Detox and juice cleanses

The claim, as heard: "A three-day juice cleanse flushes the toxins out and you drop three kilos."

What the data show. Two separate questions hide in this. On toxins: the critical review of detox diets finds that the products and programmes almost never say which toxins they remove, that none has been shown to remove any, and that the body's liver and kidneys already do this work without a juice. On the kilos: three days of juice is three days of very little energy and almost no protein, salt or fibre — so glycogen is used up and its water goes with it (about three grams of water per gram of glycogen), gut contents empty, and the scale drops two or three kilograms of which essentially none is fat. It returns in the first days of eating normally, which the programme calls "toxins coming back" and the physiology calls glycogen.

Verdict: false as stated. Evidence grade C: a critical review that finds an absence of evidence, plus physiological context for the water loss; no trial has tested the claim because there is no defined claim to test.

The grain of truth. A few days of vegetables and fruit are not harmful for a healthy adult, and people sometimes use them as a reset before changing habits. That is a ritual, and rituals can help. It is not detoxification, and the kilos are borrowed.

One sentence to remember: what a cleanse removes is water and glycogen, and both come back with the first proper meal.

The scale page explains the water arithmetic in full.