Carl Heneghan, an evidence-based medicine doctor at Oxford University in the UK, looked at the long-term evidence for Weight Watchers. He found that only eight percent of participants reach their goal and keep their weight off after five years.

These results square with what we know about maintaining weight loss: that it's incredibly difficult and only tiny minority are able to stay slim.

Correction: A previous version of this article stated that one percent of people maintained their weight loss after five years.