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Clinical Sciences: Clinically Relevant

Influence of walking volume on health benefits in women post-menopause

READY, A. ELIZABETH; NAIMARK, BARBARA; DUCAS, JOHN; SAWATZKY, JO-ANN V.; BORESKIE, SUZANNE L.; DRINKWATER, DONALD T.; OOSTERVEEN, SYLVIA

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Medicine & Science in Sports & Exercise: September 1996 - Volume 28 - Issue 9 - p 1097-1105
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Abstract

The traditional paradigm of exercise training to enhance fitness and performance has expanded to include the concept of physical activity designed to benefit health (14,20,29). While the exercise prescription to improve cardiovascular fitness is well established (2), less is known of the dose-response relationship between physical activity parameters and health benefits. Unlike fitness, improvements in health may be more related to the volume or amount of activity, and therefore to the total energy expended, than to exercise intensity (4,20).

Exercising at a lower intensity than previously recommended for fitness benefits, but more frequently and for longer durations, may reduce the risk for some diseases (20). For health purposes, it is recommended that individuals accumulate 30 min or more per day of moderate-intensity activity (4,29). Few studies have examined the effect of volume of activity on health outcomes; the necessary amounts of exercise may differ for various populations and for different health goals. Haskell (20) referred to three studies that indicate that 30 min of exercise a few times per week is sufficient to improve body composition in sedentary persons; however, these persons were all young- to middle-aged males. More frequent, low-intensity exercise over durations of several months to years may be optimal to bring about health changes such as improved serum lipids in the elderly (22). Research is needed to clarify the energy expenditures necessary to achieve various health benefits, as well as the dose-response relationship of physical activity in populations such as the elderly and women (14).

Despite the increased incidence of coronary artery disease (CAD) in older women (7), few studies have examined the effects of physical activity on risk factors for CAD in this population. The purpose of this study was to determine the influence of exercise volume, while keeping exercise intensity constant, on aerobic fitness and several parameters of cardiovascular health in sedentary women post-menopause. In a 24-wk exercise program, we compared the effect of walking for 60 min, 3 d·wk-1 versus 5 d·wk-1, at an equivalent intensity (60%˙VO2peak) on ˙VO2peak, serum lipids, and body composition.

METHODOLOGY

Recruitment and Screening of Participants

In response to media advertisements, over 700 women indicated an interest in taking part in the study. The eligibility criteria were: post-menopausal status (no menstrual periods in the past 12 months), over 50 yr of age, absence of hormone replacement therapy or medication to lower serum cholesterol, nonsmoker status, a sedentary lifestyle (exercising less than 30 min·wk-1), physical ability to exercise, and a body mass index of 34 or less. An initial screening by telephone excluded 436 women; medical forms were mailed to 230 women who passed the telephone screening. One-hundred fifty women returned their forms; of these 30 were screened out for medical reasons, including medications that interfere with serum lipids, or presence of disease that would preclude unsupervised walking (e.g., known heart disease).

The remaining 120 women were invited to attend one of two information meetings. Eighty-five women attended the meetings and then completed the next phase of screening, which included a resting 12-lead electrocardiogram (ECG) and a maximal treadmill test utilizing the Balke protocol(30). Resting and exercise ECG were evaluated by a cardiologist to determine whether participants could safely take part in the program. Additional criteria for participation included total serum cholesterol levels less than 8.0 mmol·l-1 and serum triglyceride less than 4.2 mmol·l-1. Three of the 85 women were screened out on the treadmill test (ECG irregularities), two were ineligible due to abnormally high serum ferritin values (hemochromatosis), and one did not complete the pretests, leaving 79 women to begin the study. The study protocol was approved by the faculty Committee for Research Involving Human Subjects, and all participants completed an Informed Consent form.

Study Design

For logistical reasons, entry into the study was staggered over a 3-month period (January to March). One-third of the participants completed the pre-tests each month, then were randomly assigned to walk 3 d·wk-1 (nine subjects per month, N = 27), 5 d·wk-1 (nine subjects per month, N = 27) or to remain sedentary (nine subjects in January, eight in February and March, N= 25). The study was designed to allow for subject drop-out of 25% to 30%. Based on expected changes in serum cholesterol in response to exercise training in women with moderate initial levels (5.2-6.2 mmol·l-1), an effect size of 0.41 was expected(27). To achieve this effect size at a power level of 0.80 and an alpha of 0.05, 20 subjects were required per group(10). All of the following procedures were administered prior to, and following, the 24-wk study.

PROCEDURES

Peak Oxygen Uptake

The Balke treadmill test (30) was used both to screen participants and to measure peak oxygen uptake. Prior to completing the maximal treadmill test, subjects visited the laboratory for a familiarization session. The women walked on a level treadmill (Quinton Model 645) with speed gradually increasing to 5.4 km·h-1 for 5 min or until comfortable. After stopping, they were fitted with a mouthpiece and noseclips, and completed the first three stages of the Balke protocol. On another day, subjects completed the Balke treadmill test (5.4 km·h-1, 0% grade, 1% increment per minute) to exhaustion. Expired gases were measured at 30-s intervals using an automated system (MMC Horizon System, Sensormedics), and calibration with known gas concentrations was done prior to and following each test. Heart rate was monitored with an ECG (Cardio Tracer, Birtcher Medical Systems) using a CM5 lead. The test endpoint was subjective exhaustion, and attainment of age-predicted maximal heart rate or a respiratory exchange ratio of 1.0 or greater was used to determine whether maximal effort was reached in the absence of a plateau in ˙VO2(5). Seated blood pressure was measured following a 5-min period of quiet rest before each test. A physician was present for all maximal tests.

Body Composition

Skinfold thicknesses (triceps, biceps, subscapular, iliac crest, abdominal, front thigh, medial calf) and girths (upper arm, forearm, waist, gluteal, upper thigh, mid-thigh, mid-calf) were measured according to the procedures of Ross and Marfell-Jones (32). All measurements were taken by one tester, and were made in duplicate and averaged. Where differences between the two measurements were greater than 0.4 mm (skinfolds) or 1.0 cm(girths) a third measurement was taken, and the average of the closest two recorded. Body density was estimated from skinfold measurements, using equation “Body Density (11)” derived by Jackson et al. (21). This equation incorporates age, and was deemed an acceptable alternative to underwater weighing in this population of older women, where variation in bone mineral content, and hence fat-free mass, may lead to inaccuracies in estimation of body fat from conventional methods of densitometry (26). Percent fat was calculated from body density (33). Waist-to-hip girth ratio (WHR) was calculated as the ratio of waist girth (measured at the narrowest point on the trunk) to gluteal girth (measured at the site of greatest gluteal protrusion). Body mass index (BMI) was determined from weight divided by height (kg·m-2).

Diet Counseling and Analysis

Participants were counseled by a nutritionist to not change their usual diet throughout the study. After instruction, participants completed 3-d food frequency records (two weekdays, and one weekend day) to monitor energy intake, and proportion of calories as fat, carbohydrate, protein, and alcohol. These records were analyzed by the nutritionist using a software package(Demeter Nutrient Analysis for DOS-based Computers) based on the Canadian Nutrient File database (version 1.2, 1991).

Blood Collection and Analysis

Subjects fasted and refrained from activity for the 12 h preceding blood sample collection, which took place between 6:45 and 9:00 a.m. Alcohol was not consumed for 72 h prior to blood collection. Upon arrival at the laboratory, subjects sat quietly for 20 min prior to sampling. Ten ml of blood were collected at each measurement session by a registered nurse. Serum cholesterol and triglycerides were determined using enzymatic colorimetric tests(Boehringer Mannheim, Hitachi 717) (3,25). Low-density lipoprotein-cholesterol (LDL-C) and very low-density lipoprotein-cholesterol (VLDL-C) were separated by ultracentrifugation, and high-density lipoprotein-cholesterol (HDL-C) was subsequently measured(6). LDL-C was calculated using the Friedewald equation(16). Baseline values for all serum lipids were calculated from the average of two measurements taken within a 1-wk period; reliability coefficients were as follows: serum cholesterol, R = 0.89; HDL-C, R = 0.88, and serum triglyceride, R = 0.84. Standard quality control procedures were followed in the laboratory, and known standards of cholesterol and triglyceride were routinely analyzed at 8-h intervals. Coefficients of variation ranged from 1.6 to 2.0 for total cholesterol, from 6.0 to 7.0 for HDL-C, and from 2.0 to 4.0 for triglyceride, all within the accepted guidelines (36). Hemoglobin and hematocrit levels were determined by Coulter counter (STKS Automated Hematology Analyzer).

Walking Program

Participants were asked to walk for 60 min per session, for 24 wk, at an intensity equivalent to 60% of ˙VO2peak for either 3 or 5 d·wk-1. Those subjects who could not complete the prescription at the outset were counseled to first increase exercise duration to 60 min, and then to increase intensity. As the women became more fit they were able to walk a greater distance in 60 min, thus ensuring exercise progression. Prior to the study, it was decided that members of the 3-d and 5-d groups must walk an average of at least 150 and 240 min·wk-1, respectively, to successfully complete the study.

Participants attended an information session that included shoe selection and the importance of warm-up and cool-down, as well as instruction in stretching and walking technique. Supervised walking classes were offered at the university track 5 times per week. Classes included a walking warm-up, group stretch, 1-h walking period, and cool-down. Walkers were requested to attend four supervised sessions in the first 2 wk, and at least one per week thereafter. Unsupervised sessions took place in a variety of settings including community tracks, fitness facilities, parks, and public roads.

Target heart rates were used to prescribe intensity, and these were monitored by the subjects after 30 and 60 min of walking by palpation of the radial pulse for 10 s. Heart rates were verified manually by the investigators at the supervised sessions. Walking speed was calculated from the time to complete five laps of the track, measured without the walkers' immediate knowledge in the middle of each supervised session. The energy expenditure of walking was estimated from average walking speed and weight(2). Log books were completed by the participants daily, and were examined together with an investigator each week for clarification and to confirm their accuracy. Specific details about each walking session were requested, and additional activities were recorded at the bottom of each page. Members of the control group were asked to maintain their usual sedentary lifestyle; however, they were enrolled in a walking program upon completion of the study.

Data Analysis

A one-way analysis of variance (ANOVA) was used to determine the significance of differences between groups at the pre-test, and a two-way analysis of variance with repeated measures was used to determine the significance of differences within and between groups at the baseline and after 24 wk. Planned contrasts were done to determine the significance of the interaction effects between group means and measurement occasion. The relationship of changes in serum lipids and lipoproteins to baseline values, and of changes in serum lipids and lipoproteins to changes in fitness, body composition, and anthropometric measurements were examined for all exercising subjects using correlation and regression analysis. Data were analyzed using StatView SE + Graphics version 1.04 and SuperANOVA version 1.11 (Abacus Concepts, Inc, Berkeley, CA) software for the Macintosh computer.

RESULTS

Adherence to Walking Program

Of the 79 women who began, 56 (71%) completed the study and met the compliance criteria established at the outset. Final membership was 20/25(80%) in the control group, 19/27 (70%) in the 3-d group, and 17/27 (63%) in the 5-d group. Seven women were not included in the data analysis due to inadequate compliance with the walking program, three in the 3-d group and four in the 5-d group. The other 16 women left the program, or were not included in data analysis, for the following reasons: injury(6); illness (2); motor vehicle accident (1); unable to obtain post-test(3); changed medications (1); menstrual period occurred (1); did not wish to be in the control group (1); positive stress test for heart disease at completion of the study (1).

Adherence to Regular Diet

At the outset of the study, participants consumed an average of 7011± 1775 kJ·d-1, 31 ± 7% as fat, 51 ± 7% as carbohydrate, and 18 ± 3% as protein (N = 52, four subjects did not complete diet records). There were no significant changes in any of these dietary parameters during the study. Corresponding values at the post-test were 6865 ± 2780 kJ·d-1 (P = 0.80), 31 ± 6% as fat (P = 0.50), 52 ± 7% as carbohydrate(P = 0.83), and 17 ± 3% as protein (P = 0.72). Percent intake from monounsaturated, polyunsaturated, and saturated fat did not change during the study. Alcohol intake averaged 2.8 ± 7.1 g·d-1 at the beginning of the study, and did not change from pre- to post-test. The majority of subjects did not consume alcohol(N = 28), or were moderate drinkers (N = 23) (<20 g alcohol·d-1). Two women consumed more than 29.9 g alcohol·d-1.

Subject Characteristics

Of the 56 women who completed the study, 47 (84%) had undergone a natural menopause, seven had both a hysterectomy and oophorectomy (mean age 43.0± 5.3 yr), while two had hysterectomies without removal of the ovaries. On average, the women were 61.3 ± 5.8 yr of age and 11.5 ± 7.3 yr post-menopause.

There were no significant differences between groups (5-d and 3-d walking groups and control group) in age, peak oxygen uptake, blood pressure, or any of the body composition and serum lipid variables at the beginning of the study (N = 79), or after exclusion of 23 women for the reasons described above (N = 56). There were also no significant differences in any of the above variables between those excluded (N = 23) and those who completed the study (N = 56). At the outset, the seven women dropped for noncompliance had a significantly higher peak oxygen uptake(25.9 ± 3.7 ml·kg-1·min-1) than did the 56 women who completed the study (22.3 ± 3.8 ml·kg-1·min-1) (P < 0.05).

Walking Program Characteristics

Walking program characteristics are described in Table 1. Members of the 5-d group walked significantly more than the 3-d group; however, relative intensity did not differ between the two groups. During the 24-wk program both groups walked at approximately 100% of the initial target heart rate, an intensity equivalent to 60% of baseline ˙VO2peak. Three members of the 3-d group, and five members of the 5-d group, did not reach their target heart rates. To estimate the energy expenditure of the walking programs, we considered the average walking speed of 1.66 m·s-1 as equivalent to 5 METS in a 70-kg woman(15), and calculated an average energy expenditure of 25.6 kJ·min-1(2).

Although six women did not complete the program due to injuries (four in the 5-d group and two in the 3-d group), examination of athletic therapy reports indicated that all had pre-existing musculo-skeletal conditions that were not previously reported (e.g., fallen arches, prior injury).

Effect of Walking Programs on Peak Oxygen Uptake and Blood Pressure

Peak oxygen uptake (ml·kg-1·min-1) increased by 12.1% and 13.9% in the 3-d and 5-d walking groups, respectively, both significant changes as compared with the control group (Table 2). Results are reported for 53 subjects only, as three women were not available for treadmill post-tests, two due to back pain and one with a chronic respiratory infection. One member of the 3-d group was tested at 4.8 km·h-1 at both pre- and post-tests, as she could not maintain the prescribed speed of 5.4 km·h-1. None of the women reached a plateau in ˙VO2; however, all were within 5 bpm of age-predicted maximum heart rate, or had a RER greater than 1.0 at both the pre- and post-tests, suggesting that values were near maximal (5). Resting systolic and diastolic blood pressure did not change in response to the program.

Effect of Walking Programs on Body Composition

Changes in body weight over time were not significant between the three groups (P = 0.07, ANOVA); however, the loss of 0.6 kg by the 3-d group was significantly different from change in the control group when planned contrasts were utilized (Table 3). Percent body fat decreased from 33.2 ± 7.1 to 32.1 ± 7.1 in the 3-d group, and from 33.9 ± 5.4 to 32.6 ± 5.4 in the 5-d group, both changes significantly different from the control group, who increased from 33.3± 5.1 to 33.5 ± 4.7 (Fig. 1). This represents decreases in fat mass of 4.2% and 4.0% in the 3-d and 5-d groups, respectively, and an increase of 2.1% in the control group.

Abdominal, front thigh, and calf skinfold measurements were significantly different from the pre- to post-test, between the three groups, as were waist, gluteal, calf, and arm girths (P < 0.05, ANOVA). Small but significant decreases in calf and abdominal skinfolds, and in waist, gluteal, arm, and upper thigh girths occurred in the 3-d group as compared with increases in the controls (Table 4). Front thigh skinfold and gluteal and upper thigh girths decreased, and calf girth increased in the 5-d group as compared with the controls. Waist-to-hip girth ratio was not significantly different between groups from the pre- to post-test.

Effect of Walking Programs on Serum Lipids and Hematocrit

There were no changes in serum lipids or lipoprotein ratios in response to either walking program (Table 5). Exclusion of the two women who consumed the greatest amounts of alcohol did not change the results. In the walkers, changes in HDL-C were not related to initial levels; however, there was a tendency for total serum cholesterol to decrease in those with higher initial values (r = -0.32, P = 0.058). This relationship was significant when the 3-d walking group was considered alone (r = -0.46,P = 0.046). Hematocrit and hemoglobin did not change from the pre- to post-test in any of the groups.

Relationship of Changes in Serum Lipids to Changes in Fitness and Body Composition in the Walkers

Changes in serum lipids, lipoproteins, and lipoprotein ratios were not significantly related to changes in ˙VO2peak(ml·kg-1·min-1, or l·min-1)(N = 53), weight, or percent body fat. A decrease in serum cholesterol was significantly associated with an increase in calf girth (r =-0.34, P = 0.040), as was a decrease in LDL-C (r = -0.42,P = 0.012). Changes in mid-thigh girth were significantly related to change in HDL-C (r = 0.39, P = 0.017) and in HDL:LDL (r = 0.37,P = 0.025). The ratio of CHOL:HDL was inversely related to change in midthigh girth (r = -0.32, P = 0.058). None of the other skinfold or girth measurements were related to changes in any of the serum lipids.

DISCUSSION

The results of this study suggest that the exercise prescriptions necessary to increase fitness, and to decrease risk for cardiovascular disease, differ. Walking for 60 min at moderate intensity, 3 or 5 d·wk-1, resulted in significant increases in ˙VO2peak. However, walking up to 5 h·wk-1 did not beneficially alter HDL-C or any other serum lipids or lipoprotein ratios. Small decreases in body fat in both walking groups indicate a positive effect of exercise on body composition, likely attenuated by the relatively brief program duration of 24 wk.

Adherence criteria were established at the outset of the study to ensure that the results accurately reflect the prescribed walking stimuli. Over 70% of participants met these criteria, better than the adherence rates of approximately 60% reported in other 24-wk walking interventions(13,31). Adherence was slightly less in the 5-d group than in the 3-d group.

Walking intensity, as indicated by average velocity and heart rates, was equivalent for subjects in the 3-d and 5-d groups, while volume walked differed significantly. This allowed us to compare the effect of varying energy expenditure, while maintaining a constant intensity, on fitness and health parameters. Exercise intensity and frequency were above the minimum recommended to improve cardiovascular fitness in both the 3- and 5-d groups(2). It is not surprising, therefore, that both groups of walkers demonstrated significant increases in ˙VO2peak, similar to those reported in other walking studies in older women(31,37). There is little debate concerning the role of relative exercise intensity in bringing about improvements in cardiovascular fitness (20), and a gradient of improvement in ˙VO2max has been reported as walking speed increases(13).

For the purposes of health promotion and disease prevention, it is recommended that individuals accumulate 30 min or more per day of moderate-intensity activity, equivalent to expending 840 kJ (200 kcal), for example, by walking briskly for 3.22 km (2 miles) (29). When the energy expended walking 3 or 5 d·wk-1 is extrapolated to a 7-d week, it is apparent that members of the 3-d group did not complete the recommended amount of activity while walkers in the 5-d group surpassed the recommendation. Despite achieving the recommended energy expenditure, the 5-d walkers did not achieve an improved serum lipid profile.

The 3-d walkers lost an average of 0.6 kg, while there was no change in weight for the 5-d walkers. Given an estimated weight loss of 1 kg adipose tissue per 25, 113 kJ (12), weight loss of 3.9 kg and 6.4 kg was expected in the 3-d and 5-d groups, respectively. A compensatory decline in physical activity during the remainder of the day has been reported in elderly persons during endurance training (18). There is no way to determine whether this was the case for our subjects, or whether women who walked 5 d·wk-1 were more likely to decrease their other activities than those who walked 3 d. Other possible explanations for the discrepancy between expected and realized weight loss include inaccuracies in the recall of physical activity and food intake. Correlations between self-reported and observed activity are moderate (r = 0.62), and endurance activities may be overestimated by up to 300% (23). It is unlikely that over-reporting of such magnitude occurred in the present study, as physical activity logs that monitor specific activities are more accurate than physical activity records. Recording compliance is also better when an activity-specific log book is used (1). It is also possible that an increase in energy intake not measured by the food frequency records occurred.

Women in both walking groups experienced significant fat loss; however, changes were in the order of only 1% body fat. Other walking studies have reported minimal changes in body composition in women in response to 24-wk walking programs (13), suggesting that the program length was too brief to cause a substantial loss of body fat. Weight and body fat increased in the control group, however, indicating that the walking program helped to prevent the weight gain commonly reported by middle-aged women(40). Contrary to our expectations, and to the findings of a study conducted in young men (17), skinfold and girth measurements did not decrease more in the 5-d than in the 3-d group. As stated earlier, members of the 5-d group may have been more likely to decrease activities other than walking, to overestimate walking activity, or to underreport food intake than members of the 3-d group. Calf girth increased in the 5-d group, likely due to an increase in muscle mass.

We recently reported that 6 months of regular walking reduced total cholesterol, and favorably altered the ratio of total cholesterol to HDL-C in hypercholesterolemic women post-menopause (31). In the present study, the women had normal serum cholesterol levels; the mean value of 5.9 mmol·l-1 at the beginning of the study reflects a moderate risk for coronary artery disease. The HDL-C level of 1.62 mmol·l-1 is well above average for women in this age group (75th percentile), while the LDL-C of 3.7 mmol·l-1 is average, and also represents a moderate risk for CAD. Serum triglyceride levels were below average (11). The lack of change in total serum cholesterol is in keeping with other studies of endurance exercise, in that decreases are related to elevated initial levels (27). We did find a significant inverse relationship between baseline serum cholesterol level and change in cholesterol in the 3-d walkers.

There was no change in HDL-C as a result of the walking programs, a finding in agreement with some studies of older women (8,31) and in disagreement with others (19,38). The discrepancies may be explained by methodological differences, or lack of control over confounding variables in some studies; however, a recent review concluded that exercise training programs do not cause HDL-C levels to rise appreciably in older women (34). Others have reported that an increase in HDL-C has been demonstrated in approximately half of the intervention studies in women, and that the responsiveness of preversus post-menopausal women is unknown (24).

Our study controlled for design flaws identified in many of the earlier studies. Subjects were nonsmokers, not on hormone replacement therapy or other confounding medications, at least 1-yr post-menopause, and not obese. In addition, we monitored alcohol and dietary intake, and ensured that participants adhered to the prescribed walking regimens. Although Whitehurst and Menendez (38) reported increased HDL-C following 8 wk of walking at 70% to 80% of maximal heart rate, subjects were not randomly assigned to groups, and the walking group had over 40% body fat. Increased HDL-C levels in women in the study conducted by Hardman et al.(19) may reflect the younger age of the participants(mean age 44.9 yr), and possibly their pre- or peri-menopausal status.

The lack of increase in HDL-C in the present study may be explained in several ways. Hemodilution is one possibility; however, the lack of change in hematocrit and hemoglobin suggests that plasma volume did not increase during the study (28). Alternatively, the relatively large coefficient of variation for HDL-C measurements may explain the nonsignificant results. As with the majority of studies in women, initial HDL-C levels were high and it may be that women with lower HDL-C respond better to exercise(24). We previously failed to demonstrate an increase in HDL-C after a 6-month walking program in post-menopausal women with HDL-C levels of 1.35 mmol·l-1(31); however, an increase was reported in response to a similar program in middle-aged women with initial HDL-C levels of 1.17 mmol·l-1(19). The lack of a relationship between initial HDL-C levels and change in HDL-C in the present study may reflect the homogeneous distribution of HDL-C, as few women had levels below 1.00 mmol·l-1.

Several studies suggest that the amount of activity, rather than the intensity, is a primary determinant of HDL-C. In one 24-wk study, where young women were assigned to walk 24 km·wk-1 at three different velocities, increases in HDL-C were independent of intensity(13). Women who walked slowly experienced the same increases in HDL-C as brisk walkers, although they had significantly less improvement in cardiovascular fitness. The energy expended by the women in that study was similar to that expended by our 5-d group, who walked approximately 26 km·wk-1. Given the results of the earlier study, and the proposed relationship between energy expended and change in HDL-C, we expected to see an increase in HDL-C in the 5-d walkers. The lack of change in either walking group is in keeping with the observation that a threshold energy expenditure of ≥8370 kJ·wk-1 may be necessary before elevated HDL-C levels are seen in post-menopausal women(9). Although the 5-d walkers expended approximately 7150 kJ·wk-1, this appears insufficient to increase HDL-C in older women. It is also possible that the 24-wk duration was too short, as activity programs of over 1 yr may be necessary to increase HDL-C in the elderly(22).

The women in the present study lost little weight or body fat, another possible explanation for the lack of change in HDL-C(39). Changes in HDL-C and other lipids were not related to changes in body fat or weight experienced by the walkers, however, which seems to negate this theory. It may be that changes in both variables were so negligible as to preclude any association, or it may be that loss of weight is more related to lipid changes in obese women. We did report significant associations between change in HDL-C and change in weight in our previous study; however, the participants were slightly heavier and had more body fat(31). Significant associations between changes in several girth measurements and beneficial changes in HDL-C and the atherogenic indices indicate that the latter may be related to increases in fat-free mass, and possibly to changes in muscle enzyme activities (35).

CONCLUSIONS

Walking for 60 min·d-1 at moderate intensity, 3 or 5 d·wk-1 for 24 wk, was sufficient to improve aerobic fitness in sedentary women post-menopause. The increased energy expended by walking 5 versus 3 h·wk-1 did not provide additional cardiovascular health benefits, at least over a 24-wk period. While serum lipids did not change in either group, both the 3-d and 5-d walkers lost equal amounts of body fat. Possible explanations for the absence of increased health benefits in the 5-d group, as compared with the 3-d group, include a greater compensatory decline in activities other than the walking program, or greater discrepancies in actual versus reported activity and food intake. To better differentiate the role of exercise volume on health benefits in older adults, future studies should be of longer duration. Optimal training amounts may be more readily apparent in participants at greater risk, for example, women with low HDL-C levels.

Figure 1-Estimated percent body fat (mean ± SD) before and after a 24-wk walking program for members of the control group(
Figure 1-Estimated percent body fat (mean ± SD) before and after a 24-wk walking program for members of the control group(:
N = 20), and the 3-d (N = 19) and 5-d (N = 17) walking groups. Differences are significant, pre- to post-test, between the control group and the 3-d and 5-d groups (P < 0.05).

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Keywords:

BODY COMPOSITION; DOSE-RESPONSE; EXERCISE PRESCRIPTION; HDL-CHOLESTEROL; RISK FACTORS FOR CORONARY ARTERY DISEASE; SERUM LIPIDS

©1996The American College of Sports Medicine