[Gpdd] CARE - Why shouldn't you feed guinea pigs beans?

[a.mills at portset.net]

I have read several times on the GPDD that you shouldn't give guinea pigs any types of beans and was just wondering why this was?  Dad grows runner beans and when they're in season we've always given our rabbits and guinea pigs some without any apparent ill effects.  We have also on occasions given them bean sprouts - don't know exactly what bean sprouts are but they're what people use a lot when cooking Chinese food, mum uses them in stir fry.  I'd just like to know why it's not a good idea for them to have beans.  Obviously I wouldn't want to give them something that might harm them so no runner beans for them this year!