Get pupils to collect and analyse package labels from the food and drink they consume in one day. They should compile a table showing their consumption of natural sugars (like glucose and galactose) and artificial sweeteners and compare these with the recommended intake for their age group. This data is available in many biology and food technology books.
Ways to promote healthy eating could be discussed. See the British Heart Foundation and Obesity Task Force websites for some ideas (www.bhf.org.ukyoungpeople and www.iot.org.uk) On January 16, The TES had a special supplement called School Food and this contains useful dietary information, including one article on obesity, "Fat Chances".
The Fair Trade initiative has links with PSHE and citizenship. Oxfam's "Cut The Cost" campaign is relevant; try www.maketradefair.com for details.