How can I stop my mom from always talking about her weight?

First off-my mom is healthy and we all eat well and work-out as a family. So when she complains about her weight, it's not that she's overweight, it's just that she has been more tone before and wants to get back to a toned body. However, when she is always nagging about her body I witness it and I start to do it too. I don't want to nag about my body when I feel ok with my body weight. So, how can I prevent my mom to stop nagging about her wieght when she is healthy and in pretty good shape compared to others in America?!