A normal supermarket potato carries 10 to 150 mg/kg of glycoalkaloid in the flesh, and your body clears that with no consequence 1. The acute-poisoning line is about 1 mg per kilogram of your body weight — roughly 70 mg for a 70 kg adult, a single 200 g serving of a bad tuber 2. A heavily greened or sprouted potato can hit 250 mg/kg or more, with the peel and eyes several times higher again. The margin is thin: one affected potato in a normal portion gets you there.
At that dose solanine blocks the nerve off-switch running your gut and heart rate, and tears holes in the cells lining your intestine. So poisoning reads like a stomach bug fused with something neurological: vomiting and diarrhoea, then sweating, a slow pulse, drowsiness. In 1979, 78 boys at one London school ate boiled potatoes stored over the summer; 17 landed in hospital, some comatose with brief convulsions 3.
Two things let it sneak past people. Onset is delayed one to twenty-five hours, peaking at four to eight, so nobody blames dinner 1. And the mild cases look exactly like a bug you'd wait out.
The whole defence is three sorting rules — plus one override that matters more than any of them.
The fine print — when to skip it, and what people get wrong
"Just cook it well" is wrong. Solanine is heat-stable; boiling cuts the dose by single percent, microwaving ~15%, deep-frying ~40% — none enough to save a bad potato 1. The 1979 potatoes were boiled.
"Green means solanine" is only half true. The green is chlorophyll, harmless in itself; light just makes both in parallel 1. A potato sprouting for months in a dark cupboard can carry solanine in its eyes and sprouts with no green at all. Trust the sprouts and the taste as much as the colour.
Pregnancy and small children: on anything visibly green or sprouted, default to discard, not trim. A 20 kg child crosses the threshold from a portion that wouldn't register in an adult. In pregnancy, glycoalkaloids cause neural tube and facial defects in animal embryos, and one large case-control study links frequent sprouted-potato eating around conception to roughly double the odds of a neural tube defect 2 4.
- 1Friedman M (2006). Potato glycoalkaloids and metabolites: roles in the plant and in the diet. Journal of Agricultural and Food Chemistry. link
- 2EFSA (2020). Risk assessment of glycoalkaloids in feed and food, in particular in potatoes and potato-derived products. EFSA Journal. link
- 3McMillan M, Thompson JC (1979). An outbreak of suspected solanine poisoning in schoolboys: examinations of criteria of solanine poisoning. Quarterly Journal of Medicine. link
- 4Ni et al. (2018). Maternal periconceptional consumption of sprouted potato and risks of neural tube defects and orofacial clefts. Nutrition Journal. link
Greened and Sprouted Potatoes
Trivial: glance at potatoes before cooking, trim or discard the affected ones, keep the bag in a dark cupboard. Seconds per meal, no willpower load.
EFSA CONTAM Panel 2020 risk assessment is the regulatory anchor, with a 1 mg/kg bw LOAEL derived from human case reports and small volunteer studies (EFSA 2020). Mechanism is characterised (cholinesterase inhibition + cholesterol-binding membrane disruption; Friedman 2006), and a documented outbreak literature converges on the dose range (McMillan 1979). No large RCT, which is the ceiling on a 5.
Prevents acute glycoalkaloid GI poisoning — nausea, vomiting, abdominal pain — from heavily greened or sprouted tubers, the dose range that crosses the EFSA acute LOAEL of 1 mg total glycoalkaloid/kg body weight (EFSA 2020). Per-event probability is low for any individual reader, hence small rather than large; severity of the documented tail (the 1979 schoolboy outbreak; McMillan 1979) earns the non-zero score.