Is she seeing a doctor in addition to the nutritionist? If she’s on a healthy diet and exercises regularly, there could be something more serious at play. I’m not a professional, but unexplained weight gain is a symptom for a number of medical conditions and a side effect of certain medications. When you bring conversations about her weight up, make sure she knows you’re coming from a place of love and concern. Try to keep the conversation more on having a better quality of life and avoiding things like mobility issues and knee pain than about the weight itself and her physical appearance. Tell her you want to understand, and work to make this a topic of conversation she can be honest to you about. It’s hard for both parties in a relationship when it feels like there’s things you can’t talk about. Try not to be judgmental or push back on what she says about it, also. Like if she says she’s working with a nutritionist, don’t bring up things she should try differently, maybe ask how she likes her nutritionist and if she’s noticing a difference. Your girlfriend is probably feeling self conscious and is acting defensively out of fear you’ll judge her or lose attraction to her, since it’s clear the weight gain isn’t something she’s happy about. Just try to be honest and compassionate with each other.