If you change the config you have to restart the world in order for it to apply the changes. You can't change the diet config while the world is on. Shame.

As a general rule for any mod and vanilla, you need to type "/reload" for new server configs and data packs to take affect.

Also check out the Nutritional Balance wiki for more info: https://github.com/dannydjdk/Nutritional-Balance/wiki/Customization-and-Integration

Have you typed "/reload"?

I reloaded my game but I'll give this a try instead.