How does she seem to talk about herself? Does she mention that she is fat or feels fat often? Are you concerned about an eating disorder or do you think it's not that?
I agree, she probably needs to understand nutrition- I think all kids need to learn what foods job is and why we even eat, that may help a bit.
If it's not something like an eating disorder... does she ever help prepare meals? My kids have to prepare one meal a week, in the past they have had to go to the store, purchase all the ingredients (with a budget) and come home and make it- it's amazing how kids will eat better knowing the purpose of food, and also when they have their hands in preparing it.
I wouldn't hesitate too, to call her pediatrician, give them the details of your concerns and have her go in for a check up. They should be able to identify what the cause is- if it's an eating disorder, or just not being that 'into' food.
Please don't take this the wrong way, but is she bright? Because she obviously has no grasp at all on nutrition and what she's doing to herself long-term. If she's otherwise a bright woman, this would indicate either she's just one of those people who don't like food (I dated one in college, she didn't have a reason for not eating other than she really didn't care for food all that much, which to me was just weird but there you have it -- you wouldn't find a better balanced person emotionally) or she has body issues that therapy would address. I would sit her down with some materials on nutrition and what happens to the body long-term if you don't eat well -- she's young and can survive now but it will get her at some point and if it's not a sign of a mental disorder, then as her Mom you just need to find a way to teach her what you should have taught her when it started when she was 12 -- we are literally what we eat but it often doesn't show up until we're older and it's too late to fix it. That's what she needs to learn if it's not a body image problem.