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
Most athletes consider lactic acid their enemy, and think that training helps eliminate the metabolic waste product from their muscles so the muscles will function longer and harder. But UC Berkeley physiologist George Brooks has found that training actually teaches muscle cells how to use lactic acid as a fuel to get more bang for the buck. With training, mitochondria grow, take in more lactate via a shuttle and burn it to generate more energy.
"I gave rats radioactive lactic acid, and I found that they burned it faster than anything else I could give them," Dr. Brooks said.
You may also refer to http://www.eatmoveimprove.com/2009/11/the-truth-about-lactic-acid/ for details on lactic acid metabolism.