Objective Monitoring and the Challenge of Defining Dose/Response Relationships for the Prevention of Chronic Disease

Abstract

The form of the dose/response relationship linking habitual physical activity to major health outcomes is of practical importance for those who are formulating public health recommendations on minimum daily physical activity needs. However, available information on the form of this relationship is conflicting. One might anticipate that the optimal physical activity pattern would mimic that of our hunter-gatherer ancestors. Data gathered on traditional Inuit hunters point to a large daily energy expenditure in many types of hunting; this is accumulated mainly through long days of moderate physical activity. Studies based upon occupational classifications also suggest health benefit from vigorous physical exercise at work, and this view is supported by early questionnaire analyses of leisure behaviour. However, a systematic survey of questionnaire-based reports revealed no clear picture, because of differing methods of measuring and classifying physical activity, substantial inaccuracies in self-reports, and examination of a wide variety of health outcomes. Other studies, using simple measures of fitness as surrogate indicators of accumulated physical activity, have pointed to the largest health benefits being gained from a moderate increment of fitness. Objective physical activity monitors allow a much finer gradation of physical activity patterns, and thus have the potential to clarify dose/response relationships. However, this potential has not as yet been realized. The samples tested have been small, and the health outcomes examined have been physiological changes rather than clinical events. Confidence intervals for the benefits realized at different levels of physical activity have thus been as large or larger than those found in questionnaire studies. Future developments should permit the use of objective monitors on a much larger scale; a 100-fold increase in subject numbers seems likely to overcome current problems.