In the Prussian army, the Leib-Husaren-Regiment (shortly afterwards split in two regiments) got this title in 1808 for distinction in the 1807 campaign. The Leib-Infnterie-Regiment was formed in 1808 as well, from the garrison of Kolberg, whose defense had become famous.
So it was just a distinction. The Prussian king did not figure in the list of officers of the Leb regiments, but he was regimental chief of the real guard units Regiment Garde zu Fuß and Regiment Garde du Corps.
The Leib units were not part of the guards, for example in 1815 they were not incorported in the guard corps. However, in 1808, towards the French government, they were declared to be part of the guards, in order to match the conditions of the treaty of Paris, which limited the strength of the Prussian army to 42000 men.
According to the latter, the Prussian army was allowed to have 22000 men infantry in 10 regiments, 8000 men cavalry in 8 regiments, 6000 men artillery and 6000 men guards whose composition was not prescribed.
As there were 12 regiments of infantry, one of them the Regiment Garde zu Fuß (regiment of foot guards), the king listed the Leib-Infnterie-Regiment between the units "which have to be counted as guards". Thus the allowed number of 10 line regiments remeined for the infantry.
Towards the French, the Leib-Husaren-Regiment was declared as guards as well. Later, 16 line regiments of cavalry with four squadrons each were allowed by the French government (instead of the 8 mentioned in the treaty), these were 3 cuirassier, 6 dragoon, 5 hussar (not counting the two Leib hussar regiemnts), and 2 lancer regiments.