The safety of tomato leaves is often questioned because the tomato plant, Solanum lycopersicum, belongs to the Solanaceae family, commonly known as nightshades. This family includes many edible vegetables, such as potatoes and eggplants, but also highly toxic species. Many nightshade plants produce various alkaloids, which are naturally occurring chemical compounds that can be toxic in sufficient concentrations.
The Protective Compound: Tomatine
The compound responsible for the tomato plant’s defense mechanism is a glycoalkaloid known as alpha-tomatine. This molecule, a naturally synthesized steroid alkaloid, is structurally similar to solanine, the alkaloid found in potatoes, though tomatine is generally considered less toxic to mammals. Tomatine acts as a natural pesticide and fungicide, protecting the plant from various insects, microbial attacks, and herbivores.
The concentration of alpha-tomatine is not uniform throughout the plant. It is found in the highest levels in the vegetative parts, specifically the stems and leaves. This concentration gradient explains why the green, non-fruit parts of the plant are often viewed with suspicion.
Symptoms and Practical Risk of Ingestion
Ingesting a large quantity of tomato leaves or stems introduces the possibility of Tomatine poisoning, which can cause gastrointestinal and neurological effects. These symptoms are similar to those caused by other glycoalkaloids and can manifest as vomiting, diarrhea, abdominal pain, and stomach cramps. More severe reactions may involve drowsiness, confusion, or general weakness.
However, the practical risk of severe toxicity for an adult human is extremely low. To ingest a dose considered acutely toxic, an adult would typically need to consume a massive quantity, estimated to be over half a kilogram of the dried leaves. Tomatine is also poorly absorbed into the bloodstream because it can complex with cholesterol present in other foods in the stomach, which further limits its bioavailability and toxic effect.
Most accidental ingestion, such as a child or pet sampling a small amount, results only in mild gastrointestinal discomfort. The bitter taste of the leaves and stems acts as a natural deterrent, making it unlikely that a large, toxic quantity would be consumed. If a large amount is consumed, symptoms are typically mild and resolve within a few hours or days.
Safety Status of Other Plant Components
The fruit of the tomato plant also contains tomatine, but its concentration changes as it matures. Unripe green tomatoes contain the highest levels of the alkaloid. As the tomato ripens and turns red, the plant naturally metabolizes the alpha-tomatine into a form that is no longer a concern.
This process results in fully ripe red tomatoes having a negligible amount of the compound. Even with the higher levels in green tomatoes, they are considered safe for consumption in typical culinary preparations like fried green tomatoes. The widespread consumption of the fruit, even when unripe, demonstrates that the overall concentration does not pose a general health risk in normal quantities.