One of our cats has a habit of jumping on the kitchen counter and chowing down on any leftover scraps from dinner. Personally, I don't mind this, as it's mostly "licking the plate" kind of stuff.
Does supplementing him this way adversely affect his diet to where we would want to discourage this behavior?
If it is okay, are there any foods we'll want to keep an eye out for our cats to avoid altogether?
It is not a good idea to let cats eat scraps off plates for two reasons.
It encourages bad behavior. It would be better to select any suitable scraps and put them in the cat's food bowl. This also ensures that what the cat is eating is being supervised. When selecting table scraps for you cat, pick off cooked, unseasoned meat and feed it to your cat in her food bowl.
There are many things that can be, potentially, harmful for a cat to eat from human leftovers.
Sometimes cats will vomit up table scraps, as they don't cope well with big diet changes, they cope better with gradual changes and introduction of new foods. Also the type of seasonings and spices human foods have frequently do not agree with a cat's gut and can cause irritation.
The following list is by no means comprehensive
avocado
Avocado contains persin, which is a dangerous toxin for cats. It can cause gastrointestinal upset, cardiac edema and difficulty breathing. For more detail see here https://pets.stackexchange.com/questions/939/is-it-safe-to-feed-my-pet-avocado
broccoli
Broccoli contains isothiocyanate which if given in greater than 10% of the total diet will cause gastrointestinal upsets, if it reaches 25% of the total diet, can (and most likely will be) fatal.
cooked bones
Cooked bones are not recommended to feed cats or dogs. Cooked chicken bones tend to splinter and can cause damage to the gut by perforating the bowel or causing a blockage, as they can be hard to pass through the bowel.
grapes
Grapes and their derivatives are also bad for cats. This is discussed in more detail here Is it safe for my dog or cat to eat grapes, raisins and sultanas?. Grapes can permanently damage a cat's kidneys and can cause renal (kidney) failure in cats.
liver
Liver can cause vitamin A toxicity.
milk
Milk, cats are do not produce lactase and cannot digest cow's milk. It can cause diarrhea. Addressed in more detail here Should I feed my cat milk?.
nuts
It's not advisable to feed your cat nuts. Nuts are a more complex issue addressed here https://pets.stackexchange.com/questions/940/is-it-healthy-to-feed-my-dog-or-cat-nuts.
onions
Onions and plants from the onion family, like garlic are addressed in detail here Are onions dangerous for my cat or dog and why?. To give a brief overview, onions and garlic are frequently used in cooking and are also found in many processed foods, even baby food. Onions cause hemolysis of red blood cells (RBC). It causes the RBC to break, which can lead to a potentially fatal condition allium toxicosis.
sweetener
Sweets left on plates are also not a good idea to feed cats. If they have a taste for some leftovers using Xylitol can cause hypoglycemia in cats and can lead to coma and be fatal. This is discussed in more detail here Is it OK to feed my dog or cat candy with sugar substitutes?.
sugar
As for sugar, generally, it serves no good nutritional value for a cat, it has a high glycaemic index, which can affect blood sugar levels and over time increase a cat's chance of developing diabetes, not to mention tooth decay.
vegetables
There are also risk in feeding your cat some other vegetables. For more detail see here https://pets.stackexchange.com/questions/941/is-it-ok-to-feed-my-dog-and-cat-vegetables
In one word "NO".
Lots of phytotoxins are very damaging / toxic to an obligate carnivore like a cat. Humans can eat a wide variety of plant material with no ill effects (and actually plenty of good effects), for cats it is the opposite.
I never let my cats eat anything I eat. I train them to eat only what I give them and never take any of my food (I'm a vegetarian and my diet could kill them pretty quickly). Even if you gave them stuff like meat with no spices or sauce that would encourage them to view your food as theirs and potentially take from you when you didn't offer (say you were eating something you know is toxic to them like: garlic, mushrooms, chocolate, or any of the things listed above). If you want your cat to live long and be healthy I'd discourage this habit quickly.
I give my cat the left overs and I’ve had her 5 years, in fact she waits for me to be done eating then follows me into the kitchen and waits for me to empty my plate into her bowl. I eat everything from steak covered in BBQ sauce to fish with the bones still intact. I’ve never had any issue personally. Vet says she healthy. The only bad thing I will admit is that she seems to across a bit spoilt, she won’t eat the cheaper cat foods wet or dry, she’ll let it sit there for the whole day and I’ve noticed that from a few years ago when I started to give let her have my scraps.
