There is some controversy about this.
Large amounts of garlic or small amounts given over a long period of time can affect hemoglobin. I will not give dog food with garlic in it but I would consider giving a few treats a year. There is absolutely no good reason to give it but there are plenty of bad ones. If you feed a food with garlic in it, consider checking red blood cells to make sure everything is okay. Everything should be but there isn't a guarantee that it won't affect your dog. Why take the risk?
If you are interested in some of the studies done on this subject, you can buy an article written by a nutritionist from
www.petdiets.com.
I wouldn't pay too much attention to what Dr. Pitcairn says.
There are enough untruths in it for me to not recommend it.
The book may have been accurate at one time but newer information is now available.
Doesn't he also say to feed onion and grapes?