Yes, it is dangerous to not eat enough. You actually slow your metabolism down, while actually storing fat and losing muscle (opposite of what you want to do). Also, your body doesn't receive all the necessary nutrients that it needs, so you are missing out on those as well as the energy needed for your body to perform the daily functions (heart beating, muscles contracting, etc). Here's an Ask the Expert Q&A that addresses this as well: www.sparkpeople.com/community/ask_the_expe
. Also, as this happens, your body conserves energy and doesn't actually send out the proper hunger signals, which is likely what you are seeing happen when you say you are completely full at such a low amount of calories.
If you have trouble meeting your nutritional needs, then please take a look at this article for some easy tips on increasing your calories: www.sparkpeople.com/resource/nutrition_art