You might want to check out
this page I found via
GuineaLynx which shares one person's experience with diabetic pigs. What she said about feeding was:
Diet Changes:
Most will recommend eliminating sweets such as fruit and sweetened treats from your diabetic piggies diet. Equally important is to eliminate fats - corn and seeds are very high in fat and should be avoided. There was some research done at CCT [Cavy Cambridge Trust in the UK] that suggests food dyes can "cause" diabetes also, so make sure that your piggies food contains no artificial colours. But the most important dietary consideration is promoting consumption of lots of high-fiber hay. I had to reduce the amount of green food I was offering and instead "fluff up" their hay pile when the girls wheeked, and this has had the strongest effect in helping to regulate their glucose levels.