Why the scale moved
A diet built around roughly 500 calories a day creates a large energy deficit: the body takes in far less energy than it uses. To make up the difference, it draws on stored fuel, so weight can fall quickly. Some early loss may also be water, and prolonged severe restriction can cost the body muscle as well as fat.
That arithmetic—not a special property of grapefruit—explains the diet’s appeal. Grapefruit is relatively low in calories, but eating it before meals does not make the body burn fat through a citrus-triggered enzyme. The fruit was a memorable rule attached to a much more consequential restriction.
The ritual made the rule easy to remember
A fixed instruction—eat grapefruit before every meal—gave the plan a simple routine and a recognizable identity. It could make the diet feel as though a particular food was doing the work, even though the main driver of weight loss was eating very little overall.
The distinction matters: a dramatic result from a restrictive plan is not evidence that its signature food caused it. In this case, grapefruit supplied the ritual; the severe calorie shortfall supplied the mechanism. The same shortfall also made the plan hard to sustain and potentially unsafe without medical supervision.
