One could design a mirror life form that uses mirrored amino-acids to build its proteins and also mirrored carbohydrates as building components, but which would nonetheless also have enzymes for the hydrolysis of normal proteins and carbohydrates and for their oxidation in order to produce energy, so it could eat non-mirrored food as a source of energy, even if it would need to synthesize for its growth mirrored molecules.

Mirrored amino-acids and carbohydrates are not completely missing in nature, some of them are produced by certain living beings for various purposes, but they are not used for building proteins or other macromolecules, i.e. they are not usable for growth.