Before-and-after adverts, showing pictures of people who have lost weight or become fitter, feature in thousands of magazines. But how reliable are they, asks Justin Parkinson.

Wow, what a transformation. Two volunteers go from looking pale and unfit to tanned, toned and dynamic. Before and after photos show the benefits of a change in lifestyle - eating better, exercising more and, in many cases, taking dietary supplements to help the process along.