Fad diets are an unhealthy way to lose weight. Most fad diets are based on personal testimony, not scientific research though they may try to make you believe the contrary. These diets have not been approved nor regulated so you follow them at your own risk. Any diet on which you eat fewer calories than you need to get through the day is dangerous. Calories create fuel for your body without enough fuel your body will eventually run on empty (www.about-obesity.com).
A person may experience symptoms of this with a headache, feeling of fatigue, dizziness, nausea, ...