Boiled egg white is safe — raw white is the problem
Plain boiled egg white is a safe, lean treat for most cats. The white is almost entirely protein and water, with very little fat, and once it is cooked it loses the one property that makes raw egg white a poor choice for cats. That property has a name: avidin, a protein in raw white that binds tightly to biotin, one of the B vitamins, and blocks the body from absorbing it. A cat eating raw egg white regularly can gradually run short of biotin even on an otherwise decent diet. Boiling denatures avidin, which is why the cooked version earns a yes and the raw version earns a no.
This is also why 'a little raw egg for the coat', advice that still circulates, deserves to retire. Whatever shine an owner hopes for is better pursued through a complete diet, and the cooked white delivers the protein without the vitamin-blocking side effect.
How to serve it well
Boil the egg until the white is fully firm, let it cool, peel away the shell and chop the white into cat-sized pieces. Serve it completely plain — no salt, no oil from a frying pan, no chilli, garlic or seasoning blends, several of which are unsuitable for cats in their own right. A tablespoon or so of chopped white is a generous treat portion for an average cat; think topper or training reward chopped smaller still, not a bowlful.
Offer the first serving small and watch the litter tray and his enthusiasm over the next day. Most cats digest cooked white easily, but any new protein can produce a soft stool in an individual cat, and a few cats simply dislike the rubbery texture and will leave it after one investigative bite. Both outcomes are useful to know before egg white becomes a household habit.
Where egg white fits in the bigger diet
Egg white is protein without the supporting cast. It contains none of the fats, vitamins and minerals a cat draws from whole prey or from a complete commercial diet, and it should stay a garnish on a feeding plan that already works. Cats who genuinely benefit from an occasional lean-protein topper include healthy adults whose owners want a low-fat training reward and cats who need tempting after a minor appetite dip — but a cat on a prescribed diet, a kidney diet for example, should get no extras at all without his vet's agreement, because therapeutic diets are balanced as a whole.
Do not be tempted to feed whites and discard yolks as a health statement for your cat. This is not a cholesterol intervention — cats process dietary fat differently from people, and the yolk carries useful nutrients of its own. White-only serving makes sense when white is what you have left from cooking, not as a feline diet philosophy.
Keeping toppers from quietly becoming meals
The risk with a popular topper is drift: a spoonful becomes a handful, offered by more than one person, until a measurable share of the day's calories arrives without any of the day's required nutrients. Logging solves drift. KinsLe Care Teams let the household record feeding duties and mark them done with a timestamp, so egg white offered at breakfast is visible to whoever prepares his evening meal and his portions stay honest.