Mario Anzuoni/Reuters Undereating is just as bad as overeating. Your body needs fuel from food to function. Being cold all the time, feeling sluggish, and having mood swings are all signs that you may not be eating enough even if you think you are. Weight loss plateauing, an irregular menstrual cycle, and feeling unwell are also signals that you might not be eating enough. In general, we spend more time talking about overeating than undereating.