Why do lychees cause "internal heat"?

The "heatiness" reaction after eating lychees is not mysterious; it's all the fructose playing tricks. Aside from giving a touch of sweetness to our taste buds, the fructose in lychees also competes with glucose for its working position. Although it can also serve as an energy substance, fructose must be converted into glucose in the liver before we can utilize it, and this conversion process also consumes energy. If you eat large quantities of lychees, you may feel full. However, we cannot obtain sufficient glucose at all.

With insufficient glucose, components including the brain naturally cannot function properly. Thus, people who have indulged excessively in lychees then experience "heatiness."