Do you mean that your calorie intake for one meal is 500-700, or that is your total for the whole day? If it's regularly what you eat for the day, you are not eating enough. There are others here who can explain better than I can as to the biology of why.
I also think that mindset is just as important and trying to lose weight "really fast" might be detrimental too. Think of being healthy and weight-loss as a by-product.
I have been losing while eating 1500 to 2000 calories, with at least 50 to 60% of those calories coming from fat- including full fat 35% cream, cheese, coconut oil, avocados, fat on my meat, eggs, butter, etc. If you like vegetable stock buy one with better ingredients. I sometimes buy Pacific Foods Organic broth and their vegetable broth is very tasty.
Female, age 51, 5' 9"
SW - 183 (Jan 22, 2012), CW - 159, GW - healthy.
Met my 2012 goals by losing 24 pounds.
2013 goals are to get fit and strong!