Why is rapid weight gain/loss yo yo dangerous?

Lose muscle/gain fat: First, some weight variation is normal, particularly for women with working ovaries. Varying weights within a 10 pound range is not alarming. More than this, however, and it is likely that a person is losing mainly muscle and gaining mostly fat. With each cycle, the metabolism slows a little bit more making it likely the person doesn't get as low as before, and goes up a little higher each time.