Julia Hartley-Brewer clashed with obesity campaigner Kathryn Szrodecki over whether being both overweight and healthy is possible.
A study, as yet unpublished and conducted by researchers at the University of Birmingham, claims the idea that people can be fat but medically fit is a myth.
Szrodecki is a fitness instructor despite her own obesity, and preaches to clients that they should be comfortable with their shape rather than desperately trying to lose weight.
She believes reports like this damage people's mental health and are abusive, as many young people obsess about their weight.
When Julia argued that the study is based on medical research, and this research is saying that being overweight could leave to early death, Szrodecki argued that the study is not proven and shouldn't be taken seriously.
This prompted Julia to ask whether further evidence could ever pursuade her interviewee to change her mind, and start telling clients they must lose weight.
Listen to the full interview above