This is something that I do, and I'm told it's pretty unhealthy. You need to be eating more regularly than that, and 700 calories a day isn't enough. The recommended daily amount for women is 2000, so you're under half. It really depends on your body and your metabolism though. It would be worth seeing your GP for some more information. They'd be able to tell you if you need to be eating more, or more often, or if you're sustaining yourself healthily already