Carb diets are not really healthy. To have a healthy balanced diet you need a mixture of protein, carbs and fats in your diet.
Carbohydrate is the body's primary energy source. It is not only important to provide fuel for the working muscles, but is also essential for the central nervous system. As the brain uses glucose as its energy source, a lack of this can result in weakness, dizziness and low blood sugar. Reduced blood sugar during exercise decreases performance and can lead to mental and physical fatigue and low blood sugar levels across the day result in poor energy and heightened cravings for sugary foods.
Ideally, each meal in the day should contain low Gl carbohydrates, avoiding more sugary carbohydrates that can result in inflammation in the body and lead to fluctuating energy levels and further sugar cravings.
Fast weight loss tends to result in a rebound effect, which is where calorie intake dramatically increases and weight goes up. Normally, when people try to lose weight too quickly calorie intake is very low. Unfortunately, this is very difficult to sustain and people gain weight very quickly afterwards. The low calorie intake can also slow down the metabolism, making further weight loss harder.
I ripped up for 4 days before a competiton and ate nothing but protein, believe me it was horrible.