Feeling healthy v. objective fitness


As of September 2017, new Sweat Science columns are being published at www.outsideonline.com/sweatscience. Check out my bestselling new book on the science of endurance, ENDURE: Mind, Body, and the Curiously Elastic Limits of Human Performance, published in February 2018 with a foreword by Malcolm Gladwell.

- Alex Hutchinson (@sweatscience)


Which is more important: feeling healthy, or being fit? That’s the question addressed by a new British Journal of Sports Medicine study from Steven Blair’s group at the University of South Carolina. Blair is best known for pioneering the research that suggests that fitness is more important than fatness as a predictor of health outcomes. In this case, he and his colleagues take a look at “self-rated health” (SRH), which has been touted lately as a valuable health-assessment tool.

SRH is assessed by simply asking:

How would you rate your overall health?

The answers are poor, fair, good or excellent. The researchers compared the predictive values of SRH to aerobic fitness, assessed using a maximal treadmill test (with the modified Balke protocol, which is apparently highly correlated to VO2max). The study followed 18,488 men who were tested between 1987 and 2003, 262 of whom died during the 17-year follow-up period. They controlled for age, BMI, physical activity, smoking, alcohol, heart disease, diabetes, cancer, and several other confounders.

Not surprisingly, being fit and having a high SRH reduced your chance of death dramatically. The real question is what happens when you separate the two factors. Aerobic fitness was strongly protective against mortality even when all other factors were controlled. SRH and mortality were were still inversely correlated, but the association was “only marginally significant (p=0.09)” once aerobic fitness was controlled for.

Given the previous research topics of this group, I suspect they set out to show that all the apparent predictive power of SRH is just a byproduct of aerobic fitness. It seems safe to conclude that fitness is indeed the more important of the two, but it’s interesting that SRH retained some predictive power. Their thoughts on why this might be:

One plausible explanation is the afferent information that conveys messages from the organism to the brain. These messages are usually not brought to consciousness because they function at lower levels of the central nervous system. However, this afferent information is perceived by the individual as sensations, feelings and emotion and is the sense that reflects the physiological condition of the entire body. Another theory explaining a person’s perceptions of their health involves a family of proteins called cytokines. Cytokines are involved in inflammation processes and play a major role in infectious conditions and also the pathogenesis of many chronic diseases. Research is beginning to show that the inflammatory process and certain cytokines are associated with tiredness, impaired sleep, depressive mood and poor appetite.

On one level, this seems a bit needlessly complicated. After all, most of us can probably make a reasonable assessment of how healthy we are without relying on unconscious afferent feedback and cytokines! Still, the idea that our “sensations, feelings and emotion” can reflect our underlying health status is also interesting — and inarguable. After a few late nights, skipped workouts and junk-food binges, we tend to feel like crap. This isn’t just guilt and fatigue: it’s the body sending a distress signal.