the feeling of "胃部空空如也" is an indirect result of 血糖低in other words, you cannot feel "血糖低", but low blood sugar level can stimulate a long chain of mechanisms which finally result in stomach contracting and digestive juice secretion. the feeling of "胃部空空如也" is stomach contracting & digestive juices secretion. So, a glass of water will dilute digestive juice and transiently inhibit stomach contracting which makes you feel not so hungry for a moment. However, if the long chain of mechanisms which links "血糖低" and feeling of "胃部空空如也" is somehow blocked, one will not feel hungry even if blood sugar level is very low; by contrast, if the long chain of mechanisms will be somehow stimulated without the signal of "血糖低", one will still feel hungry even if his blood sugar level is very high