
I've copied and pasted the below from a google book I have on nutrition, puts 'dosage' into perspective:
By looking at the amount of blood cell damage the researchers can see the effects of feeding animals relatively low levels of onions or garlic. In the testing of onions and garlic on blood cell oxidation, onions have about 15 times the ability of garlic to damage red blood cells.
Testing Garlic in Pet Food
Although it is very rare to find cases of garlic toxicity in cats because they are finicky about what they eat, research looking at the damage to red blood cells in cats after feeding garlic showed cats are sensitive to garlic. For this reason, the use of garlic in cat foods was discontinued in the early 1990s.
It was not until the late 1990s that research was done on the effect of garlic on dogs. In 2000 a research paper was published which tested a garlic extract on dogs. The dogs did not show any observable toxicity symptoms, but there was a definite effect on the red blood cells. In the conclusions the researchers stated: "we believe that foods containing garlic should be avoided for use in dogs." This led to a flurry of warnings and panic that garlic should also be removed from dog foods. The problem with the researchers' statement (and many of the subsequent quotations of the study) is that they did not consider the relevance of the level of garlic extract used in the experiment, compared to the level included in dog foods.
In their research they fed a garlic extract equivalent to 60 g of garlic to dog weighing approximately 12 kg. A 12 kg dog will normally eat between 150 to 200 g of food. Therefore, if the food was about 30% garlic the researchers' concerns would be valid.
The Reality of Garlic in Dog Food
When garlic is added for flavor, the maximum usage level is around 3 g per kilogram of food. Our 12 kg dog eating 200 g of food would eat approximately 0.6 g per day. To achieve the health benefits of garlic, the usage level is around 1.5 g of garlic per kilogram of food. A 12 kg dog would eat about 0.3 g a day. It is very apparent that these levels are nowhere close to the levels used in their experiment, and at these levels research had not shown any effect of garlic on red blood cells. The confusion comes from not considering the dosage rate.
Question: What is the difference between a nutrient, a drug, and a toxin? Answer: Dosage.
That is an old saying among nutritionists, and it's true. To say something is toxic without some reference to the level needed to cause the toxic effect is misleading, especially in the fields of nutrition and health.
Lots of nutrients we, and our pets, consume are potentially toxic. An example is the trace mineral selenium. Selenium is usually added to pet foods at the level of 0.2 ppm (parts per million). Increase that level to 1 ppm you get additional health benefits. Increase it to 10 ppm and the level becomes toxic, possibly even deadly.
The list of potentially toxic items could go on and on. I could include nutrients like salt, vitamin D, or Zinc. You name it, and it could be toxic at some level.
Garlic is Healthy