Verdict: In moderation
Plain bread is generally not toxic to cats but offers no nutritional value and can lead to obesity or gastrointestinal upset if consumed in large quantities. Raw yeast dough, however, is dangerous as it can expand in the stomach and produce alcohol, causing serious health issues. Always avoid bread containing toxic ingredients like garlic, onion, or xylitol.
Yeast dough ferments in the warm, moist stomach, producing ethanol which causes alcohol poisoning, and carbon dioxide gas which leads to bloat. Xylitol causes a rapid release of insulin, leading to hypoglycemia. Sulfides cause oxidative damage to red blood cells.
Ethanol (from yeast fermentation), Xylitol, Sulfides (from garlic/onion)