How to Decrease Max Health and Lose Hearts in Minecraft

Let's create the scoreboard below, which will score according to the amount of health we have.

scoreboard objectives add set_health health

By default, our health is 20, represented by 10 hearts.

Place a Repeat command block set to Always Active with this command:

execute as @a[scores={set_health=19}] run attribute @s minecraft:generic.max_health base set 19

As soon as a player's health in the world reaches 19, the attribute command is executed and will reduce the maximum health according to the player's current health.

For 18 health, you will need to do the same command, just replacing 19 with 18 in the commands above. For 17 it's the same thing, and so on.

You can place other Chain command blocks set to Always Active connected to the Repeat command block and line them all up. The video below shows the idea well.

If you want to return to the default maximum health, just run this command:

attribute @p minecraft:generic.max_health base set 20