Minecraft wiki entry for Hunger, associated text: When the hunger bar is at 20 () and there is any saturation left, health regenerates 1 every 1⁄2 second.‌[Java Edition only]
When the hunger bar is at 18 () or more, health slowly regenerates 1 every 4 seconds.
When the player is damaged with 18 ( × 9) or more, health regenerates 2 for one time.‌[Bedrock Edition only]
When the hunger bar is at 17 () points or below, the player does not naturally regenerate health.
If the hunger bar is at 6 () points or below, then the player cannot sprint.
When the hunger bar is at 0 (), the player's health depletes at a rate of 1 every 4 seconds (this makes sleeping impossible). On Easy difficulty, the player's health stops dropping at 10, on Normal, it stops at 1, and on Hard, it keeps draining until either the player eats something or starves to death. Resistance and armor do not reduce starvation damage.[1]