When a seed is eaten, the outcome depends on its structure, chemical composition, and whether it is mechanically broken down before swallowing. Biologically, a seed is an embryonic plant encased in a protective outer layer called the seed coat. This interaction involves a complex interplay between plant defense mechanisms and human physiology.
The General Digestive Fate of Seeds
Most small, accidentally swallowed seeds (such as those from berries, tomatoes, or cucumbers) pass through the human digestive system largely unaffected. This is because the seed coat is composed mainly of cellulose, a complex carbohydrate. Humans lack the necessary enzyme, cellulase, to break down cellulose, meaning this protective layer remains intact throughout the gastrointestinal tract.
The digestive tract, with its powerful acids, enzymes, and churning action, is an extremely hostile environment for plant matter. Any seed that survives this process does so because its durable coat protects the delicate embryo within. Contrary to a common myth, a seed cannot germinate and grow inside the human body, as the conditions required for sprouting are absent. The seed is simply eliminated with other waste, often appearing undigested in the stool.
When Seeds Are Intentional Food Sources
The deliberate consumption of seeds for nutritional benefits contrasts with accidental ingestion. Seeds like chia, flax, hemp, and pumpkin are rich in healthy fats, protein, and fiber. To access these nutrients, the seeds must often be chewed or mechanically processed.
Flaxseeds, for example, are most beneficial when consumed in a ground form, allowing the body to absorb the omega-3 fatty acids, specifically alpha-linolenic acid (ALA), and lignans. Chia seeds, while often eaten whole, absorb water and form a gel-like substance in the stomach due to their high soluble fiber content. This supports digestive health and promotes a feeling of fullness. This intentional breakdown releases the seed’s nutritional components for human absorption, rather than allowing them to pass through undigested.
Identifying Seeds and Pits with Toxic Compounds
A health consideration involves seeds and pits that contain cyanogenic glycosides, chemical compounds that release hydrogen cyanide (HCN) when metabolized. This mechanism is a natural defense developed by the plant. Seeds from the Rosaceae family, including apples, cherries, peaches, apricots, and plums, contain a specific glycoside called amygdalin.
Toxicity is not a concern if the seed or pit is swallowed whole, as the hard outer layer shields the amygdalin from digestive enzymes. The risk emerges when the seed or pit is crushed, chewed, or ground, allowing the amygdalin to mix with enzymes in the digestive tract and release the toxic compound.
The amount of cyanide released depends on the quantity of seeds consumed and whether they were broken down. While one or two chewed apple seeds are unlikely to cause harm, consuming a large number of crushed seeds or pits can lead to symptoms of cyanide poisoning, such as nausea, headache, and vomiting. For small children, even a few crushed kernels could pose a serious risk. Therefore, it is advisable to remove and discard the pits and seeds of these fruits before consumption or blending.