The immune function undergoes markedly adverse changes with age that might be explained by decreased function of, or diminished regulation of, the immune system (27). The decline in immune function associated with aging increases the incidence of infectious disease, malignancy, and autoimmune disorders (19). In particular, respiratory infections, such as pneumonia and influenza virus infection, including upper-respiratory tract infections (URTI), are common and serious illnesses among elderly individuals (6).
Salivary secretory immunoglobulin A (SIgA) is putatively the first line of defense for the human body against pathogenic microbial invasion (13). Therefore, the SIgA is associated with URTI incidence (5,9). In fact, SIgA has been shown to inhibit attachment and replication of certain pathogens to prevent colonization. It can also neutralize toxins and viruses. Because SIgA secretion reportedly decreases concomitantly with aging (15,30), the reduced SIgA level might increase pathogenic microbial invasion and thereby engender a higher frequency of URTI in elderly individuals.
The influence of physical activity on immune function has received considerable attention in recent years (23). The salivary SIgA level is reportedly linked to physical activity: the weekly amount of time spent in sports activity (25). Furthermore, moderately active subjects are less susceptible to infectious disease than are inactive subjects (11,14,18). Therefore, the possibility exists that physical activity enhances salivary SIgA level, thereby more effectively preventing infection.
Very few studies have addressed the relationship between SIgA secretion and physical activity in an elderly population. Our previous studies have shown that continuous exercise training enhanced salivary SIgA levels in older people (1,8). These exercise interventions might increase mucosal immune function in elderly people. However, it is unclear whether free-living daily physical activity influences mucosal immune function in elderly people.
Pate et al. (22) suggest that instead of a vigorous exercise program, small changes that increase daily physical activity might reduce the risk of chronic diseases. This approach is particularly suitable because it is easy to conduct for individuals who are primarily sedentary, or for those who lack time for scheduled exercise.
Ambulatory activity such as walking is the most convenient and popular mode of physical activity and is therefore an obvious choice to accomplish set points. Accordingly, walking has been used as a public health intervention, particularly among people with low physical activity (28). The American College of Sports Medicine (ACSM) and the Centers for Disease Control and Prevention (CDC) issued a lifestyle recommendation that includes accumulation of at least 30 min of moderate-intensity physical activity per day (22,34). Approximately 3400 steps of continuous walking in elderly subjects correspond to 30 min of moderate-intensity physical activity (33). In addition, the Japan Health Promotion & Fitness Foundation (JHPFF) issued a recommendation urging elderly individuals (both men and women) to walk 6700 and 5900 steps per day, respectively (7). In fact, these recommendations were derived from epidemiological data that demonstrated a relationship between daily physical activity and several chronic diseases, including coronary heart disease, hypertension, non-insulin-dependent diabetes mellitus, osteoporosis, and colon cancer (22). However, it remains unclear whether such recommended daily physical activity might be effective from the point of view of mucosal immune function among elderly people. Therefore, demonstration of the relationship between daily physical activity and mucosal immune function might contribute to establishment of effective physical activity recommendations in terms of improvement of mucosal immune function in elderly individuals.
This study was undertaken to examine the relationship between free-living daily physical activity (step count and energy expenditure) and mucosal immune function (salivary SIgA) in elderly individuals. We hypothesized that salivary SIgA level would be higher in older people with habitual, moderate physical activity than in sedentary ones.
A total of 284 healthy elderly volunteers (age 65-86 yr) participated in the study. Potential participants were given a detailed explanation of the risks, stress, and potential benefits of the study before they signed an informed consent form. All participants had passed a complete medical examination within the preceding year and had received written permission from a specialist sports doctor to be included in this study. No subjects had been treated with any drugs that are known to affect immune function. No patients reported allergies or recent infections (prior 3 months) in the determination. The ethical committees of the Institute of Health and Sport Sciences and the Institute of Clinical Medicine of University of Tsukuba approved this study. The study conforms to the principles outlined in the Declaration of Helsinki. Moreover, this study conforms to the policy statement regarding the use of human subjects and written informed consent published in Medicine & Science in Sports & Exercise®.
Saliva samples were collected between 8:30 and 9:30 a.m., as in a previous study (1). Participants came to the laboratory without breakfast. They sat and completely rinsed their mouths with distilled water (30 s, three times) and then rested for at least 5 min. Saliva production was stimulated by chewing a sterile cotton ball (Salivette; Sersted, Vümbrecht, Germany) at a frequency of 60/60 s. Obtained saliva was separated from the cotton by centrifuging at 3000 rpm. After measurement of the sample volume, saliva samples were frozen at −40°C and stored until the end of the study period. Salivary SIgA concentrations were measured using enzyme-linked immunosorbent assay (ELISA) according to the procedures of a previous study (1). To analyze SIgA levels, data were expressed as the SIgA-secretion rate (μg·min−1). The SIgA-secretion rate was calculated by multiplying the absolute SIgA concentration (μg·mL−1) by the saliva flow rate (mL·min−1).
Measurement of daily physical activity.
To assess the physical activity, we used an electric pedometer (Kenz Lifecorder; Suzuken Co. Ltd., Nagoya, Japan). With regard to this pedometer, previous studies have shown accuracy for the assessment of counting steps (24) and availability for the assessment of the energy expenditure in free-living conditions (12). Participants were instructed to wear an electric pedometer for 14 consecutive days during all waking hours, except during bathing. Participants were instructed to go about their normal lives unrestricted and were asked not to look at the pedometer to see how many steps they had taken each day. Pedometer placement was standardized on the belt or waistband, according to the manufacturer's recommendation.
The electric pedometer measures acceleration in the vertical (z) direction. The acceleration signal is filtered by an analog bandpass filter and is digitized. A maximum pulse for 4 s is taken as an acceleration value. Activities were categorized into 11 activity levels (0.0, 0.5, and 1.0-9.0) based on the accelerometric signal pattern. The activity levels were subsequently converted using an algorithm to calculate energy expenditure (kcal). The energy-expenditure values derived from the accelerometer (expressed as kilocalories) were converted into kilojoules using a standard conversion factor: 1 kcal = 4.184 kJ. The measured energy expenditure was normalized for body weight (kJ·kg−1·d−1). The estimated metabolic equivalents (METs) from the recorded activity level were calculated using an equation described in a previous study (12). They were then categorized into four activity levels defined as inactive (< 1.8 METs), light (1.8-2.9 METs), moderate (2.9-5.2 METs), and vigorous (> 5.2 METs) activity, as described in previous reports (2,12). Data from electric pedometers were summarized into daily averages for step count (steps per day), energy expenditure (kJ·kg−1·d−1), and activity duration (min·d−1) in these specific intensity levels.
Data analysis and statistics.
Descriptive data are presented as means ± SE. Independent t-tests were used to test differences between sex in age, body composition, pedometer-determined variables, and salivary data. Differences in age, BMI, pedometer-determined variables, and salivary data were tested among quartiles using a general linear model (ANOVA) procedure following data stratification of pedometer-determined steps per day using the 25th, 50th, and 75th percentiles for distribution, as described in a previous study (32). Post hoc analysis (Student-Newman-Keuls test) was used to compare specific differences between quartiles when significance was found. Furthermore, effect size was calculated as the difference between the lowest and highest quartile means, divided by the total cohort standard deviation, as described in a previous study (32). Calculating the effect size presents differences in relation to expected interindividual variations (31). An effect size of 0.2 is considered small, 0.5 is moderate, and 0.8 is large (3). In addition, eta square (η2), which denotes the proportion of total variance attributed to an effect (independent factor), was employed to evaluate the meaningfulness of the differences. An η2 value of 0.01 is considered small, 0.06 is moderate, and 0.15 is large (29). For all analyses, a P value of less than 0.05 was inferred as statistically significant.
Table 1 shows descriptive variables, daily averages for pedometer-determined variables, and salivary data for the total cohort and for male and female participants. Males displayed higher values of energy expenditure (P < 0.01) than females, but step counts and times of increased intensity levels were similar between genders. Saliva flow rates were significantly higher in males than in females (P < 0.05), whereas SIgA concentration was lower in males than in females (P < 0.05). As a consequence, no significant differences in SIgA-secretion rates were found between genders.
Table 2 shows a comparison of descriptive variables, daily averages for pedometer-determined variables, and salivary data across quartiles of pedometer-determined ambulatory activity. Results of cross-quartile statistical comparisons and calculated effect size (between the highest and lowest quartiles) are also presented. Pedometer-determined physical activity was classified as Q1 (≤ 4166 steps per day), Q2 (4167-5967 steps per day), Q3 (5968-7673 steps per day), or Q4 (≥ 7674 steps per day). Energy expenditure per day differed significantly among pedometer-determined quartiles (P < 0.0001), displaying an increasing pattern with higher quartiles. With regard to activity durations in specific intensity levels, duration of inactivity differed significantly among quartiles (P < 0.0001), displaying a decreasing pattern with each higher quartile. In contrast, durations of light and moderate activity showed an increasing pattern with each higher quartile. Vigorous activity showed no significant difference between Q2 and Q3. With regard to variables of physical activity, all significant differences detected corresponded with a large calculated effect size (> 0.80).
Figure 1 shows the saliva flow rates (Fig. 1A) and SIgA concentrations (Fig. 1B) across quartiles of pedometer-determined ambulatory activity. Saliva flow rate differences were not statistically significant across quartiles. Nevertheless, SIgA concentrations differed significantly between Q1 and Q3; they were higher in Q3 than in Q1 (P < 0.05). The SIgA-secretion rate also differed significantly between Q1 and Q3; it was higher in Q3 than in Q1 (P < 0.05) (Fig. 2). Significant differences in detected SIgA secretion rates between Q1 and Q3 corresponded with a moderate calculated effect size (0.50) (data not shown).
The major finding of this study is that elderly people who accumulated moderate-intensity daily physical activity have higher salivary SIgA secretion than their predominantly sedentary peers. Elderly people in Q3, who accumulated approximately 7000 steps per day, had the highest level of salivary SIgA when stratified by pedometer-determined steps per day using quartiles. This value might be regarded as a moderate daily physical activity target for elderly people to improve mucosal immune function despite declining with aging. To our knowledge, the present data are the first to demonstrate that moderate daily physical activity in free-living elderly people is associated with high salivary SIgA levels.
Physical activity beneficially modifies immune function; in particular, chronic moderate-intensity exercise training might enhance immune function (17,23). On the basis of that speculation, our previous studies have attempted to boost salivary SIgA secretion in elderly subjects through moderate exercise training (1,8). Results of those interventions indicate that moderate exercise training enhanced SIgA secretion in elderly subjects. However, little was known about the relationship between free-living daily physical activity and salivary SIgA levels among elderly individuals. Demonstration of a relationship between daily physical activity and SIgA would contribute to the establishment of guidelines of physical activity for increasing immune function among elderly people. The salivary SIgA level was significantly higher in Q3 than in Q1 when elderly subjects were stratified by pedometer-determined steps per day using quartiles. The elderly people in Q3 also seemed to have the highest salivary SIgA level among their peers. In terms of pedometer-determined physical activity, elderly subjects each accumulated approximately 3000 steps in Q1 and 7000 steps per day in Q3. The JHPFF has reported that males and females older than 70 yr accumulated respective averages of 4787 and 4328 steps per day (7), so the elderly people in Q1 might be classified as sedentary and inactive. The elderly subjects in Q3 accumulated an average of approximately 7000 steps per day of ambulatory activity, which is similar to the JHPFF-recommended value (7). Additionally, healthy elderly subjects who attended a structured walking program two to three times during the week accumulated approximately 6600 steps per day of ambulatory activity (33). Accordingly, elderly people in Q3 would cumulatively perform such physical activity at home, at work, or during walking exercise. Our previous study has shown that 3 months of brisk walking exercise (30 min·d−1, 5 d·wk−1) increased SIgA secretion in elderly subjects (8). Brisk walking, defined as walking ≥ 3.5 miles per hour, is one example of moderate physical activity (34). Parise et al. (21) have suggested that brisk walking in elderly subjects corresponded to approximately three or more METs according to their calculated walking speed. In the present study, elderly people in Q3 accumulated 22 min·d−1 of moderate-intensity activity (2.9-5.2 METs). Therefore, positive outcomes of moderate physical activity (including brisk walking for 22 min·d−1) should include long-term maintenance of high salivary SIgA level in elderly people.
Nieman (16,17) has proposed that the lowest risk of URTI is found among individuals who are moderately active, and that risk of URTI is increased for both physically inactive and highly active individuals, suggesting a J-shaped relationship. On the basis of that finding, the highest immune function is found among moderately active individuals and is lower for both inactive and highly active individuals. Several previous studies have examined physical activity associated with URTI, using cross-sectional design; their results support the J-shaped relationship (11,14). With regard to salivary SIgA, previous studies have reported a significant correlation between SIgA levels and exercise duration (25), and intensive exercise has been shown to reduce salivary SIgA levels (20,23). However, no cross-sectional studies have examined the relationship between physical activity and salivary SIgA among elderly people, even though SIgA levels are associated with prevalence of URTI (5,9). In the present study, SIgA-secretion rate also showed an increasing pattern with each higher quartile. Therefore, our results seem to be in line with results of preceding studies. In the present study, the coefficient of variation for pedometer-derived step count per day, which ranged from 969 to 20,139 steps per day, corresponded to 45.7% for the total sample (data not shown), suggesting that the subjects displayed a relatively wide range of activity levels. However, the subjects did not include individuals who engaged in very intensive physical activity, reducing the SIgA level. Therefore, we were unable to evaluate the effects of high levels of physical activity on SIgA level sufficiently. Additional research is necessary to examine a relationship between immune function and physical activity in elderly athletes.
In the present study, no statistical difference was observed in SIgA levels between Q1 and Q4, whereas elderly subjects in Q4 had higher physical activity levels. Kohut et al. (10) have shown that vigorously active individuals had higher serum IgG levels and IgM antiinfluenza response compared with moderately active and sedentary elderly individuals. Accordingly, vigorous levels of physical activity, in addition to moderate levels, could be associated with enhanced immune response in elderly people. Also, it is possible that the SIgA secretion is affected by other factors in addition to physical activity. Immune function is known to be influenced by nutritional status (26) and psychosocial factors (4,10). So, nutritional deficiency and psychosocial stress include interference with the production of humoral antibodies and of mucosal secretory antibodies, increasing susceptibility to infection (4,26). Further research on monitoring nutritional status and psychosocial factors would clarify the association of immune function and physical activity.
Walking is a convenient and popular mode of physical activity. In recent years, popular recommendations for ambulatory activity have been derived from evidence of cross-sectional studies (22). Moreover, a limited number of interventions have documented health improvements attributable to increased steps per day. Nevertheless, ambulatory activity recommendations for young and middle-aged adults have not included clear advice for elderly individuals. We have shown, with a large number of elderly individuals who reflect a wide range of physical activity, that free-living physical activity was indeed associated with SIgA secretion among elderly subjects. Our results are useful and contribute to establishment of recommendations for elderly people's health, especially for bolstering their immune function.
In summary, we conclude that moderate physical activity (including approximately 7000 steps per day and 22 min·d−1 of moderate-intensity activity at home, at work, and during walking) might enhance salivary SIgA levels among elderly people.
We thank the subjects for participating in this study, and Takeshi Otsuki, Kai Tanabe, Katsuji Aizawa, and Keisuke Koizumi for critical comments (Graduate School of Comprehensive Human Sciences, Doctoral Program of Sports Medicine, University of Tsukuba). This study was supported by a grant from the Tsukuba Advanced Research Alliance Project of the University of Tsukuba and a Grant-in Aid for Science Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (13558003, 18650189 to T. A).
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