Is a Natural Toxin in Your Dog's Bowl Fueling Chronic Disease?

Damaged fruits and vegetables commonly found in pet food contain naturally occurring toxins. 

All potatoes (except sweet potatoes), tomatoes, eggplant, and bell peppers are members of the nightshade family of plants.  All nightshade plants produce toxins in response to stresses like bruising, UV light, microorganisms (bacteria and viruses), mold, and attacks from insects, worms, and herbivores.  After harvest, the toxin levels can increase during storage and transportation because of light, heat, processing, and sprouting.  The light sources that can stimulate toxin production include sunlight, some artificial light, and irradiation used in food safety measures to kill pathogenic bacteria.   

These natural toxins are called solanine and chaconine. The World Health Organization has them on a short-list of baddies.  Chemically speaking, they are in a class of substances called glycoalkaloids (“GA”).  The highest concentrations of GA are found in potato sprouts, peel, and greenish parts, as well as in green tomatoes.  

History has many real-life examples of this toxin causing acute illness and death in humans. Reports include school children, soldiers, and families.  Scientific research on GA effects on animals has been going on for decades though it is limited mostly to the higher exposure levels which would cause immediate harm to laboratory animals and people.

An analysis of 52 pet foods containing potatoes found glycoalkaloids in concerning amounts.  We’ll talk about that study in a moment.

What does history tell us about glycoalkaloid toxicity for people?  And what implications does this have for our pets?  Let’s walk through the research, studies, and the facts.

Acute Glycoalkaloid Toxicity

Glycoalkaloid poisoning elicits a wide variety of symptoms - ranging from gastrointestinal disorders, through confusion, hallucination and partial paralysis to convulsions, coma and death - but is thought to stem from one or both of two quite distinct modes of action.

Studies suggest that the first assault inflicts damage on the cellular membranes.  It starts in the gastrointestinal tract, creating a leaky gut by damaging the cells there.  GA then seeps out of the gut’s mucosal barrier and is transported to other tissues or organs (e.g. blood, liver) where it continues to disrupt cellular membranes following absorption.  

A second wave of systemic damage is then caused by inhibition of the enzyme acetylcholinesterase, which is responsible (through a chain reaction) for the structural integrity and signaling of every cell in the body.  Neurological symptoms such as weakness, confusion and depression, which have been seen in human patients suffering from GA poisoning, are likely manifestations.

It is known that the two GA compounds occurring in nightshade plants noted above, (solanine and chaconine), interact synergistically, which results in a marked intensification of the overall activity.[1] 

Chronic Disease from Mild Poisoning is a Widely Held Suspicion

It also appears that glycoalkaloids accumulate in the body, particularly the liver, implicating sustained low-level exposures to glycoalkaloids in chronic disease.  There is a widely held suspicion that mild poisoning is more prevalent than assumed.  However, because the symptoms (e.g. abdominal pain, vomiting, diarrhea) are similar to those of other common gastrointestinal ailments, it is rarely diagnosed or treated.[2]  No animal studies have been conducted to investigate this potential problem of chronic low exposure.

One study on three humans in the 1980s looking at acute exposures found that "a fraction in excess of 90% [of a solanidine dose administered via IV] was sequestered somewhere in the body 24 h after dosing. After this time, the rate of elimination from the body was low, about 1-2% per day,corresponding to an overall half-life of 34 to 68 days."

Acceptable Limits of Total Glycoalkaloids

So how much is too much?  That answer varies so greatly as to be largely unestablished.  There are no limits on the amount of total glycoalkaloid (“TGA”) toxin allowed in pet food.

A lab that analyzed a pet feed product after a dog who ate it suffered digestive problems, found levels in the feed of 294 mg of TGA per 1 kg of feed.  That’s notably higher than the levels generally reported as tolerable to humans.

The permissible level in human food in some European countries is 100 mg TGA in 1 kg of fresh potato.  That’s approximately 15 mg of TGA in an average potato.  But most humans don’t live on potatoes the way dogs live on commercial pet feed.  If an average adult eats one potato per day, he would be exposed to 0.1 mg TGA per pound of body weight.  (This assumes a 150 lb human and 150 g potato.)   

Other countries double that permissible level to 200 mg TGA per 1 kg fresh potato.  Researchers, however, suggest that these levels are too high because “the narrow margin between toxicity and lethality is obviously of concern.”[3]

Although serious GA poisoning of humans is rare, susceptibility of humans to GA poisoning is both high and very variable: oral doses in the range 1-5 mg/kg of body weight are marginally to severely toxic to humans whereas 3-6 mg/kg body weight can be lethal.  

The permissible levels in human foods have not been supported through toxicological studies and do not take into account the synergistic effects that have been shown to occur when solanine and chaconine are present in various proportions. 

Glycoalkaloid Poisoning of Pets Is Extremely Concerning

Pets eating pet feed (i.e. not human grade pet food) including kibble and canned products that include nightshade ingredients are at increased risk of chronic and acute GA poisoning for three reasons:

1. The parts of the potato that pets eat have a high concentration of glycoalkaloids.
2. The ingredients and finished pet feed products are handled in such a way as to increase the concentration of GA in processing and storage.
3. The diet of most pets does not vary like humans. Dogs consistently eat the same feed daily. Thus, dogs eating commercial pet feed containing potato and other nightshades are at greater risk of bioaccumulation and chronic disease.

This is specifically what the study of 52 pet feeds discussed.  According to these researchers:

“Most commercial dog and cat food contains products from many sources including potato waste from food manufacturers. Often this waste consists of the undesirable parts of the potato including skins and sprouts. Furthermore, if the skins are exposed to sunlight and the material is not fresh, sprouts will develop. These are the conditions under which GAs will become most highly concentrated.” [4]

Glycoalkaloids in certain potato varieties have been shown to increase over 10 times when exposed to sunlight for 2 weeks. The GA values also vary by the part of the potato (e.g. tuber, peel, sprout, leaves).  Below is a table of the highest reported GA values in various potato parts which are commonly used ingredients in pet feeds. The concentration can be anywhere from 9 to 90x that of a permissible human level of 100 mg/kg of fresh potato.

The analysis of 52 pet foods containing potatoes found several wet and dry products for dogs and cats from different manufacturers with TGA concentration over 40 ppm (40 mg/kg) and two with TGA over 100 ppm (100 mg/kg).  The highest sample approached 140 ppm.  The two samples found to contain TGA over 100 ppm were both dry dog foods and were from different manufacturers.

So what might a 50 lb dog's exposure be if she ate a kibble containing 100 ppm of TGA?  Assuming a 50 pound dog eats about 300 grams (10.6 oz) of dry food per day, that dog would be ingesting .6 mg/lb of bodyweight of glycoalkaloids.  That's 6x the level thought to be generally tolerated by adult humans. If she bioaccumulates TGA as researchers suspect humans do, then these toxins could account for a whole range of GI, neuromuscular, cognitive, liver, blood and immune system abnormalities and injury.

The researchers discuss why levels in pet food are concerning:

“Since potatoes are not a primary ingredient in the product, the potatoes used must have had relatively high concentrations of TGA. This would suggest that greened potatoes, sprouts or varieties with naturally high TGA levels were used in the food production. While it is unclear at what level TGA can cause health risk in animals, the levels found in this study indicate possible concern. Since the majority of pet diets consist of commercially sold pet food, the generally accepted warning limit of 200 ppm for potatoes intended for human consumption, may not be applicable for a warning limit in pet food.”[9]

More research is needed to evaluate the risks of GA in pet food in today's $42B US pet food industry. This study appears to be the only publicly available research on GA in pet food. It was conducted in 2000. Based on today's metrics, these 52 pet foods would have represented about 12% of 2000's $9.37B US pet food industry. Today 52 pet food products represent less than 3%.

How Does A Pet Parent Avoid Glycoalkaloids?

Since glycoalkaloids are present in inedible plant matter from potatoes, tomatoes, peppers and eggplant among other nightshades, the best way to protect your pet is to avoid purchasing pet feeds containing these plant based ingredients. 

Human grade pet foods are regulated to mean that they contain edible ingredients.  So if you want to feed your dog nightshade vegetables, then make sure you're choosing a human grade commercial pet food.  Look for the words "Human Grade" on the label.  (Websites are unregulated.)

While there are beneficial plant compounds and essential vitamins and minerals in some nightshade plants (like lycopene in tomatoes), other nightshade plants like white potato, have more nutritious alternatives.  Instead of white potato (which is also heavily treated with the herbicide glyphosate) try sweet potato.  Sweet potato is not a nightshade plant.  It is also packed with vitamins, minerals, beneficial plant compounds and carries a much lower chemical load than white potato.

If you prepare foods at home for your pup and want to use nightshade ingredients, then make sure that the ingredients are in excellent condition and have been stored properly.  It's best to peel white potatoes and discard the peel along with any sprouts.  

Endnotes & Other Reading Materials: 

[1]  D.B. Smith, J.G. Roddick, and J.L. Jones. Potato glycoalkaloids: Some unanswered questions. Trends Food Sci. Technol. April (7) 126–131 (1996). https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=7dcfc6fd86d1b8886bb55c7636c3243d2fe5fd01

[2] Ibid

[3] Ibid

[4] Glycoalkaloid Content in Pet Food by UPLC–Tandem Mass Spectrometry. Robert S. Sheridan and Jennifer L. Kemnah https://cdn.shopify.com/s/files/1/0500/2490/6914/files/48-10-790.pdf?v=1685809532

[5] M. Friedman. Potato glycoalkaloids and metabolites: Roles in the plant and in the diet. J. Agric. Food Chem. 54: 8655–8681 (2006).

[6] B. Zywicki, G. Catchpole, J. Draper and O. Fiehn. Comparison of rapid liquid chromatography- electrospray ionization tandem mass spectrometry methods for determination of glycoalkaloids in transgenic field-grown potatoes. Anal. Biochem. 336: 178–186 (2005)

[7] M. Friedman. Analysis of biologically active compounds in potatoes (Solanum tuberosum), tomatoes (Lycopersicon esculentum), and jimson weed (Datura stramonium) seeds. J. Chromatog. A 1054: 143–155 (2004).

[8] D.B. Smith, J.G. Roddick, and J.L. Jones. Potato glycoalkaloids: Some unanswered questions. Trends Food Sci. Technol. April (7) 126–131 (1996). https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=7dcfc6fd86d1b8886bb55c7636c3243d2fe5fd01

[9] Glycoalkaloid Content in Pet Food by UPLC–Tandem Mass Spectrometry. Robert S. Sheridan and Jennifer L. Kemnah https://cdn.shopify.com/s/files/1/0500/2490/6914/files/48-10-790.pdf?v=1685809532

Reading materials:

SOLANINE AND CHACONINE First draft prepared by Dr T. Kuiper-Goodman and Dr P.S. Nawrot Bureau of Chemical Safety Health and Welfare Canada Ottawa, Ontario, Canada https://inchem.org/documents/jecfa/jecmono/v30je19.htm 

A Review of Occurrence of Glycoalkaloids in Potato and Potato Products DUKE GEKONGE OMAYIO, GEORGE OOKO ABONG* and MICHAEL WANDAYI OKOTH https://cdn.shopify.com/s/files/1/0500/2490/6914/files/Nutrition_Vol_4_No3_p_195-202.pdf?v=1685809532

COMMITTEE ON TOXICITY OF CHEMICALS IN FOOD,CONSUMER PRODUCTS AND THE ENVIRONMENT https://cot.food.gov.uk/sites/default/files/2020-08/made%20accessible%20TOX-2020-19%20EFSA%20glycoalkaloids.doc_accessibleinadobepro_0.pdfhttps://www.who.int/news-room/fact-sheets/detail/natural-toxins-in-food

Risk assessment of glycoalkaloids in feed and food, in particular in potatoes and potato-derived products https://efsa.onlinelibrary.wiley.com/doi/full/10.2903/j.efsa.2020.6222