Glycerol can act as a substrate for glucose synthesis by conversion to dihydroxyacetone phosphate and an intermediate in gluconeogenesis.  In  order  for citric  acid  cycle pyruvate, lactate,  intermediates  and  the  carbon  skeletons  of mainly  amino  acids  to act as precursors  for gluconeogenesis and these  compounds must  first  be  transformed  to  oxaloacetate.  several  of  the  carbon  skeletons  of  the amino  acids  give  increase  to oxaloacetate  straightly.  Others feed into the citric acid cycle as intermediates and the cycle then converts these molecules to oxaloacetate.  Lactate is changed to pyruvate through the lactate dehydrogenase reaction and some amino acids also provide enhance to pyruvate.  Therefore,   for these precursors the first step in the gluconeogenic pathway is the conversion of pyruvate to oxaloacetate.