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Preface: Does an Empty Stomach Clear the Mind?

5D Library › Fasting, Ketones and the Metabolic Basis of Attention · 中文

Preface: Does an Empty Stomach Clear the Mind?

Many traditions hold that going without food sharpens the mind. Daoist adepts practised bigu (辟谷), “avoiding grains”. Buddhist monks and nuns take no solid food after midday. People who fast for religious reasons, and a growing number who skip breakfast for other reasons, often say that the hours before the first meal are the clearest of the day. Others say the opposite: that hunger makes them slow, irritable and unable to think of anything but food.

This book asks what the biology of fasting can and cannot say about that experience. It follows the body through the hours and days after a last meal: the liver’s store of glycogen running down, fat being released, the liver turning fatty acids into ketone bodies, and the brain, which uses about a fifth of the body’s energy at rest, gradually switching part of its fuel from glucose to ketones. Along the way it looks at four ideas that have made fasting famous:

  1. Autophagy. Yoshinori Ohsumi’s work on how yeast cells digest parts of themselves when starved, which won the 2016 Nobel Prize in Physiology or Medicine.
  2. The fuel switch. George Cahill’s studies of starving people, which showed how the body protects its protein and how the brain comes to run largely on ketones.
  3. The thermodynamics of ketones. Richard Veech’s argument that ketones are a more efficient fuel than glucose, which this book checks with a short calculation and partly corrects.
  4. NAD+ and sirtuins. The redox coenzyme that links fuel to signalling, and the enzymes that depend on it.

It then puts numbers on the most common fasting schedules (16:8, one meal a day, and a 48-hour fast) with a small model written in Python, reads the classical Chinese and Buddhist sources on fasting, and ends with a four-week observation log and a pre-registered trial that asks the question directly: does a 16:8 schedule change attention?

Note

Medical notice. Please read before anything else. This book is general scientific information. It is not medical advice, it does not recommend that any particular person fast, and it does not replace a doctor, a dietitian or a pharmacist. Fasting changes blood sugar, blood pressure, fluid and salt balance and the way medicines act.

Do not fast without medical supervision if you:

  • have diabetes and take insulin, a sulfonylurea or any other medicine that lowers blood sugar;
  • take an SGLT2 inhibitor (for example dapagliflozin, empagliflozin or canagliflozin), which can cause ketoacidosis even when blood sugar is normal;
  • are pregnant, trying to become pregnant or breastfeeding;
  • are under 18;
  • have, or have had, an eating disorder, or are underweight;
  • have kidney disease, liver disease, gout, heart disease or low blood pressure, or take medicines for blood pressure, heart rhythm or mood;
  • are frail, elderly, recovering from illness or surgery, or have been told by a doctor not to fast.

Stop a fast and eat if you feel faint, dizzy, confused, very weak or unwell, or notice palpitations; seek medical help if these symptoms do not settle quickly. Fasts longer than 24 hours, and anything longer than the 16:8 schedule in Chapter 8, are described in this book as science, not offered as practice.

Note

This book does not set weight targets, does not recommend any diet, product or supplement, and gives no supplement doses. If counting hours without food starts to feel like a test you must pass, or you notice guilt, fear or preoccupation around eating, stop the practice and talk to a doctor or someone you trust.

How to read the boxes

As in the other books of the 5D Library, different kinds of statement are kept apart.

Evidence

Evidence boxes report published findings, with their sources and, where it matters, with the kind of study (cell, animal, or human; observational or randomised).

Calculation

Calculation boxes give formulas and short Python programs. Every program runs as printed with a standard Python 3 installation, and its output is shown underneath it.

Hypothesis · not established

Hypothesis boxes hold the author’s ideas. They are labelled “not established”, and they say what result would show them to be wrong.

Source text

Source text boxes quote the classical texts, with the author’s translation.

Note

Note boxes give cautions and explain choices.

Three warnings about the evidence

Fasting research has three habits that make popular claims run ahead of the data, and this book tries to resist all three.

First, much of the most striking evidence comes from yeast, worms and mice. A mouse burns energy per gram of body about seven times faster than a person (Chapter 2 does the arithmetic), so a one-day fast for a mouse is not a one-day fast for a human. Second, many human studies measure a marker, such as a gene’s expression in blood, rather than the process itself; autophagy in human brain tissue, in particular, has not been measured in living people who fast. Third, when fasting schedules are compared in randomised trials with the same amount of food, their advantages often shrink. Each chapter says which kind of evidence it is using.

The plan of the book

  1. The fuel switch. Cahill’s curve: what the body burns in the first hours, days and weeks without food, and why the brain’s switch to ketones protects muscle.
  2. Autophagy. What Ohsumi found in yeast, what is known in mice, and what is and is not known in people.
  3. Ketones and the brain. How β-hydroxybutyrate crosses into the brain, how much of the brain’s fuel it can supply, and its second life as a signalling molecule.
  4. The thermodynamics of ketones. ATP per molecule of oxygen, the free energy of ATP, and what Veech’s efficiency argument actually rests on.
  5. NAD+, redox and sirtuins. Why the ratio of NAD+ to NADH matters, how fasting and ketones move it, and what the human trials of NAD+ precursors have and have not shown.
  6. Schedules in a model. A Python model of liver glycogen and ketones under 12:12, 16:8, one meal a day and a weekly 48-hour fast, and what the published trials say.
  7. Bigu and not eating after noon. The classical record, from the Zhuangzi and the Inner Classic to the Mawangdui manuscripts and the monastic rule, and “the fasting of the heart”.
  8. Practice. A four-week log for observing your own metabolic switch, with stop rules.
  9. A pre-registered study. A randomised crossover trial of 16:8 eating and attention, with its analysis and decision rules fixed in advance.

This edition

This is version 1.0.0, the first complete edition: a preface and nine chapters, in Chinese and English. Corrections and references the author has missed are welcome through the 5D Library website.

Disclosure

The author and the group that includes 5D Library operate ketohackers.com and nadplusnadplus.com, websites about ketogenic eating and NAD+, and therefore have a direct commercial interest in the subjects of this book. The book names no brand or product, recommends no supplement and gives no doses. Every model and the pre-registered study protocol in this book are published openly for anyone to test.

Fan, Y. (2026). Fasting, Ketones and the Metabolic Basis of Attention: Autophagy, β-Hydroxybutyrate and the Bioenergetics of Intermittent Fasting [断食、酮体与注意力的代谢基础] (Version 1.0.0). 5D Library. https://doi.org/10.5281/zenodo.23684203 · Licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0).