Why is pyruvate build up bad, but lactic acid is good (or better)?

Question

I know lactic acid is bad for the body, it's what causes the muscles to be sore. But why can't pyruvate build up in the body, why does it have to be converted into lactic acid?

Answer 1

Hi Chang.

The simple answer is that the conversion of pyruvate into lactate oxidizes NADH back into NAD+, thus allowing the process of energy formation via anaerobic glycolysis to continue. Additionally, the liver in mammals can reuse lactate to help synthesize more glucose to fuel glycolysis.

To understand this a little better, it may help to review the processes of cellular energy production. Remember that the "ideal" way of making energy is for glycolysis to proceed into the citric acid (Kreb's) cycle, which produces at total of 38 ATP (energy). However, the citric acid cycle requires the presence of adequate oxygen.

When the cells lack adequate oxygen, such as when you work your muscles harder than your your body can keep up with, the citric acid cycle cannot continue. In these anaerobic circumstances, the body adapts by making energy through glycolysis alone. I say this is not "ideal" because glycolysis only produces 2 ATP per molecule of glucose rather than 38 ATP. Also, the pyruvate that would normally enter the citric acid cycle starts to build up. This is when the conversion to lactate occurs. Lactate is much more useful to the body then pyruvate (glucose synthesis), and as stated before, this reaction also returns NADH to NAD+, allowing it to be used again for glycolysis.

I hope this answer clears this up a bit!