People often hear that eating lychee causes heatiness; heatiness is a concept from traditional medicine. So why does eating a large amount of lychee cause discomfort?

First, the flesh of lychee (the false seed coat) contains a very high sugar content, reaching 20%, which can create a hyperosmotic environment in the mouth and throat, leading to dehydration, dryness, and cracking of the mucous membranes, and also easily fostering bacteria, causing mouth ulcers. This is a typical "heatiness" sensation.
But more importantly, unlike the longan, a crop from the same family, lychee's unique and dangerous aspect is that it contains Hypoglycin A (Hypoglycin A) and Methylcyclopropylglycine (MCPG). These two toxins, when ingested in large amounts on an empty stomach or when glycogen reserves in the body are insufficient, can cause serious consequences. They inhibit fatty acid beta-oxidation, blocking the liver's gluconeogenesis pathway.
When the high sugar content of lychee is metabolized by the body or consumed through insulin secretion, the toxins block the gluconeogenesis pathway, preventing the conversion of non-carbohydrate substances into blood sugar, leading to severe acute hypoglycemia and metabolic disturbances. This condition is known as lychee poisoning.
Therefore, the folk saying that eating lychee "causes heatiness" actually describes the hyperosmotic pro-inflammatory response caused by high sugar, as well as the acute metabolic disturbances induced by toxins.