In heavily exercising muscles, which metabolic process is activated to deal with high NADH levels?

• A. Alcohol fermentation
• B. Krebs cycle
• C. Lactic acid fermentation
• D. Glycolysis

Answer: (C)

During intense exercise, muscles rely on anaerobic processes to quickly produce energy in the form of ATP, especially when oxygen supply is limited. In this scenario, lactic acid fermentation is the metabolic pathway that becomes activated to manage high levels of NADH, which accumulates as a byproduct of glycolysis.

As glucose is broken down to produce ATP, NAD+ is reduced to NADH. When the demand for ATP is high and the aerobic pathways are insufficient to keep pace, the buildup of NADH can hinder glycolysis, as regeneration of NAD+ is crucial for continuing this process. Lactic acid fermentation helps to regenerate NAD+ by reducing pyruvate (the end product of glycolysis) to lactic acid. This allows glycolysis to continue, providing ATP even when oxygen levels are low.

The other metabolic processes listed do not primarily address the immediate need for ATP production in the same way during intense exercise. Alcohol fermentation occurs in yeast and not muscle cells, the Krebs cycle requires oxygen and operates at a slower pace for steady-state energy production, and while glycolysis is active, it cannot proceed without sufficient NAD+, which lactic acid fermentation provides by converting NADH back to NAD+. Thus, the activation of lactic acid fermentation is key