In the short term, most people who go on low carb diets do lose weight and they lose it very quickly. However, the majority of weight loss comes from loss of water and muscle tissue, not fat which is important to lose to keep the weight off.

Also, while trying to lose weight permanently, losing precious lean muscle tissue is like sabotaging the body. Muscle tissue is metabolically active, and burns calories even when at rest. A decrease in the amount of muscle tissue will lead to a decrease in the number of calories needed each day to maintain weight, making it much harder to keep weight under control when the diet is stopped.