To our knowledge, this is the first study to explore associations of sleep duration, vegetable consumption, and their interactions on all-cause mortality in a large population-based perspective cohort study of Chinese older adults. The results revealed that both short (≤ 5 hours/day) and long (≥ 10 hours/day) sleep durations were associated with increased risks of mortality, compared to 7–8 hours/day. We also found that the high frequency of vegetable consumption was associated with a substantially lower mortality risk. These associations were stronger in female, oldest-old, urban, and illiterate participants. Moreover, a significant multiplicative interaction between vegetable consumption and sleep duration on mortality risk was detected. A low frequency of vegetable consumption combined with short or long sleep hours could have a double jeopardy of mortality, although a high frequency of vegetable intake could reduce mortality risk among participants with short sleep duration. The findings are valid in all subgroups except for young-old aged 65–79 in which both the sleeping duration and the frequency of vegetable consumptions were not statistically associated with mortality.
Our finding is consistent to the existing literature [34–39]. For instance, a meta-analysis found globally adverse effects of a long sleep duration on mortality and moderately adverse effects of a short sleep duration only on mortality in North American populations [2]. Several other studies on Chinese middle-aged or older adults in Hong Kong [37] and, Shanghai [4] found that a prolonged rather than short sleep duration was more significant associated with increased mortality even controlling for participants’ health status. One study using the 2005 and 2008 waves of the CLHLS confirmed such pattern in older women and at oldest-old ages [18]. Besides, several meta-analyses provided solid evidence that higher consumption of vegetables is associated with a lower risk of all-cause mortality [40, 41]. Some empirical studies also confirmed that vegetable consumption was inversely associated with risk of all-cause mortality among the older adults in China [42, 43]. Although the interaction effect of vegetable consumption with sleep hours revealed by our study on mortality has not been empirically verified in the existing literature, it partially coincided with many prior studies on the directional associations of vegetable consumption to sleep duration or to mortality respectively [9–14, 40–43]. The existing literature has also documented that higher prevalence of mortality risk linked to sleep disorder or low vegetable consumption was more likely to be found among disadvantaged older adults, such as women, the oldest-old, the less educated and so on due to their low socioeconomic status and poor health status [44].
Several mechanisms have been suggested to explain the associations among sleep durations, vegetable consumption, and mortality. An adequate sleep duration is a basic guarantee for normal immune function. However, as for long sleep, some studies demonstrated that long sleep has been associated with fatigue, lower immune function, changing in cytokine levels, depression, underlying disease process or physiological reduction of the photoperiod, which could lead to increased mortality [4, 45]. As for short sleep, insufficient sleep hours could damage the immune system, which is more likely to cause body dysfunctions [46] and increase risk for chronic health conditions [47, 48], such as hypertension [49], cardiovascular disease [50], type 2 diabetes [51], and obesity [52]. Besides, limited sleep hours may also gradually change certain neuroendocrine systems and sensitize individuals to stress-related disorders, like depression [53]. All these adverse health outcomes are main risk factors for mortality among older adults. Vegetable is an important source of micro-nutrients with antioxidant, polyphenols, carotenoids, vitamin C, fiber, potassium, flavonoids and other and other biologically active properties [42, 43], which are proved to act synergistically through numerous biological mechanisms to prevent against a wide range of chronic diseases and premature mortality [13, 14]. For instance, vegetable intakes have been shown to reduce blood pressure, cholesterol levels, inflammation and to stimulate vascular and immune functioning [54]. Vegetables may also have a beneficial effect on modulating steroid hormone metabolism and concentrations. Additionally, antioxidants in vegetables may help to neutralize reactive oxygen species [55].
Though mechanism underlying the association between interaction effect of sleep duration and vegetable consumption on mortality risk remains unclear, some potential pathways can be suggested. First, since both an extreme sleep duration and a low frequency of vegetable consumption would increase risk of mortality respectively, it is reasonable to infer that interaction of an extreme sleep duration and a low frequency of vegetable could trigger a double jeopardy of mortality. Second, some mechanisms have been proposed of the reciprocal association between sleep disruption and vegetable intake that may subsequently lead to reverse health outcomes or even mortality. On one hand, laboratory studies suggested that disrupted sleep would change appetite-related hormones ghrelin and leptin, which may increase the preference for energy-dense foods probably leading to lower consumption of vegetable [56]. Some experimental studies also found that both short and long sleep duration may impair energy homeostasis through unhealthy dietary patterns, causing lower vegetable intake [57, 58]. On the other hand, sleep serves as an important role in protecting the human body against the harmful impacts of free radicals caused by a high metabolic rate during waking times [55]. It was previously suggested that metabolic rate during sleeping time appears much lower than awake time, when providing an opportunity to re-generate the enzymes influenced by free radical [59]. In this regard, vegetables with antioxidant components are expected to maintain the regular sleep-wake cycles by improving mitochondrial function and energy metabolism through decreasing protein content, which may lead to a reduction in the synthesis of brain sleep inductors and sleep parameters [60].
Third, mortality protective benefits of frequent vegetable intake may mitigate mortality risk brought by short sleep duration. As noted in the literature, short sleepers are more likely to take energy- and fat-dense food [61]. Saturated fat intake is associated with risk of increased all-cause mortality and risk of mortality from atherosclerotic vascular disease [62]. Frequent vegetable intake among short sleepers may reduce their fat intake and mitigate the burden of fat on mortality risk. Some studies found that a combination of high fruit and vegetable and low saturated fat intakes is protective against mortality, while consuming either low saturated fat or high fruit and vegetables alone is not associated with lower mortality risk, compared to those having neither saturated fat nor fruit and vegetables [63]. Adherence to frequent vegetable intake or greater vegetable intake may help adjust extreme sleep durations to the recommended level [64] and improve sleep quality without changing sleep duration [ 65, 66]. Our study used only baseline vegetable intake and sleep duration to predict mortality risks in the six-year follow-up period. It is possible that participants’ short sleep duration prolonged to the recommended level or improved in sleep quality in the follow-up time with an adherence to frequent vegetable intake diet. Improved sleep duration in turn leads to fewer risk of death.
Another interesting finding is that young-old adults appear to be escapers from risk or double jeopardy of mortality by extreme sleep hours and/or a low frequency of vegetable intake. The lack of association between sleep duration or frequency of vegetable consumption among young-old adults is somewhat consistent with previous studies. For example, a prior Chinese study showed short sleep was not associated with mortality among young-old adults aged 65–79 [18]. It is possible that dramatic amplitude reduction and increase in frequency of delta waves, which are closely linked to sleep duration and its subsequent reverse health outcomes, are more likely among the oldest-old, rather among young-old adults [18]. Another Swedish study found that the sleep duration was not associated with 13-year mortality among adults aged 65 and over [67]. One recent meta-analysis further showed that shorter sleepers (groups with less than 7 hours/day) were not associated with mortality compared to those with 7 hours/day in 40 studies [68]. One Japanese study revealed that mortality risk was not statistically different among young-old adults aged less than 75 years between those who had a healthy diet (mainly composed of vegetables, seaweed, fish) and those who had a greasy diet (mainly composed of meat and fat) [54]. Young-old adults may still maintain some biological advantage compared to oldest-old that enable them to be immune from threats of short sleep and low frequency vegetables intake. Nevertheless, the reasons of lack of association among young-old adults are unclear, and more research is warranted to shed light on it.
This study bears the following limitations. First, it relied on self-reported sleep duration, which may have measurement errors. However, it is challenging and expensive to assess sleep duration by using actigraphy for thousands of participants from large epidemiological survey. Similarly, our assessment of vegetable consumption was mainly based on the self-reported information on frequency intake of vegetables. No detailed dietary consumption information was collected including the amount/type of vegetables, which may limit the generalizability of our results. Second, sleep quality was not modeled. There is plenty of evidence showing that sleep quality is strongly linked with mortality [18], which may moderate the linkage between mortality with sleep duration and dietary pattern. Thus, the present study examined very basic sleeping and dietary patterns among Chinese older adults. Tea- or coffee- drinking behaviors and their timing could affect sleep patterns. Future research should conduct standard methods to capture more informative patterns of dietary and sleeping behaviors. Third, the study has several unmeasured confounders including sleep related disorders (e.g. insomnia), sleep medication use, and other underlying diseases may also affect both sleep duration and mortality. However, prior studies found that even after controlling for insomnia, daily napping, sleep apnea, use of sleep medications and many other confounders, an extreme sleep duration was still associated with increased all-cause mortality [58]. Thus, the biases of our present study would not be substantial.
Despite limitations, our study made contribution to the existing literature. Our study extended the findings of sleep duration effect on mortality to older adults at very old groups aged over 80 years with a focus on the interaction between vegetable intakes and sleep durations, while most of relevant studies focused on younger age groups. We examined the interactions among various subpopulations and found that the effects differ across the subpopulations and more pronounced among more vulnerable populations such as women, the oldest-old, and the illiterate. These findings could have important implications for implementing person-centered intervention programs and policies among Chinese older adults for achieving healthy aging. The difference in the interactions among subpopulations also suggests a stratification of the analysis is recommended.