Aliens could starve to death on Earth

Firstly, some fundamental biological interactions can’t happen. Just like with our glucose example, our food would have zero calories to a Mirror Image alien. So at the very least, Mirror Image aliens would starve to death here.

But, just because a compound is an opposite chirality doesn’t mean it glides through passively. Some opposite chirality compounds interact with the wrong mechanism, causing massive damage in the process.

You can see this in the infamous drug Thalidomide, which has two chiral versions of the compound and was produced in a way that made both types. One of them was a brilliant drug for combating morning sickness. The other was toxic and caused congenital disabilities and an increased risk of stillbirth. Sadly we only found out about this after it went into wide use.

So if a Mirror Image alien was to come into contact with our proteins, sugars and amino acids, it is possible that it could poison or even kill the alien. Whilst these compounds have the correct chirality to enable our life, they could mess up all the internal workings of the alien, throwing a spanner into their works.