Before You Begin

This book is for education and general information. It is not medical, psychological or nutritional advice, it does not diagnose or treat any condition, and reading it does not create a relationship with any clinician. The author is a writer, not a physician or licensed health professional. What follows summarizes published research as the author understands it.

Talk to a qualified clinician before you change anything important, and especially before starting a new exercise, diet, fasting, supplement, cold or heat exposure, or breathing practice, if any of these apply to you: you are pregnant, trying to conceive or breastfeeding; you have heart disease, high blood pressure, diabetes, kidney or liver disease, a seizure disorder, an eating disorder or a history of one, or any other chronic illness; you take prescription medicines; you are over 65 or new to exercise; or you have symptoms that have not been explained.

Never start, stop or change a medicine because of this book. That includes contraception, hormone therapy, antidepressants and other psychiatric medicines, opioids and medicines used to treat addiction, and medicines for blood pressure, blood sugar or weight. Advice about children is general; follow your child's own clinician.

In an emergency, get help first. Chest pain, signs of a stroke, severe shortness of breath, or thoughts of harming yourself need immediate care: call your local emergency number (911 in the United States). In the US you can call or text 988 to reach the Suicide and Crisis Lifeline.

The evidence grades are judgments, not guidelines. The marks ●●●, ●●○ and ●○○ are the author's summary of how strong the published evidence is. They are not clinical practice guidelines, and they cannot tell you what is right for your body. Research, screening schedules, vaccine recommendations and drug approvals change; the information here was checked in September 2026, and some of it will be out of date by the time you read it. Check current guidance with a clinician.

Results vary. People differ in their genes, health, medicines and circumstances, and nothing in this book guarantees any result. Dana, Marcus, Priya and Tom, who appear throughout, are composites created for illustration, not real patients, and nothing that happens to them is evidence.

No sponsors. The author has no financial relationship with any company whose products, drugs or devices are mentioned. Brand and drug names appear only to identify them, not to endorse them.

The author and publisher disclaim liability for any loss or harm arising from the use or application of anything in this book.