The Slim Fast diet has been around for over two decades. This simply shows that the Slim Fast Diet is really a popular form of diet and it is definitely the most popular diet shake ever existed. The Slim Fast diet is actually a diet plan that aims to replace all meals and provide a balanced nutrition diet. This diet allows dieters to follow the regular 3 times a day meal, including snacks. Out of the two regular meals (breakfast and lunch), you would be required to use Slim Fast shakes as a replacement. Dinner would be the only meal, which consists of real food eaten in moderate proportions.