Why did Napoleon never bring his old guard into battle during the Battle of Borodino?

Why did Napoleon never bring his old guard into battle during the Battle of Borodino? What does the expression mean, “800 leagues from Paris, I cannot risk my last reserve”?

Napoleon was afraid of losing his “old guard”, because it represented the most efficient part of his army. In case of what exactly the regiments of the old guard could save the life of the emperor and save him from complete defeat. Nevertheless, many historians believe that if Napoleon had thrown the old guard against the Russian army in the battle of Borodino, then most likely Kutuzov’s army would have been defeated.