Food can be great at Glastonbury. Usually the veggie stuff and around the green fields can be pretty healthy. There are some places where I just think use your common sense - pasta vans serving pasta with tomato based sauces, morrocan style lamb with cous cous, some of the chinese and veggie places can be a good bet. Good old jacket spud with beans or HEX cheese.
As previous poster said, if you are bringing food with you then cans and a camping stove - that also works for the usual staples (mugshots, pasta and sauce, ainsley couscous etc)
For me though, Glastonbury is a holiday and a giant party (I'll be celebrating even more since I only just got a ticket in the resale so have only known I'm going for a few weeks - was gutted when we missed out on original T-Day). For me that means no campsite cooking - there's tons of lush food that I want to enjoy. I will take some fruit and HEX cereal bars for snacks but that's it. I will mix up my food options, sometimes I will go for a comparatively healthy choice, but if i really want something unhealthy I will have it.
There's no way I'm going to Glastonbury without getting a Pieminister pie (and I live in Bristol so there's stockists galore round here, but still HAVE to have one at Glasto)
Drinks wise - you can bring spirits (decanted into plastic bottles of course) mixed with diet coke/lemonade etc. Otherwise, all drinks will be pretty high syn.
If you aren't normally that active, remember that Glastonbury is a huge site, so you will be walking loads which means loads of useful exercise and body magic.
And hopefully it will be a scorcher so you will be naturally drinking tons of water which can help with your losses.
Last year at my post Glastonbury weigh in I lost 2lbs!!