What is caused when too little calories are taken in?

Weight loss happens when you eat less calories than you use up.

If you have weight to lose, this is probably a good thing.

But if you eat a lot less calories than you need, your body might go into "energy saving mode".

You get cold, slow and tired.

Your immune system weakens.

Eventually, you start breaking down muscle tissue to remain alive.

You get emaciated.

Ultimately you starve to death.