Efficacy of the diet
This is generally the first question asked by majority of people before starting the diet. As the case is with the cabbage soup diet, it is a temporary diet. Weight loss is achieved in a short period, but this diet cannot be continued for a longer duration due to absence of nutrients and calories.
In the short term most people would lose weight very quickly. However, the loss of weight will come from loss of water and muscle tissue not from fat reserves. The combination of foods "allowed" would force most people to nearly starve each day. This could actually be counter-productive, forcing the body into starvation mode will slow metabolism and encourage the body to hang on to fat reserves!
Talking about the nutritional value of the diet, the ingredients of the cabbage soup, in themselves, are mostly healthy - though if stock is used as well as soup mix the finished soup could be quite high in salt. Overall the cabbage diet is deficient in protein, carbohydrate and essential fats. Since the diet is only supposed to be followed for seven days, this shouldn't cause people in good health any long-term problems, but people, who have special dietary needs, including diabetics, should definitely consult a doctor before starting the cabbage soup diet. No one should follow the diet for more than seven days.