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A high cardio-respiratory fitness is required to participates in the Ironman triathlon in Hawaii.Marco Garcia/The Associated Press

The Ironman Triathlon World Championship was recently held in Kona, Hawaii. This annual event brings together more than 2,000 participants who want to subject themselves to a four-kilometre ocean swim, a 180-km bike ride and a 42-km marathon run in conditions of high heat and humidity. There may be no fitter place in the world than Kona in the week leading up to the big event.

The annual Ironman Sports Medicine Conference preceded the race, and many of the keynote presentations were focused on high-performance athletes. A key reminder that I took away from the meeting, however, was the profound importance of cardio-respiratory fitness for health, and the potential for even modest doses of exercise to extend your life.

Cardio-respiratory fitness is a marker of the functional status of the cardiovascular, respiratory and skeletal muscular systems. Put simply, it reflects the capacity of the body to transport and utilize oxygen. A high cardio-respiratory fitness is critical for a triathlete who wants to compete in Kona. But even if you never aspire to be an endurance athlete, increasing your cardio-respiratory fitness can help to extend your life.

This message was reinforced in a recent report based on the Aerobics Center Longitudinal Study that examined the relationship between leisure-time physical activity and mortality in more than 55,000 adults. It found that running even five to 10 minutes per day at a relatively slow pace (more than six minutes per kilometre) was associated with a markedly reduced risk of dying from all causes.

Is there something special about running? Probably not, and it is more likely the improved cardio-respiratory fitness achieved by running is what helps to extend your life. That's good news because many different forms of exercise can improve your cardio-respiratory fitness, so long as large muscle groups are engaged. So if running is not for you, choose another activity, such as walking, hiking, cycling or swimming.

Compelling scientific evidence has established cardio-respiratory fitness as a strong and independent predictor of the risk of dying from all causes. In fact, several studies suggest that poor cardio-respiratory fitness is at least as important as the other major risk factors such as obesity, high blood pressure and diabetes. However, the importance of cardio-respiratory fitness is sometimes overlooked from a clinical perspective. This may be due in part to the fact that it is easier to assess some of the other traditional risk factors, for example, by measuring waist circumference.

Cardio-respiratory fitness is measured directly during an incremental exercise test to exhaustion. This test typically involves running or cycling at progressively higher workloads, while the amount of oxygen used by the body is measured through a mouthpiece or airtight mask. This test, which is also known as a maximal oxygen uptake or "VO2max" test, requires specialized laboratory equipment and considerable effort on the part of the subject. Cardio-respiratory fitness can also be estimated based on submaximal tests such as a stress test.

One way to express cardio-respiratory fitness is using metabolic equivalents, or "METs," which are multiples of the oxygen uptake required to support metabolism at rest. (One MET is precisely defined as an oxygen uptake of 3.5 millilitres of oxygen per kg of body mass per minute.) A healthy adult of average fitness might have a cardio-respiratory fitness of 10 METs (or an ability to work 10 times higher than rest), whereas endurance athletes can have values that exceed 20 METs.

One review that examined the association between cardio-respiratory fitness and mortality considered 33 studies involving more than 100,000 men and women. It found that every 1-MET increment in cardio-respiratory fitness (corresponding to approximately 1 km/h higher jogging or running speed) was associated with a 13-per-cent reduction in the risk of dying from all causes. To put this change into context relative to some traditional risk factors, the authors explained a 1-MET increase was comparable to the effect of a 7-centimetre decrease in waist circumference, a 5-milimetre Hg decrease in blood pressure or a 1-millimole reduction in fasting blood sugar.

Cardio-respiratory fitness is influenced in part by non-modifiable factors such as age and gender. However, the main determinants are lifestyle-related, and in particular, habitual physical activity. One of the best ways to increase and maintain your cardio-respiratory fitness is to meet the recommendation of 150 minutes of moderate- to vigorous-intensity aerobic physical activity per week, such as brisk walking.

Most Canadians do not meet the minimum physical activity recommendation, often citing "lack of time" as the main barrier. Brief sessions of interval training may provide an option for time-pressed individuals to improve cardio-respiratory fitness, but the trade-off is that the exercise needs to be very intense. A recent study from our laboratory found that subjects who performed only three minutes of "all out" intermittent cycling per week, within a total weekly time commitment of 30 minutes (including warm-up and cool down), improved their cardio-respiratory fitness by 12 per cent, or approximately 1 MET, after six weeks of training.

Brisk walking is effective, though. In fact, walking speed is an objective measure of physical capability that has been used to predict survival. An Australian study analyzed walking speed and mortality in over 1,700 older men. It found that men who could walk at speeds greater than three kilometres per hour were less likely to encounter death. The authors estimated that the Grim Reaper's likely maximum speed was five kilometres per hour, since no men in the study who could walk at this pace were caught by death. Those who wish to outpace death are therefore advised to move faster than the Grim Reaper's maximum pace.

Health Advisor contributors share their knowledge in fields ranging from fitness to psychology, pediatrics to aging.

Dr. Martin Gibala is a professor and chair of the Department of Kinesiology at McMaster University. You can follow him on Twitter @gibalam

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