Such a common issue. So many threads on this topic already, but each has its own spin. Each person has a different and unique scenario to work through.
First of all, great job on speaking up and asking what others think. Sometimes getting multiple perspectives really helps you understand how you can best help the situation for both you and your girlfriend.
I'm all over the comments here already, but I wanted to give my own fresh thoughts.
Is it best to let your loved ones do whatever they want? Does it matter if they are adults? Perhaps throw some comments out and she how she responds? Perhaps drop clues and hints and try to nudge her into more acute awareness of how her decisions may not be best for long term health? Is the 'heavy hand' approach applicable here? Is that too controlling? What if she feels judged?
When a person feels the need to give a loved one some advice, it can often be one of the most challenging and just downright difficult situations to work through. I think at the base of this is the human nature of feeling judged and controlled, combined with the need that most people have to feel accepted and to be heard. This can span across so many things that it's dizzying.
I have several of these type of scenarios going on in my life right now. How do I tell me mother-in-law that I don't want to eat the food she wants to cook for us? With some of the things she grew up eating... she thinks is good because well... "I'm still alive". is what she says. It's sort of a long term battle at the moment, but progess is being made all around. She is in our life forever, so we are choosing to massage the situation slowly, leading by example, showing her through continued practice that we do actually have some good insight to offer and that we're serious about it. Slowly but surely, she is beginning to understand.
When it comes to personal matters, it gets pretty dicey, which is most of the time. The topics that seem to spark the most controversy are religion, politics, sex, family, finance, and health.
In your case, I think you just care. I think you really love your girlfriend and want to continue to build a long term relationship, else it wouldn't make a lick of sense for you to come on here and seek help in this way. You'd have already 'dumped her' like Quilt says.
Badgering and pestering people almost never works, even with close family and friends. It violates too many aspects of human nature and people skills. Ever heard of the little ol' saying... "A man convinced against his will is of the same opinion still."?
Being judgemental or harsh with your approach also usually does not work, and can breed comtempt and jeopordize trust in the relationship. We are such tricky beings, us humans.
Personally, I have watched some of my closest people destroy their own health out of either ignorance, intentional poor decisions, or a combination of both. It's awful. It's awful enough to maybe make a person do something drastic out of love, like what Quilt describes.
Dakota - there's a lot of meat here to chew on from this community. I think leading by example, and explaining your concerns to her in an open, honest communicative way using whatever people skills you can muster will be your best approach. Then just keep the goal in mind and don't be dogmatic about what you think is best for her health. If, in all your efforts, you are seeing a bleak and discouraging road ahead of you, then you truly may want to consider the long term implications of a committed relationship when you believe that you may be at odds with each other on one of the key elements of daily life, health and wellness.