My girlfriend eats one meal a day or not at all?

Some days she doesn't eat anything all day, every other day she will eat like a sandwhich all day. I'm actually really worried about her because health indications are showing. She feels ill a lot, she gets headaches when she doesn't eat. And her stomach hurts but she claims she isn't hungry. I feel like these are all indications that her body is begging for more nutrients. She is already vitamin D deficient and won't take vitamins unless I 'make' her. She won't listen to me either, she will just roll her eyes or say I'm not hungry but I'm actually truly worried about her..
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Update:

Scientists say that by doing nothing all day the amount of calories your body needs to just exist is 1500. I can tell you that my girlfriend takes no where near this daily. And she is active running around at her job..
My girlfriend eats one meal a day or not at all?
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