kelloggs recommend subbing one meal a day with a bowl of cereal for 2 weeks to drop a jeans size. basically instead of having a bowl of cereal and 2 "normal" meals, you have 2 bowls of cereal, and 1 normal meal, as long as it's a well balanced meal and you still make sure you get 5 portions of fruit/veg.
However it's only sensible if your BMI is over 25, which yours is almost certainly not, so try doing it on occasional days, or for one week, and see if that helps. I do it for a few days before a big party or a holiday if I'm feeling a bit flabby, along with some gentle toning excercises, and it works for me. But make sure you dont do it for too long, and make sure your third meal is a good one.