When people worry about green potatoes being poisonous, they should actually be worried about the toxic effects of all potatoes. Potatoes are, to a certain degree, poisonous, though not in a deadly manner, and usually not enough to make anyone sick. The leaves of potatoes do contain high levels of a certain poison, and are direct relatives of nightshade, a poisonous plant. Green potatoes, the actual spuds, may contain just a little more of this poison than do white ones, and reflect improper storage or growing methods.
The green color is not the issue. This is merely a sign that a potato has been exposed to light, prompting it to produce more chlorophyll, and chlorophyll is not toxic to humans. What creates the problem with green potatoes is that exposure to light also produces a substance called solanine, a glycoalkaloid, which has been shown to be toxic when consumed in great quantities. Scientists have sought to determine exactly what quantity of green potatoes it would take to cause human toxicity, and it turns out that an adult would have to eat about 4.5 pounds of light-exposed potatoes (approximately 2 kg) to reach toxic blood levels of solanine.
There are some theories about whether regular consumption of potatoes that have gone green might reduce levels needed to produce poisoning. It is thought that it takes the body over a full day to clear solanine from the body, and so eating potatoes that are green on a daily basis could have some cumulative affect or accumulate toxic levels more quickly. Still, in the US, it has been over 50 years since any one has succumbed to potato poisoning, and it’s generally thought that people are at low risk for solanine toxicity from potato consumption.
This may leave cooks wondering whether they ought to eat green potatoes or discard them. For greatest safety, it is probably best to discard potatoes that are green, to avoid consuming solanine. Though a bite of these potatoes is unlikely to be fatal, consuming poisons on purpose isn’t a great plan for healthy living.
Scientists have tried to assess exactly how much poison results from green potatoes, and here people may be in for a surprise. Some potatoes will retain their white color but have been exposed to light, and still have higher than normal solanine levels, and some greener potatoes have lower levels of this toxin. Color isn’t necessarily the best indicator of potato toxicity. One study in 2006 published in the Journal of Agriculture and Food Chemistry found that improperly stored potatoes accumulate most solanine in the peel. This might suggest that peeling the potatoes is the best way to get rid of this toxin, no matter the color of the potato underneath.