Dose-response issues concerning physical activity and health: an evidence-based symposium

Abstract

Regular physical activity is widely accepted as a behavior to reduce all-cause mortality rates and to improve a number of health outcomes. On October 11–15, 2000, Health Canada and the United States Centers for Disease Control and Prevention sponsored a scientific symposium to determine whether there is a dose-response relationship between physical activity and several health-related outcomes and to identify areas for future research. The invited experts reviewed and evaluated the existing literature according to an evidence-based methodology used previously by the National Institutes of Health (5) (Table 1). The Consensus Committee (Panel), consisting of individuals with experience and knowledge in health areas of concern from outside the field of exercise science, reviewed and evaluated the evidence presented, assigned it to an evidence category, and identified topics for future research. The manuscripts upon which the consensus was based and the report of the Consensus Committee are published concurrently in this issue of Medicine & Science in Sports & Exercise. Table 1: Categories of evidence.aMost of the evidence currently available seems to be related to the effects (benefits or risks) of regular physical activity rather than to the relationship between dose and response. Therefore, the Panel decided to summarize the evidence for the effects of participation in regular physical activity, because the Panel found this useful and necessary to properly understand the dose-response data. The key terms used during the Symposium and in this report are described in detail by Howley (4). Briefly, health is defined as a human condition with physical, social, and psychological dimensions. Physical activity is defined as any bodily movement produced by contraction of skeletal muscle that substantially increases energy expenditure. The dose of physical activity, or exercise, needed to bring about a particular health benefit response is described by the characteristics of frequency, duration, intensity, and type of activity. Frequency is described as the number of activity sessions per time period (e.g., day or week). Duration refers to the number of minutes of activity in each session. Intensity describes, in relative or absolute terms, the measured or estimated effort (energy cost) associated with the physical activity. Physical activity may be of a leisure time or occupational type activity. Physical fitness is defined as a set of attributes (i.e., cardiorespiratory endurance, skeletal muscle endurance, skeletal muscle strength, etc.) that relate to the ability to perform physical activity. The product of frequency, duration, and intensity yields the total energy expenditure associated with the physical activity and is a measure of the volume of exercise. The gross cost of an activity is the total energy expenditure, which includes resting metabolic rate and the cost of the activity itself. The net cost is that associated with the activity alone. BACKGROUND Concepts and Methods Appropriate measurements of physical activity are needed to judge whether there is a dose-response relationship between physical activity and health. Ideally, the important components of physical activity include measurements of frequency and duration (time) and intensity (absolute and relative). With these measurements, it is possible to calculate the dose (or volume) of exercise. Unfortunately, many studies have not collected detailed measures on intensity, duration, and frequency but have used subjective assessments such as “little, moderate, and heavy.” The dose equals the energy expended in physical activity and is one of the potential mediators of the health benefits of physical activity. Therefore, relatively accurate measurements of both the time spent in physical activity and intensity of physical activity are needed. This is a problem because the methods used in field studies (questionnaires, physical activity records, and recall diaries) are imprecise, particularly for estimating low levels of physical activity. These methods rely on self-reports of the individual’s perceived intensity of physical activity. They may not reflect the absolute intensity across all age and sex groups required for converting the perceived levels of exercise intensity into METs, a commonly used unit used to estimate the metabolic cost (oxygen consumption) of physical activity. One MET equals the resting metabolic rate (∼kcal·min-1 or 3–5 mL O2·kg-1·min-1). The lack of a gold standard for the precise quantification of the energy expenditure of physical activity has hampered the development of universally acceptable and accurate field assessment techniques. A variety of direct and indirect methods are presently used to assess physical activity. Portable oxygen uptake devices and improved sensors of bodily movements and temperature continue to be investigated, but more work and better consensus regarding their uniform application are needed. Older and/or obese individuals are likely to perceive a given absolute level of physical activity as more intense than younger, fit, less obese individuals. To convert perceived levels of physical activity intensity into meaningful absolute intensity levels (and therefore into energy expenditure of physical activity), it is helpful to have conversion factors. The Panel recommends that the conversion factors provided in Table 2 be widely used by investigators to calculate indices of volume, as well as the absolute and relative levels of physical activity. This table allows the data reported in relative intensity to be converted into absolute intensity. Table 2: Classification of physical activity intensity.It is important to consider the intensity of physical activity as part of the dose-response relationship because of its known effect on fitness. Fitness, commonly assessed in terms of peak workload or maximal oxygen uptake (V̇O2max), is improved more by high- than low-intensity physical activity. If fitness is an intermediate factor between physical activity and health benefits, then measurement of the intensity of physical activity may be important in assessing dose response. V̇O2max has been used as a measure of the “dose” of physical activity. Given the significant constitutional, interindividual variability in V̇O2max, this may not be ideal. The measurement of fitness itself, however, is more accurate than current field measurements of physical activity. There is some evidence that fitness could be more strongly related to some health outcomes than physical activity. These observations, however, may be due in part to the greater accuracy in the measurements of fitness. Other measurements of fitness include the assessments of submaximal exercise endurance and the heart rate response to a given workload. Two further issues need to be considered when testing for a dose-response relationship between physical activity and health outcomes. The first relates to the increased risk to adverse health outcomes as the volume and intensity of exercise are increased. In this case, the net benefit of higher levels of physical activity may not be as great as predicted (Fig. 1). The second issue relates to the likelihood that the overall physical activity level (both activities of daily living and occupational physical activity) of populations is decreasing. Consequently, estimating the volume of physical activity needed to reduce health risks from data generated by older studies could be problematic. FIGURE 1: The relationship of exercise intensity to biological change (for two dose-response profiles) and risk injury. Net health benefit for moderate- and high-intensity exercise displayed; from ref. 3.Although the resistance training and flexibility issues are important to global health outcomes, the Symposium focused mainly on physical activity with an emphasis on aerobic activities. Fractionalization of Physical Activity Fractionalization of physical activity can be understood as: 1) comparing one continuous session of exercise with several short sessions of the same total duration; or 2) comparing a session of moderate-to-hard exercise with a session of longer duration, lower intensity, but equivalent energy expenditure, i.e., “trading intensity for duration.” In randomized short-term studies, which compare the training effects of one long exercise session per day to several short (≥10 min) exercise sessions per day at the same intensity, two short sessions appear generally as effective as a single session of the same total duration. Moreover, improvements in V̇O2max are found equal for both programs in randomized long-term training studies (Category B). When considering the issue of “trading intensity for duration,” short-term studies suggest that heavy exercise is more likely to induce negative energy balance than light exercise (Category C). In long-term studies, there is evidence that high-intensity training elicits a greater increase in V̇O2max than low-intensity training for the same total energy expenditure (Category B). Acute and Chronic Effects of Physical Activity Acute effects of physical activity refer to health-related changes that occur during and in the hours after physical activity, which are not further improved with additional physical activity. Chronic effects associated with physical activity occur over time due to changes in the structure or function of various body systems, independent of the exercise response and the to exercise training be in because of sessions with more (i.e., Therefore, there may be an between the and effects of physical activity. In some exercise may have an effect that with on the response to exercise. 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