supported by clear evidence. In a 2025 crossover study, people who ate a low-carbohydrate, high-fat meal before a 24-hour fast reached fat-burning by around twelve hours, while people who ate a high-carbohydrate meal before the same fast did not reach it at all within the 24 hours (Nutrition and Metabolism, 2025). The meal before the fast decided the entire outcome. The mechanism is simple. A high-carbohydrate meal refills your glycogen, your stored sugar, so your body has more to burn through before it reaches the fat-burning switch. A low-carbohydrate, high-fat meal leaves those stores lower, so you cross into the useful part of the fast much sooner. In other words, the fast does not begin when you stop eating. It begins with the meals before you stop. Here is the setup, step by step. One to two days before your fast, begin shifting your meals toward fat and away from carbohydrate. This gradually drains your sugar stores ahead of time. On the day immediately before, keep it very low in carbohydrate, with little to no starch or sugar, moderate protein, and fat as your main fuel source. It is worth explaining the moderate protein point, because it is counterintuitive. Excess protein can be converted into glucose through gluconeogenesis, which can slow your entry into fat-burning. This is not a reason to fear protein in general. In the specific window right before a fast, though, keeping protein moderate rather than high helps you reach ketosis faster. Hydration is the other piece of preparation. Going into a fast already well-hydrated makes the entire experience easier and safer. Drink well in the day before, rather than trying to catch up once you have started. If the fat-loading approach is not practical for you, you can still fast normally. You will simply reach the fat-burning state later, typically around the usual 24 to 36 hour range rather than around twelve hours. The preparation is an optimization, not a requirement. When it comes to actually crossing into the fast, timing helps. Starting your fast after your last low-carbohydrate meal in the evening means you sleep through the first several hours. Sleeping through the early stretch is one of the easiest ways to make a fast feel manageable, because you reach the more comfortable, fat-adapted phase having barely registered the beginning. Expect the first morning to test you. Hunger tends to arrive in waves and then pass, and learning that it passes is part of the skill. It is a sensation, not an emergency. Black coffee and plain tea are generally fine during a fast and can help steady you, as long as they contain no calories. Anything with calories effectively restarts the clock on the metabolic switch. A useful thing to observe in yourself, especially early, is the nature of that hunger. Much of the early hunger in a fast is a passing wave tied to your usual eating schedule rather than a true physical need. Watching a single wave rise and fall without acting on it teaches you more about your own hunger than any amount of reading. Entered well, a fast is far more comfortable and far more effective. The preparation is where most of the difference is made, long before you feel your first pang of hunger.
Safety
This article is educational and not medical advice. Fasting is not appropriate for everyone, including people who are pregnant or breastfeeding, those with a history of disordered eating, people with type 1 diabetes, and anyone on medication that affects blood sugar. Talk to your doctor before beginning any fasting protocol.
References
Pre-fast carbohydrate crossover, Nutrition & Metabolism, 2025 (PMID 40234969).