In what sense does exercise keep you "young"?

Exercise is good for the body; exercise is good for the mind (Mayo Clinic). People like to say it can even help you to stay young.

But wait, is it literally true that exercise can keep us young?

The New York Times describes a new study that indirectly addresses this question. In the study, researchers examined a group of older cyclists, and measured their physical and mental physiological functions including cardiovascular fitness, endurance, muscle mass, strength, reflexes, balance, and memory:

"To accomplish that goal, the scientists recruited 85 men and 41 women aged between 55 and 79 who bicycle regularly....

The scientists then ran each volunteer through a large array of physical and cognitive tests. The scientists determined each cyclist’s endurance capacity, muscular mass and strength, pedaling power, metabolic health, balance, memory function, bone density and reflexes."

The primary goal of the study was to try to identify "physiological markers that can be used to reliably predict the age of an individual." The researchers chose a group of cyclists to mitigate variations in lifestyle factors. In other words, they selected a cohort of people who all exercised a lot so that variations in activity would not contribute to variations in the markers.

One question they asked is how do these various physiological functions correlate with age within the group of cyclists. A second question was how do the cyclists' physiological markers compare to those of more sedentary (i.e. normal) people who do not exercise so much.

Interestingly, they found that none of the physiological function markers showed a high correlation with age. Despite the fact they exercised to a similar degree, there were still a lot of individual physiological differences in cyclists of the same age. The marker with the best correlation with age was VO₂ max with a correlation coefficient of ~0.4. According to Wikipedia, "VO₂ max is the maximum rate of oxygen consumption as measured during incremental exercise, most typically on a motorized treadmill. It is a measure of aerobic physical fitness."

Thus, physiological markers like VO₂ max or blood pressure provide some information about age, but they are imperfect predictors, and as a result the authors concluded that "the relationship between human aging and physiological function is highly individualistic."

In the second half of the paper, the authors compared the physiological functioning of the cyclists to that of a typical group of people with a more sedentary lifestyle. They found that the cyclists possessed the physiology normally associated with a much younger person.

For example, there is a crude heuristic for estimating your age from your maximal heart rate ("The maximum heart rate (HR_max) is the highest heart rate an individual can achieve without severe problems through exercise stress, and generally decreases with age.... In this test, a person is subjected to controlled physiologic stress (generally by treadmill) while being monitored by an ECG." ). Researchers have fit a linear relationship to maximal heart rate data from a sample of normal people spanning a range of ages. They found that the equation that fit the data the best was (crudely speaking):

HR_max = (220 - age), or age = (220 - HR_max)

Thus, a typical 80 year old should have a maximal heart rate of 140 beats per minute (bpm). By comparison in the group of cyclists, the approximate average maximal heart rate of the 55 year olds was 175 bpm (which corresponds to an age of 45) and the average maximal heart rate for the 80 year olds was 160 bpm (which corresponds to an age of 60). Thus, the cyclists had a "heart age" of someone roughly 10-20 years younger.

Does this mean the cyclists will live 10-20 years longer than the average person? The answer is we don't know (but probably not). In this particular study, there was no attempt to correlate physiological functions to longevity (i.e mortality was not examined). However, one can speculate that if you possess the physiological age of someone younger, then you may live longer too. Indeed others have shown that there is a relationship between VO₂ max and all-cause mortality although once again there is a lot of individual variation in this relationship [link].

So exercise, and in this specific example cycling, does indeed help keep us physiologically young which may indeed extend lifespan.

Figure 1. A person who exercises regularly may have the physiological functioning of someone much younger.