Survival of the Fittest

Survival of the Fittest

In 1859 Charles Darwin published his highly controversial work “On the origin of species” and shortly after, the phrase “survival of the fittest” was born. The term is used to describe intergenerational survival of traits better adapted to the environment. When used literally, in relation to an individual’s longevity based on cardiorespiratory fitness, “survival of the fittest” takes on a whole new and potentially life-saving meaning.

Research over the last 20 years has proven time and time again that cardiovascular fitness is an excellent predictor of future mortality. Why then does it seem that so few medical professionals and patients care about this crucially important and modifiable factor?

Perhaps it is because the magnitude of the effect is simply not known or is underestimated. Smokers frequently have their smoking habits questioned in medical consults, by friends, family and even by the media, yet rarely will a medical professional, other than an exercise physiologist, ask how someone’s exercise tolerance is going.

Perhaps the most important study regarding exercise tolerance and ‘all cause’ mortality was published in the New England Journal of Medicine in 2002 patients both with and without cardiovascular disease. This allows the prognostic utility of exercise capacity to be directly compared with well-known and established risk factors. The study looked at 6213 patients referred for exercise stress testing and looked to determine which factors proffered the best prognostic information. The study showed that the best predictor of increased risk of death was poor exercise tolerance.

That’s correct, poor exercise tolerance was a greater risk factor for death than years of smoking, congestive heart failure, history of myocardial infarction, pulmonary disease, hypertension, left ventricular hypertrophy, diabetes, hyperlipidaemia and exercise induced ST segment depression.

The relative risk of someone in the lowest quintile of fitness dying compared with those in the highest quintile was 4.5 for normal subjects and 4.1 for those with established cardiovascular disease. That means that the least fit had an over 400% likelihood of death when compared with the most fit during the follow up period. The likelihood of mortality during the 6.2 year follow up period for patients who could not achieve five metabolic equivalents was double that of those who could exceed eight. Every 1-MET increase in performance was associated with a 12% improvement in survival.

Other studies assessing fitness interventions exist and importantly the findings are exactly what you would expect. The National Exercise and Heart Disease Project looked at an exercise intervention in patients who had suffered a myocardial infarction. For every 1-MET increase in exercise capacity patients had an 8-14% reduction in mortality in the 19 year follow up period.

It is important to note that a 1-MET increase in exercise capacity is very modest and in the NEJM study the most striking differences in outcomes were found between the lowest and second lowest quintiles of exercise capacity. It is therefore reasonable to assume that only modest improvements are needed to have very significant positive outcomes. Patients don’t have to aim for elite status, just getting out of the bottom 20% will have significant positive outcomes.

Exercise is a free intervention which can have a positive effect on all of our lives. It is certainly at least as effective as many of the medications we prescribe. Perhaps we should begin to consider it more when we discuss risk factors with our patients (and ourselves).

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