A healthy mind in a healthy body or mens sane in corpore sano, as Decimus Junius Juvenalis put it in his time, is a phrase that seems to imply some causal relationship between exercise and a sound mind. However, when Juvenalis (one of the great Roman satirists of his time) made himself immortal with these words, he at least seemed to care to make a causal statement. Juvenal's complete statement is actually:

"If and only if there is anything worth praying for at all, then if you must,pray for a healthy mind in a healthy body, a valiant hearth without fear of death,that reckons longevity the least among Nature's gifts, that's strong to endure all kind of toil, that's untainted by lust and anger, that prefers the sorrows of labours of Hercules..."

First rule of methodology: Be careful with causality

Proving causality has always been tough in the social sciences, as one needs a well-designed experiment in order to make more of a mere correlation. I will argue here that although exercise is healthy it does not necessarily increases your psychological well-being. More research is needed to elucidate the mechanisms behind the relationship between exercise and well-being. Although it would be great if we could aid people by letting them exercise regularly, more knowledge is needed in order to interpret the ambiguous results obtained through clinical trials in which depressed patients are treated via an exercise program.

The World Health Organization advises at least half an hour of extensive exercise per day in order to stay in healthy shape. It is thought that people who do not exercise on a regular base are more often anxious, depressed, and have lower self-esteem than people who do not exercise (Lawlor & Hopker, 2001). Exercise as an intervention has been studied extensively. Most studies focus on clinical patients comparing the effects of exercise treatment programs to other intervention strategies. Most studies expect to find a causal relationship where exercise positively influences well-being. However, this is hardly studied directly. Hence, a more critical view is vital for examining possible causality between exercise and well-being. Moreover, more knowledge of the underlying mechanisms of the effects of exercise on the brain is necessitated, a few examples of which will be given in this article.

The effect of exercise on depression

The psychological definition of well-being in exercise literature is very ambiguous, as it can be defined in multiple ways. In exercise literature, the main factors of psychological well-being that are being studied are stress reactivity, anxiety, depression, self-esteem or body image, and, finally, restful sleep. The current essay particularly revolves around the relationship between exercise and depression.

In recent years, clinicians have used physical exercise as a treatment for depression. Therefore, the benefits of exercise and depression are predominantly tested within the use of clinical trials, in a quasi-experimental setting in order to test their causality hypothesis. In general, most studies find a consistent but moderate positive correlation. Furthermore, it seems that exercise as a treatment for depression gives better results than no treatment at all, although it is as effective as other forms of treatment such as medication or cognitive therapy (Moore & Blumenthal, 1998).

For depression and anxiety, most studies use data from clinical patients instead of a random trial, drawn from the general population. From these data it is often seen that subjects with high scores on anxiety or depression and low scores on exercise benefit the most from exercise programs (Scully, Kremer, Meade, Graham, & Dudgeon, 1998). An explanation for this observation is that highly depressed or anxious people are more likely to get well anyway, since they are in the top segment of how depressed or anxious someone can be.

Brain endorphins, or why mice keep running in their little wheel

What if the perceived causality between the healthy mind and body is erroneous? Suppose that, in general, happy people are just more fond of exercise, meaning that there is only a correlation? Or factors involved with exercising could operate independently and the effects of exercise are found in factors that accidentally come with exercise. More specifically, people suffering from depression could in fact benefit from exercise programs due to the fact that they engage in a social activity, exercising with other people and receiving attention in a constructive and positive way.

These critical views of this popular view that exercise would cause a more sane mind allows for a more comprehensive model of causality. Namely, physiological mechanisms and environmental factors could influence both exercise behaviour and effects on psychological wellbeing. There are many very interesting theories on physiological mechanisms of exercise could aid in the understanding of psychological well-being. One of those will be discussed here and is very interesting when seen in the context of psychological well-being.

Cordain Gotschall, Eaton, and Eaton (1998) hypothesized from an evolutionary perspective that human genotype responsible for physical activity is roughly the same as our ancestors that were hunter-gatherers. Our energy expenditure only accounts for 38% of our energy intake. It seems that current-day society demands less physical activity from us then the Paleolithic society 10.000 years ago. There are several studies that focus on activity requirements. From these it is concluded that people (and especially women) are exercising less when they get older. What could be the reason for this decline? Tergerson and King (2002) studied motivations and barriers that explain exercise and compared men and women. They found that men and women differ in their motivation; Women focus on their health, while men intend to gain strength.

A physiological mechanism that could account for both motivation to exercise and the positive effect on psychological well-being could be that of changes in brain endorphins (Dishman, 1997). More specifically, it is thought that beta-endorphin, a hormone released from the pituitary gland during vigorous exercise may be responsible (Goldfarb & Jamurtas, 1997). Exercise is healthy for us and is thus rewarded in our brains. Most organisms seem to be biologically driven to stay physically active. Caged rodents are often seen to run for several hours a day in their little wheel, although they certainly do not need to run a mile in order to obtain food or escape natural predators. The changes in the brain’s endorphins affects mood and can make exercise very addictive. One could probably even say that people engaged in professional sports probably are addicted for achievement!

The effects of exercise are not observed immediately

More research is needed in order to confirm that exercise can truly counter depression. One of the goals is to establish a better understanding of the amount of exercise needed in order to see its effects. This is referred to as the dose-response relation in exercise treatment. Analyses of multiple studies (so called meta-analyses) have been conducted in order to determine the effective duration as well as the most effective type of exercise as an intervention. Many meta-analysis studies have encountered difficulties in comparing different results because the research designs differ remarkably. A good meta-analysis by Lawlor and Hopker (2001) selected 16 out of 72 relevant studies, including those with several exercise paradigms, several depression scales and designs with healthy and non-healthy subjects. Their findings were that several exercise paradigms had similar effects on depression. While other studies have found that for treatment of anxiety aerobic chronic exercise is more beneficial than non-aerobic exercise such as strength training (Petruzello, Landers Hatfiel, Kubitz, Salazer, 1991). Most meta reviews suggest that exercise programs are beneficial in depression after 4 to 6 weeks of intervention (Scully, Kremer, Meade, Graham, & Dudgeon, 1998). Unfortunately, only one of the studies reviewed by Lawlor and Hopker (2001) performed a follow up test of depression and found that the exercise intervention had no long-term effect.

In conclusion

In order to gain a more proper understanding of the causal relationship between exercise and depression (if any), one should examine several environmental and physiological mechanisms that are involved, as well as the use of random trials, drawn from the general population. For example, at the Vrije Universiteit in Amsterdam, a large study is now conducted in order to study the heritability of exercise behavior as well as the genetic factors involved with depression. A multidisciplinary study is unavoidable if we do not wish to be trapped into assuming that a correlation is the same as a causal relationship.

There is no reason to believe that exercise is not healthy of course. Do not let this article prevent you from going to the gym because of some scientific doubts on the psychological advantages (unless you were looking for an excuse to get out of exercising. By all means, stay home and relax). To answer the first question: does exercise make you happy? Not necessarily, and as an intervention for depression, surely it will not hurt. Just remember to stay critical, and most of all, inquisitive. Science still has a lot to unravel in our world and a lot of bright minds will have to put their heads together, for solving questions as these.

References

Cordain, L., Gotschall, R.W., Eaton, S.B. & Eaton, S.B. III. (1998). Physical activity, energy expenditure and fitness: An evolutionary perspective. International Journal of Sports Medicine, 19, 328-335.

Dishman, R.K. (1997). Brain monoamines, exercise, and behavioural stress: Animal models. Medicine and Science in Sports and Exercise, 29, 63-74.

Goldfarb, A.H. & Jamurtas, A.Z. (1997). Beta-endorphin response to exercise. An update. Sports Medicine, 24(1), 8-16.

Klein, M. H., Greist, J. H., Gurman, A. S., Neimeyer, R. A., Lesser, D. P., Bushnell, N. J. & Smith, R. E. (1985). A comparative outcome study of group psychotherapy versus exercise treatments for depression. International Journal of Mental Health, 13, 148-77.

Lawlor, D. & Hopker, S.W. (2001). The effectiveness of exercise as an intervention in the management of depression: systematic review and meta-regression analysis of randomized controlled trials. BMJ, 322, 1-8.

Martinsen, E.W., Hoffart, A. & Solberg, O. (1989). Comparing aerobic with non-aerobic forms of exercise in the treatment of clinical depression: A randomized trial. Comprehensive Psychiatry, 30, 324-331.

Moore, K.A. & Blumenthal, J.A. (1998). Exercise training as an alternative treatment for depression among older adults. Alternative Therapies, 4, 48-56.

Petruzello, S.J., Landers, D.M., Hatfield, B.D., Kubitz, K.A. & Salazar, W. (1991). A meta-analysis on the anxiety-reducing effects of acute and chronic exercise. Outcomes and mechanisms Sports Medicine, 11(3), 143-182.

Scully, D., Kremer, J., Meade, M.M., Graham, R. & Dudgeon, K. (1998). Physical exercise and psychological well being: A critical review. British Journal of Sports Medicine, 32, 111-120.

Tergerson, J.L., King, K.A. (2002). Do perceived cues, benefits, and barriers to physical activity differ between male and female adolescents? The Journal of School Health, 72(9), 374-380.