Which of the following substances appears to be MOST responsive to exercise training?

High-density lipoprotein (HDL) cholesterol is recognized as the substance most responsive to exercise training due to the significant positive impact physical activity has on its levels. Engaging in regular aerobic exercise is linked to an increase in HDL levels, which is considered beneficial as HDL helps transport cholesterol away from the arteries and back to the liver, where it can be processed and excreted. This relationship highlights exercise as a key factor in improving cardiovascular health.

Other substances such as total cholesterol, low-density lipoprotein (LDL), and triglycerides may also be influenced by exercise, but the responses are not as pronounced or consistent as with HDL. For instance, total cholesterol can fluctuate based on a range of dietary and lifestyle factors beyond just exercise. LDL levels, often associated with increased cardiovascular risk, may decrease with exercise but not as reliably as HDL increases. Triglycerides can also be lowered through exercise; however, the effect is less specific, as they are heavily influenced by nutritional intake and body weight.

Thus, HDL cholesterol's distinctive and robust response to exercise training elevates its significance in understanding the benefits of regular physical activity on lipid profiles and overall heart health.