With apologies to the 1979 cult classic “Attack of the Killer Tomatoes”, green potatoes are the real killer. Maybe killer is too strong a word. How about poisonous? Most of us have heard that green potatoes can make you sick, but is there any truth to this, or is it just a myth? The answer is yes, and I have the science and a personal experience to back it up.
I did my undergrad work in Colorado, in the middle of a huge valley filled with potato farms. I was also friends with two odd brothers, Tim and Steve. Great guys, just odd. They both ran track and cross-country, were math whiz’s (Tim once took a calculus test drunk, and scored perfect. Me, sober? Not so perfect.). . . . and frugal. They would walk through the potato fields after they were harvested and pick up the left overs, which were often green. They’d feast on them, and over time they both became sick with nausea and diarrhea. It didn’t deter them from doing it again, though. I told you they were odd.
The reason they became ill were because of two glycoalkaloids that are present in potatoes, solanine and chaconine. In green potatoes, the green coloring comes from chlorophyll, and when exposed to light the potatoes increase production of both chlorophyll and glycoalkaloids. The chlorophyll is for growth and energy, and the glycoalkaloids are presumably a defense mechanism to poison scavengers . . . like Tim and Steve.
The glycoalkaloids present toxicity by two different mechanisms: inhibition of the enzyme acetylcholinesterase and disruption of epithelial barrier integrity in the GI system. That’s a mouthful, but I’ll explain. Acetylcholinesterase is an enzyme that hydrolyzes the neurotransmitter acetylcholine. In the peripheral nervous system (PNS), acetylcholine activates skeletal muscle, and can also control heart rate, digestion, and respiration through its effects in the autonomic nervous system (ANS). In the central nervous system (CNS) it stimulates arousal and reward. Disruption of this delicate cycle can cause sweating, vomiting, and bronchospasms. Of these, bronchospasms are the most serious, as they are the sudden constriction of the muscles in the bronchioles, causing difficulty in breathing, like an asthma attack or allergic reaction. (1)
The second mechanism, by disrupting the epithelial barrier in the GI system, is equally intriguing, and perhaps the main cause of symptoms due to potato glycoalkaloid toxicity. Our GI system is lined with sheets of thin epithelial cells, which forms our mucosa. If this barrier is breached, either by bacteria, physical injury, or toxins, the effects can range from mild to fatal. In the case of the solanine and chaconine, they share the same structural backbone of cholesterol, which makes up about one third of the intestinal epithelial membrane. Solanine and chaconine can thus either associate with cholesterol in the membrane or insert themselves diretly into the membrane. The net effect is a disruption in the morphology of the intestinal barrier, causing physical discomfort. (2)
The physical discomfort, such as cramping and diarrhea, can often be confused as irritable bowel syndrome (IBS) or irritable bowel disease (IBD). And while most likely not the cause of such, the glycoalkaloids can exacerbate the conditions (2). The other challenge in diagnosing the poisoning is the long half-life of the glycoalkaloids, the amount of time needed to clear one-half of the drug from the body. Solanine and chaconine have half-lives of approximately 25 and 35 hours, respectively (1). This means that while someone may not be consuming a toxic amount of potatoes or glycoalkaloids in one sitting, the total concentration in the body is not decreasing fast enough, and subsequent potato meals continue to build the total concentration. This is what we would call chronic toxicity. So if a person presents to their doctor with IBS-like symptoms after having eaten potatoes for a week, they may not associate it with the potatoes, since they ate them a week ago and were fine. Humans have a tendancy to look for the immediate, or accute, toxic events. So if you have IBS, IBD, or similar symptoms, try laying off the spuds for a while.
The next question is: how much is toxic? In humans the total glycoalkaloid concentrations deemed toxic is approximately 1-5 mg/kg body weight (3). Since I always use an average of 70 kg (154 pounds), this would be about 70-350 mg of glycoalkaloids. The lethal amount is estimated to be between 3-6 mg/kg, and given an average body of 70 kg again, this would be a total of 210-420 mg. So how much of these glycoalkaloids are in our potatoes, and what affects the concentration? The glycoalkaloids are formed naturally in the potato, with an increase in concentration when they are exposed to light while growing, or sprouting. This is one reason, besides increasing the total production, that potatoes are hilled, and continuously covered with dirt. Glycoalkaloid concentration is thus highest in, and just under, the skin, and particularly in parts that are green. On average the concentrations in potatoes, and their parts are, in mg/kg (3):
- Whole potato 10-150
- Peel (outer 10%) 150-1068
- Sprouts 2000-7300
- Flowers 2150-5000
- Leaves 230-1000
- Bitter-tasting potato 250-800
This is why you don’t eat potato leaves. A large salad could kill you. And if a potato tastes bitter to you, that’s those alkaloids, and its their way of saying “don’t eat me.” You should listen to these warnings. The concentrations in potatoes themselves are extremely variable, as we have hundreds of different varieties. In general though, the types you buy in the store have been bred to have low glycoalkaloid content, while the older, heirloom styles that we grow at home tend to be higher. Most regulatory bodies consider concentration of less than 200 mg/kg in potatoes as safe. Cooking does not seem to affect the total concentrations to a large degree, with the possibility of frying.
Potatoes happen to be one of the most highly processed foods around, and the source of many snack-style foods. Fried potato skins (common in the US) have concentrations from 560-1450 mg/kg (3). If someone were to eat enough of these, say 1 pound (~ 0.5 kg), someone could become severely ill. At home, while bent over the toilet or sitting on it, they may just be thinking that they ate too much, and oblivious to the possibility of being poisoned by something so delicious. Other factors that we need to consider is how we store potatoes. Our ancestors always stored them in cool, dark places. They were so smart. Under these conditions the glycoalkaloid concentration barely changes. Under current supermarket conditions, room temperature and under fluorescent lights, glycoalkaloid concentration can double (4).
It wouldn’t be right to subject you to all of this information without a case study, and there is a doozy. In 1978 there was a mass poisoning that affected 78 schoolboys in South London. All had been served, and eaten, boiled potatoes. Onset of vomiting and diarrhea started between 4 and 14 hours, which would indicate a disruption in the epithelial layer of their GI systems. And in several severe cases depression of the CNS occurred, with seizure-like symptoms. GI distress lasted for six days! (5).
So enjoy those potatoes, it is the fourth most common food, behind wheat, rice, and corn, after all. Just cut off the green parts.
[Homepage featured image of green potatoes by Groene Aardappels (CC 3.0)]
1. Mensinga, T. T., A. J. Sips, C. J. Rompelberg, K. Twillert, J. Meulenbelt, H. J. Top, and H. P. Egmond. “Potato Glycoalkaloids and Adverse Effects in Humans: An Ascending Dose Study.” Regulatory Toxicology and Pharmacology 41.1 (2005): 66-72.
2. Patel, Bijal, Robert Schutte, Peter Sporns, Jason Doyle, Lawrence Jewel, and Richard Fedorak. “Potato Glycoalkaloids Adversely Affect Intestinal Permeability and Aggravate Inflammatory Bowel Disease.” Inflammatory Bowel Diseases 8.5 (2002): 340-46.
3. Smith, D. “Potato Glycoalkaloids: Some Unanswered Questions.” Trends in Food Science & Technology 7.4 (1996): 126-31.
4. Machado, Rita M., Maria C. Toledo, and Lucila C. Garcia. “Effect of Light and Temperature on the Formation of Glycoalkaloids in Potato Tubers.” Food Control 18.5 (2007): 503-08. 5. “Solanine Poisoning.” British Medical Journal 2.6203 (1979): 1458-459.