Understanding Garlic

By Dr. Dave Summers, Nutritionist

Garlic has recently come under scrutiny for its use in dog foods. Misleading information about the dangers of garlic in relation to dogs has appeared on the internet and in other media.

The reason for the misinformation starts back over a hundred years ago with onions, which are the same family as garlic. It was noticed that cattle, sheep, and horses that consumed wild onions showed toxicity symptoms. Then in the 1930s it was shown that dogs consuming cultivated cooked onions also showed toxicity symptoms which included; diarrhea, vomiting, blood in urine, abdominal pain, lethargy, and/or loss of appetite.

It wasn’t until the 1980s that some of the same toxicity symptoms were seen in cats consuming food containing onion. Cats are six to eight times more sensitive to onion than dogs. It takes about 1.5 grams per kilogram of body weight for a cat to show onion toxicity, and about 10 grams per kilogram of body weight in a dog. The reason the symptoms were first seen in dogs is because dogs are far more likely to eat onions then cats.

In the 1980’s the cause of the toxicity was shown to be a form of blood anemia. Something was causing the red blood cells to be destroyed by oxidation. The culprit, or I should say culprits, are a group of compounds called thiosulfates.

The blood cells of cats are particularly sensitive to oxidative damage, which is why cats are much more sensitive to thiosulfates than dogs. To complete the story, dogs are more sensitive to oxidative damage to blood cells than humans, which is why it is safe for us to eat onions.

Measuring Toxicity

Onions are part of the plant family called Allium which includes leeks, chives, shallots, and garlic. All of these plants contain varying levels of different forms of thiosulfates. If there was only one form of thiosulfate it would be possible to estimate the potency of the different plants. Unfortunately, the only way to see the potency of the different plants is to have animals eat them and measure the number of destroyed red blood cells in the animal. Don’t worry - they don’t need to eat enough to cause observable toxicity symptoms.

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.

Dosage

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.

Another example of where confusion is caused by talking about toxicity without considering dosage rate is Poinsettias. “Poinsettias are toxic; don’t let your dog near them.” Not true.

Chocolate is another example. Chocolate can be deadly to dogs in high dosages, especially highly-potent chocolate such as unsweetened baker’s chocolate or cocoa powder. If your Labrador steals a single milk chocolate off the table, it will likely suffer no ill effects. If your Teacup Poodle eats a whole box of dark chocolate, she should visit the veterinarian immediately.

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

Garlic is added to dog foods because it has many health benefits, even at the very low levels used. Its main benefit is improvement to the health of the digestive tract. The other medicinal properties of garlic include: anti-microbial, antioxidant, antibiotic, fights cancer cells, decreases blood cholesterol, helps to prevent strokes and decreases blood pressure.

In fact, most of the research into the effect of feeding garlic to dogs is done because the researchers want to better understand the benefits of garlic, not the dangers.

Be assured that garlic is safe at the level used in dog foods, and remember that talking about toxicity without putting in some context of a “normal” consumption level is misleading.