What happens to lactic acid I eat?
First, there are two different isomers of the lactic acid, the L(+)- and the D(-)-form. Both differ in the position of the OH-group in the molecule:



Both turn polarized light in different ways, the D(-) to the left and the L(+) to the right. The physiological form of lactic acid for the human body is the L(+) form, which is taken up in the gut and then metabolized by the L-Lactatdehydrogenase to form pyruvate and NADH.

For the D(-)-Lactate is has been thought the the D-Lactatedehydrogenase is only present in some lower animals and bacteria and that this lactate isoform is mostly excreted. This seems not to be true and there is some D-Lactatedehydrogenase in humans (and other mammals) as well. 

See these publication for more information:


D-lactate in human and ruminant metabolism.