One of the main reasons that aerobic athletic performance declines with age is that the body becomes less effective at utilizing oxygen. The maximal ability to utilize oxygen can be measured by VO2max, an assessment of how much oxygen your body can use per kilogram of body weight. As a result, a high VO2max will indicate that a person can effectively utilize oxygen, which is often seen in well-trained endurance athletes.

Unfortunately, after the age of 30, VO2max will begin to decline. For non-athletes, VO2max tends to decline by about 10% every decade. However, athletes who continue their rigorous training can reduce the decline to only 5% every decade. The main reason that VO2max declines with age is because the maximal heart rate (max HR) decreases as well.

Cardiac output = stroke volume x maximal heart rate

Because cardiac output is the product of stroke volume and maximal heart rate, a decrease in max HR will directly decrease cardiac output, and in turn, oxygen delivery to the muscles. This translates to a lower VO2max and decreased athletic endurance with age. Fortunately, vitamin K2 appears to improve cardiac output by maintaining healthy mitochondrial function.