The Three Day Diet is a very low calorie diet where you only consume around 1000 calories per day for 3 days from foods like coffee, toast, tuna, vegetables and lean meat. While you can lose up to 10 pounds in 3 days, the diet is not healthy long term as it slows your metabolism and causes weight to be rapidly regained once normal eating resumes. The authors believe the diet could work if calories are moderately reduced with healthy foods rather than severely restricted.