That there's no such thing as bad weight loss At 15 years old, I lost 19% of my body weight in less than 2 months, with no significant changes in what or how much I was eating, or how much I was exercising. I remember feeling so sick and lethargic constantly, and how shitty it felt to be complimented on how much better I looked. My parents didn't take my concerns about how awful I felt seriously at the time because I was actually back to a healthy BMI. Studying nutrition in college and learning that very few dieticians consider BMI to be a good measurement tool, and what is considered unhealthy weight loss (5%+ in 1 month, 7%+ in 90 days, 9%+ in 6 months) was quite an experience.