Let's say I drink some Kefir Milk, what happens to the lactic acid in the Kefir Milk that has entered my digestive system?
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:
Lactic acid has been found to act as a fuel for the muscles. Refer to: http://www.nytimes.com/2006/05/16/health/nutrition/16run.html?_r=0
You may also refer to http://www.eatmoveimprove.com/2009/11/the-truth-about-lactic-acid/ for details on lactic acid metabolism.