Most breast cancer survivors in this sample participated in some type of physical activity, which included household and occupational activities, leisure time physical activity, and intentional exercise. Approximately 78% participated in at least two days of moderate or greater intensity activity, but only 41% reported physical activity levels at the currently recommended frequency and duration (43
). This percentage is slightly lower than U.S. women in the general population; national data shows that approximately 50% of women between the ages of 35 and 44, and 46% of those between 45 and 64 engage in leisure time physical activity of moderate to vigorous intensity for 30 minutes on at least 5 days per week (44
Results of this study showed that total energy expenditure from all types and intensity levels of activity was associated with better physical functioning, overall health, and less pain and depression. Results were very similar for frequency of activity. These results are more consistent than those reported from the HEAL study, which found associations primarily with physical functioning and selected fatigue variables. Our results are consistent with our hypotheses and other research showing an association between exercise and symptoms and health outcomes in cancer survivors (17
). The finding that both energy expenditure and frequency were related to outcomes suggests that both mechanisms related to the total volume of physical activity and distraction from distress may be operational.
Our results highlight two unique points. First, we found relationships between outcomes and total physical activity, which includes activity such as occupational and household activity as well as exercise. Most previous studies have looked only at exercise (planned physical activity performed with an intent to increase fitness). Second, the analysis of both total energy expenditure and frequency of activity showed that the largest differences in outcomes occurred between survivors with the lowest levels of energy expenditure and frequency of activity, and those with slightly more. Our results are similar to those reported by Brown et al. (45
). They found that respondents to the BRFSS surveys who reported any physical activity were less likely to report poor physical or mental health in the past 30 days than respondents who were completely sedentary. This suggests that increasing activity in the sedentary cancer survivor population should be a higher intervention priority than helping those who are already active enhance their physical activity. Currently, few studies target only sedentary cancer survivors, which, in the long run, may be the most cost-effective and best use of resources, although this requires further study.
The results for duration of activity (average time per day spent in moderate or more intense activity) were surprising. A linear relationship was expected, with greater duration associated with better outcomes, but this study found that, in general, outcomes improved with increasing duration for the first three quartiles, but were poor for participants with the highest duration of physical activity. These survivors may have extensive household or occupational activity, causing them to be active to the point of exhaustion, resulting in decrements in their quality of life.
Studies of moderate-intensity activity for breast cancer survivors have been shown to improve their physical function and quality of life (17
) with some suggesting a possible dose response effect (46
). The findings of this study are also consistent with studies showing fitness benefits from multiple short bouts of activity (47
). Such a regimen may produce improvements in symptoms without compromising fitness benefits, and may be easier for some survivors to adopt and maintain (47
). To date, most studies on exercise for cancer survivors have used exercise programs of a fixed frequency, duration, and intensity, which does not allow for testing the effects of these variables in a randomized fashion. Most of these studies have prescribed moderate to vigorous exercise 3 to 5 days per week, with sessions lasting 20 to 30 minutes (49
). More research is needed, including randomized studies, to better determine is the frequency and duration of activity necessary to achieve optimal outcomes among cancer survivors.
A surprising finding is that there was no consistent relationship between the physical activity variable and fatigue as indicated by low scores on the SF-36 vitality subscale. This is particularly surprising given the existing evidence of the positive effects of exercise interventions on fatigue, as summarized in a recent Cochrane review (50
). We used only a single unidimensional measure of fatigue, the SF-36 vitality subscale, so it is possible that it is not an adequate fatigue measures. However, other studies of fatigue in cancer survivors have found it correlates highly with other commonly used fatigue measures (12
). Our study may have failed to show such a relationship because it measured total physical activity, including household and occupational activity, as well as exercise. A large study of breast cancer survivors and physical activity found that while sports and recreational activity was related to sensory aspects of fatigue, household activity was not (46
). Intentional exercise or leisure time physical activity may provide distraction that household activity does not, thus leading to different effects on fatigue. It is also possible that household activity is not done at a consistently moderate or vigorous intensity, and thus does not affect fitness which may relate to fatigue.
Although total energy expenditure and frequency of activity was found to be associated with symptoms and health status, the cross-sectional design of this study makes it difficult to determine causal effects. It is unclear whether experiencing symptoms and poor health leads survivors to be less active or whether physical activity improves these outcomes. Indeed, both directions of influence are likely present to some extent. While controlling for medical variables partially addresses this, the direction of causation is still questionable in a cross-section study. A longitudinal study examining the effect of physical activity on symptoms and health status would provide stronger support for the influential effects of specific dimensions of physical activity, allowing an examination of changes in physical activity and outcomes.
In summary, the higher total energy expenditure and more frequent physical activity was more consistently associated with outcomes than was greater duration of the activity. When encouraging cancer survivors to participate in exercise, emphasizing increases in frequency of moderate intensity activity follows safe practices, and may have positive implications for symptoms and health status. However, our results are based on a fairly small sample, and should be interpreted accordingly. Because our measure of physical activity included all forms of physical activity, not only planned exercise, a more controlled study with a different measurement approach may reveal different effects. This issue could be further examined using randomized design comparing activity regimens that vary in frequency and duration. More systematic investigation is needed on the effects varying dimensions of activity explored in these studies. Future studies should investigate the optimal frequency and duration of various types of physical activity for improving symptoms frequently experienced by breast cancer survivors.