Fitness, Health, Risk

February 8, 2010

Popular discourses on fitness, health and risk are filled with confusion, misunderstanding, and sometimes willful ignorance. Let this quick post be your guide to sorting out the three concepts.

Fitness. Generally speaking, fitness is capability to perform a certain task. Therefore, “fit” is fairly useless as a descriptor, because it does not specify what the person in question is fit to do. “Are you fit?” can always be answered with “fit for what?” There exist many “fitness” standards – for cardiovascular fitness, strength, flexibility, and so forth. Arguably, the bare minimum of fitness is being fit enough to sit on the couch without keeling over and dying. However, neither this nor any other fitness standard gives a good estimate of health, more on which below.

Health. Unlike fitness, health is a state of being, generally understood as a state in which the body is not suffering from disease. Depending on your relationship with postmodernism, you can add qualifiers such as “socially constructed disease,” etc. “Being healthy” is somewhat more informative than “being fit,” although again the operationalization of health varies considerably. In some cases, health is summarized as just a handful of measures, while in others, it is used to mean the state of the entire body. Using the latter maximal definition of health, or some approximation thereof, we can evaluate fairly accurately whether an individual is healthy (in fact, medical professionals do this daily with their patients), but unfortunately we may not be able to ascertain an individual’s risk for future problems.

Risk. Unlike health or fitness, risk does not reside within the individual. In discussing the risks of developing future health problems, it is important to clarify two points. First, what is the outcome for which risk is being computed? Second, is the risk to be computed as is, or net of other effects? The importance of the first question is self-evident: risk for early mortality is quite different from risk for chronic diseases, which is quite different from risk for low self-reported health. The importance of the second question is more elusive. Risk can be computed simply by correlating a predictor variable with an outcome variable: this is “as is” risk. This basic correlation can be controlled for confounding factors – gender, socioeconomic status, health behaviors, and age, to name a few – yielding (theoretically) the true net effect of the predictor on the outcome. For example, the “as is” risk of mortality is greater among underweight people relative to “normal weight” people (using standard BMI categories), but controlling for factors such as smoking and disease explains this association and results in a much smaller “net” risk. Risk, by its nature, cannot be computed from a single case. It requires referencing studies of large and representative samples to make a well-validated estimate. By the same line of logic, risk cannot be assessed from either fitness or health without appealing to robust statistical findings.

The final point to keep in mind is that these three concepts carry very different standards of proof. At one extreme, “fitness” for a task can be proven simply by doing that task. At the other extreme, the estimated risk of a certain outcome for a given person is always open to revision by newer and more sophisticated definitions and analytical techniques, but cannot be “tested” through personal anecdote. Beware when discussants switch concepts to down- or up-shift the standard of proof to their own advantage.

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