This study showed that the prevalence of depressive symptoms among older Taiwanese adults in 2005 was 20.6%. It is comparable with the 20.2% reported in another study from 2003 with a nationally representative sample of 2,570 older Taiwanese adults . The current study showed that physical activity performed during leisure time is associated with a lower prevalence of depressive symptoms in this national sample of Taiwanese older adults. No association was evident for NLTPA. These findings support those from a previous study, where only LTPA was related to lower depression  but this was based on a younger (18-64 years) female-only sample. Additionally, among the three components of LTPA, only intensity, especially at higher intensity, seems to be associated with reduced risk of depressive symptoms, which is consistent with the findings of a recent systematic review .
The various settings in which physical activity takes place provide different exposures, or degrees of exposure, to the range of bio-psychosocial mechanisms operating in exercise. It has been suggested that NLTPA, such as domestic, commuting and occupational activities, are more likely to be obligatory, repetitive, or routine, while LTPA may offer a sense of enjoyment, fulfillment, connection and social interaction . It has been suggested that improvements in mental health after engaging in LTPA are partially related to social relations and mutual support derived from LTPA undertaken in a group or social setting . Furthermore, NLTPA, such as domestic or work-related physical activity, are often performed at a shorter duration and/or insufficient intensity than that required for yielding health benefits. The current physical activity guidelines proposed by the US  and the UK governments , and the American College of Sports Medicine  state that older adults should engage in 150 minutes a week of moderate-intensity, or 75 minutes a week of vigorous-intensity physical activity, or an equivalent combination of moderate- and vigorous-intensity physical activity.
Few studies have focused on the association between components of energy expenditure of physical activity and depressive symptoms in older adults. Energy expenditure is the product of frequency, intensity, and duration. The inter-relations among the three components contribute to the complexity of dose-response analyses in physical activity and morbidity. Without appropriate analyses, such as adjusting the volume of physical activity energy expenditure or the other two parameters in a multivariate model, findings may be distorted . Although each of the three components contribute to the determination of energy expended through physical activity, the present study showed that only intensity emerged as an independent predictor of depressive symptoms after adjusting for frequency, duration and other confounders. Participation in LTPA, particularly at higher intensity, is associated with a reduced risk of depressive symptoms regardless of the frequency or the duration of activity. This is consistent with the physical activity recommendations for older adults suggesting that physical activity needs to be moderate or vigorous intensity for health benefits [5, 6, 8].
Previous research has led to inconsistent results regarding physical activity intensity and depressive disorders. Some reviewers have suggested that physical activity intensity may not be an underlying factor for preventing or mitigating depressive symptoms given that mental health benefits have been observed at various levels of intensity [44, 45]. Two meta-analyses indicated that lower intensity supervised physical activity intervention may be more effective in reducing depressive mood than moderate intensity interventions among the elderly [12, 46]. The dual-mode model of exercise and mood [47, 48] may provide some clarification of these mixed results, suggesting that both low and high intensity can produce positive affective changes from pre-exercise to post-exercise. It is worth noting these hypotheses were not specifically aimed at, or tested in, older populations. Furthermore, most observational and intervention research of physical activity intensity has examined physical activity of different durations. This makes it difficult to compare the results of different studies and to disentangle the influence of intensity from the effect of duration .
Interpretations of the current results need to be treated with caution given the cross-sectional research design. Although LTPA is related to a lower risk of depressive disorder, it is not possible to ascertain if low activity levels contribute to depression, or if the presence of depressive symptoms results in decreased levels of physical activity due to feelings of low energy and motivation . The potential reciprocal associations should be taken into account when interpreting the results. Furthermore, it is worth noting that as well as being self-reported and thus susceptible to recall and misclassification biases, the measure of NLTPA does not include transport-related physical activity. The relationship of this form of activity with depressive symptoms cannot be determined from the current study. Well-designed prospective cohort studies, using objective measures of physical activity such as accelerometry, or extended randomized controlled trials, are needed to establish whether LTPA at higher intensity can prevent or delay depressive symptoms in older adults, and to elucidate why intensity is relatively more important than the other two components. It is also important to specifically compare physical activity interventions at various intensities by matching on total energy expenditure or on frequency and duration, to identify optimal intensity. Nonetheless, the current study provides important preliminary data on the complex issue of dose-response effects in the physical activity/depression relationship, particularly in older adults. Furthermore, it adds to the very limited evidence base for physical activity and older adults in East Asian countries and specifically with a Taiwanese population.