Aliens' Chirality Means No Food on Earth

We humans all get energy out of D-glucose. But, our proteins cannot deal with its asymmetrical cousin L-glucose. Just as our left hand cannot shake hands with our right hand, aliens could very well work with L-glucose and thus, they would not be able to find food on Earth (D-glucose).

Maybe this helps explain why no aliens have yet been detected on Earth.

Link to relevant article.