Big NO NO: any bars that contain seeds, nuts, dried fruit, dairy, sugar/honey and fat. Same goes for yoghurt drops and similar shop treats. If you want to buy treats, go for dried herbs or forages, but give them only in small quantities. Dried plants are higher in calcium.
Foods that should not be fed at all:
Potatoes - too starchy.
All bulb plants including the green bits of leeks and chives.
Mushrooms.
The leaves, stems and seeds of all nightshade plants like tomato and (bell) peppers.
All other pulses except mangetout or green beans , like French beans.
Some varieties of avocado can be highly poisonous to guinea pigs.
Exotic fruit can cause diarrhea and severe tummy upset, so never feed in any quantities.
Avoid highly "bloaty" cabbages like white and red cabbage as well as Brussels sprouts. Other, less bloating cabbages, can be fed every few days if introduced slowly and as part of a mix of veg, so piggies never eat them in big quantities. These are broccoli, sweetheart/pointed cabbage, savoy cabbage, the green leaves of cauliflower. If you have a piggy with a delicate digestive system, leave all cabbages out.
Foods that should be fed only very occasionally:
spinach (very high in calcium), kale, the outer leaves of brussels sprouts.
Only every few days:
Fruit like apple, pair, seedless grapes, melon, strawberries (including tops), small bits of organic banana (with skin).
Carrot and Romaine lettuce (high vitamin A content that builds up in the liver and can cause liver failure later in life).
Tomatoes
Green beans
Sweetcorn (with leaves and husks)
Parsley
Spring greens
Veg high vitamin C veg: (bell) peppers of all colours - one slice, fresh parsley and dill, broccoli. Ideally, on eof these should be part of the daily mix of veg.
This list is not comprehensive.
If anybody is interested in more information about specific foods and what they contain, look up the food chart on Guinea Lynx:
http://www.guinealynx.info/chart.html