101 Facts About Ageing #45: Cardiorespiratory Fitness Declines With Age

As Daniel Patrick Moynihan, an American sociologist, politician, and diplomat once said: “Everyone is entitled to his own opinion, but not his own facts”. And we wholeheartedly agree. A shared set of facts is the first step to building a better world with longevity for all. In that spirit, we are creating a series that covers 101 indisputable facts about ageing, health and longevity.

We discussed in fact #42 how greater cardiorespiratory fitness is strongly associated with reduced mortality. Unfortunately, cardiorespiratory fitness tends to decline with age. Cardiorespiratory fitness can be gauged by measuring the body’s maximum ability to utilise oxygen during exercise – if you make a person work harder and harder, they will eventually reach a point of maximum oxygen consumption.

By measuring the concentration of oxygen in the air they inspire and expire, we can calculate the rate at which a person is utilising oxygen (VO2max). This rate is limited by their cardiorespiratory fitness – factors like how much oxygen is reaching the alveoli in the lungs, the total surface area of the alveoli over which oxygen can enter the blood, how quickly the heart can deliver that blood to the muscles, and how quickly the muscles can utilise that oxygen. As people with more body weight will require more oxygen, this value is normalised to body weight to obtain VO2 max in millilitres per kilogram of body weight per minute.

The above graph shows the trend for the decline in average VO2max with age in women, depending on level of physical activity. For males, VO2max is about 25% higher at any given age. Generally speaking, VO2max declines at a rate of about 10% per decade after the age of 25. Unfortunately, although being more physically active increases your VO2max at a given age, it doesn’t slow the rate at which VO2max declines.

What causes this change? As already mentioned, the rate of oxygen consumption is limited by the rate of oxygen exchange in the lungs, how quickly the heart can deliver that blood to the muscles, and how quickly the muscles can utilise that oxygen, all of which are impacted by ageing (though their relative contributions to VO2max are not fully agreed upon).

• Maximum cardiac output declines because the intrinsic rhythm of the heart becomes slower, the heart responds less well to adrenergic stimuli, the arteries and walls of the heart become stiffer, and the contractile function of the cardiac muscle is reduced.
• The rate of oxygen exchange in the lungs decreases because of an increase in alveolar dead space (the volume taken up by alveoli that are not supplied with blood), accumulated damage to the alveoli making gas exchange less efficient, and structural changes that reduce the overall flux of gas into and out of the lungs.
• The rate at which muscles utilise oxygen drops because of a reduction in total muscle mass with age, reduced blood supply to the muscle tissue, and a reduction in the number and the quality of the mitochondria, which are the cellular components responsible for using up the oxygen in order to produce energy for the cell in the form of ATP.