A lot of fad diets sell a lot of books by this simple trick: they have the dieter cut out carbohydrates. When the body stores starch, the starch granules are surrounded by water molecules. When the dieter stops eating starch, the body takes the granules out of storage to use for fuel and urinates the water.

Water weighs a lot, so in the first week or ten days of the diet, the dieter sees a wonderful and encouraging loss of weight. S/he tells friends, who all buy the book too.

Some fad diets obscure this process by putting emphasis on what is eaten, rather than what is not eaten. I remember the Beverly Hills diet, which IIRC correctly, had dieters eating lots of pineapple. Everybody talked about the pineapple and about Beverly Hills, and nobody noticed the missing carbs.

The keto diet sounds like the same thing, only with its high fat and protein, more risky than most.

Some diets have you drink a lot of water, so you won't wonder why you are urinating so much.