Many Women Have Been On 'More Than 20' Diets

How are they working?

Badly.

Why? (e.g., see here, here, here and here)

Women are being warned about the dangers of yo-yo diets after a UK-wide survey revealed that more than one in five women have been on at least five diets and regained weight and many have been on at least 20 diets without keeping the pounds off.

The survey of more than 2,300 people, conducted by Slimming World and YouGov, reveals that 21 per cent of women have yo-yo dieted at least five times, 11 per cent have done it at least 10 times and six per cent have dieted and put the weight back on again more than 20 times.

Yo-yo dieting is explained here.