Can trace amounts of onions, or other foods poisonous to dogs, be harmful?

Question

I'm asking this in response to the question: Are onions dangerous for my cat or dog and why?

The above question concludes that onions and garlic are poisonous to dogs.

But what about trace amounts? If I feed table scraps to my dog that have only been seasoned with onions or garlic, is that still dangerous?

Some examples of what I'm referring to:

• A piece of steak that has been marinated with a marinade that had onions in it.

• A hamburger that had onions on it(with the onions taken off)

• Pizza crust(which almost certainly has trace amounts of sauce which normally contains garlic)

Answer 1

There are a variety of factors that influence this:

• The size and breed of dog will have an impact on how much of a toxic substance a dog needs to ingest before the effects become visible.
• In many cases, whether or not the toxin is ingested on an empty stomach will also be significant. (So, a small amount of a toxin present in a large quantity of food will usually pose less of a threat than the same amount of toxin ingested alone.)
• Frequency of exposure can also play a big role. Many toxic compounds will stay in the system for some time before they are broken down and/or passed. Others will cause damage that may not be immediately symptomatic but can become so if the body is not given time to repair itself between exposures.

For chocolate/cocoa, it looks like 1.3g/kg of baker's chocolate will generally cause symptoms. (eg. a dog weighing 10kg would need to ingest at least 13g of baker's chocolate for symptoms to present.)

For grapes and raisins, even 10g/kg can be problematic.

For onions, 15g/kg is enough for some dogs to show signs of the toxin. (The amount for cats is even less: 5g/kg.)

Also, since you specifically asked about pizza crusts:

All forms of onion can be a problem including: dehydrated, raw or cooked onions, table scraps containing cooked onions or garlic, left over pizzas, chinese dishes, any feeding stuff containing onions. - Some Food Toxic to Pets

The quantity of macadamia nuts required to cause symptoms seems to vary widely. Apparently as little as 2.4g/kg can be enough to induce symptoms.