1. Why Does a Potato Make a Toxin in the First Place?
Potatoes belong to the nightshade family, and plants in this family make glycoalkaloids as a defense against pests and germs. [1] A potato cannot run away from a hungry insect or a fungus, so it fills its tissues with chemicals that taste bad or do harm.
The chemicals are not spread evenly. Most sit in the peel, and the highest amounts are in the sprouts and the “eyes” (the small dents where sprouts grow). [1] That fits the idea of defense: the parts that must survive to grow a new plant are the best guarded.
2. How Much More Toxin Is in a Sprout Than in the Flesh?
The European Food Safety Authority (EFSA) pulled together measurements from many studies. The amounts below are milligrams (mg) of glycoalkaloids per kilogram of fresh potato. [2]
- Whole tuber: about 10 to 150 mg/kg
- White flesh: about 12 to 100 mg/kg
- Peel: about 150 to 170 mg/kg
- Thin outer skin: about 300 to 640 mg/kg
- Sprouts: about 2,000 to 7,300 mg/kg
These ranges come from different studies and potato varieties, so treat them as a sense of scale, not exact values. Even so, sprouts hold roughly twenty to several hundred times more than the flesh. Japan’s agriculture ministry gives about 7.5 mg per 100 g for the parts people normally eat, which is 75 mg/kg and sits inside the EFSA range. [3]
3. How Little Sprout Could Upset a 20 kg (44 lb) Child’s Stomach?
For sudden stomach trouble, EFSA used a reference point of 1 mg of glycoalkaloids per kilogram of body weight, based on case reports, outbreaks and volunteer studies. This is a reference point from studies—not a dose that will make everyone ill, or a guarantee that smaller amounts are safe. Typical symptoms are vomiting, diarrhea and stomach pain. [2]
Here is a thought experiment. A child weighing 20 kg reaches 20 mg. At 75 mg/kg (average edible parts), that is about 270 g of potato. At 2 to 7.3 mg per gram of sprout, it is roughly 3 to 10 g of sprout, about the weight of a few coins. This is arithmetic that mixes figures from different sources. It does not mean a bite that size makes anyone sick, and it does not mean a smaller bite is guaranteed safe. People differ, and so do potatoes.
Germany’s Federal Institute for Risk Assessment (BfR) takes a more cautious line. It derived a no-observed-adverse-effect level of 0.5 mg per kilogram of body weight per day, and suggests that table potatoes contain no more than 100 mg/kg. It calls these figures provisional because the data are incomplete, and advises that small children eat only peeled potatoes. [1] Because the amounts can vary and the data are incomplete, the usual advice is to remove sprouts completely, not just “most of them.”
4. Can You See or Taste Whether a Potato Has Too Much?
Partly. When a potato is stressed by light, damage, poor storage or sprouting, its glycoalkaloid level can rise quickly, and the potato can taste bitter. [4] BfR says a bitter taste and a burning feeling in the mouth can warn that the content is high, and advises against eating bitter potato dishes. [1]
Green skin is a clue too, but the green itself is not the poison. It is chlorophyll, the same pigment that colors leaves, which builds up when a potato sits in light. [4] BfR notes that green parts go along with a higher glycoalkaloid content. [1] One limit: the taste warning comes only after the food is in your mouth, so the better habit is to look first and cut away green parts and eyes generously. [1]
5. Does Cooking Get Rid of the Toxin?
Not reliably. EFSA lists reported reductions that vary widely: peeling about 25 to 75 percent, boiling or blanching peeled potatoes 5 to 65 percent, frying peeled potatoes 20 to 90 percent, and baking unpeeled potatoes 20 to 50 percent in an oven or 3 to 45 percent in a microwave. [2] A range that wide means nobody can say “cooking removes this much.” Part of the drop in boiling may simply be toxin moving into the water: BfR notes that some passes into it and advises against reusing potato cooking water. [1]
So the dependable step happens before cooking: cut away sprouts, eyes and green areas, and peel when in doubt.
6. What Happened When a School Cooked Its Own Potatoes?
In January 2015, an elementary school in Nara Prefecture, Japan, cooked potatoes grown on school grounds. Of 51 people who ate them, 31 developed symptoms such as nausea and stomach pain. Leftover mashed potato contained 19 to 39 mg of glycoalkaloids per 100 g, according to Japan’s agriculture ministry. [3] The ministry also says green parts can contain 100 mg or more per 100 g. The school’s leftovers had a lower measured amount, but people still became ill. How much each person ate was not reported, so this case cannot tell us what dose caused the illness. The effect of a given amount depends on how much was eaten and on body weight.
Small potatoes grown at school or at home can be immature, and looks alone are not a reliable guide. Peeling them and removing eyes and green areas is the safer way. [1]
7. How Do You Keep the Toxin From Building Up in Stored Potatoes?
Since light, damage and sprouting raise the level, storage matters. BfR advises keeping potatoes cool, dark and dry, and not eating old, shriveled, green or heavily sprouting ones. [1] Japan’s ministry prefers about 10°C (50°F); above 20°C (68°F) potatoes sprout and rot more easily. [3]
Well-grown, well-stored potatoes are not a worry. BfR says they usually contain about 20 to 100 mg/kg, and reports no deaths in the last 50 years, though mild poisonings may go unrecognized because the symptoms look like many other illnesses. [1] If someone feels sick after eating potato, tell an adult and get medical advice.
8. How Small Is 3 to 10 Grams?
Ask an adult to help you weigh something harmless on a kitchen scale, such as uncooked rice, until it reads 3 g, then 10 g. That is the sprout amount from section 3. Notice how small it is. Then check the potatoes at home: are any green or sprouting, and where are they stored? Do not taste or eat sprouts or green parts for this activity.
To dig deeper, read the BfR question-and-answer page and the EFSA assessment below. Why do an “effect level” and a “no-effect level” give such different numbers, and which one would you use to set a rule?