We did some research. And here are the results.

We asked scientists at a leading university to test the Special K
Challenge. They looked at how eating a bowl of Special K for breakfast
and either lunch OR dinner for a maximum of 2 weeks affected
body shape and fat percentage.

In the study, overweight volunteers got to select any cereal from the
Kellogg’s Special K range. Fruits and vegetables were advised as
alternatives to more indulgent snacks. Their 3rd meal was nutritionally
balanced following healthy eating principles such as being low in fat
and high in fibre; containing plenty of vegetables; plus including a source of
protein such as meat, fish or other alternatives.

At the end of 2 weeks, up to 75% of the volunteers had slimmer waists
and hips. The university’s scientists concluded that the majority of this
was due to fat loss. However, please keep in mind that the Challenge isn’t
an eating plan meant for long-term use; it’s simply a 2-week kick-start to a
healthier lifestyle.