My wife followed a variation of the potato diet, eating only potatoes for 3-4 days each week, and losing several kg in bodyweight. She was interested in doing it after I mentioned the results from several others, who lost over 100lbs from following this diet.
The basic rules are that you can only eat plain potatoes on the designated potato days, with some seasoning but no condiments e.g. ketchup. You’re also restricted to black coffee, tea or water. I was concerned about the lack of protein and certainly the lack of fat, but in the spirit of the diet I only requested she eat an additional half can of tuna and either have a glass of kefir or add coleslaw for the fats.
She lost weight but as you see in the video more than half of what she lost is lean body mass, which in the long run means she will burn fewer calories each day and be more susceptible to long term weight gain. The use of bodyweight squats were also insufficient at her weight, and she would have needed to do several resistance exercises to better preserve lean tissue.
In general the less fat you have to lose the more you have to pay attention to diet and exercise. The success stories of the potato diet revolve around morbidly obese people who have large amounts of weight to lose. My wife was 65.3kg when she started this, and at 177cm it would be unhealthy for her to weigh any less than 58kg.
Please consider your long term health when deciding which eating plan to follow, and be sure to get enough protein and go to the gym. As I do not want to encourage anyone to attempt this diet I have omitted information about the recipes she used and any before and after pictures.