Ample evidence exists promoting physical activity to prevent inactivity-related chronic diseases, but most Americans still exercise below minimal standards (18,27). Performing moderate- to vigorous-intensity continuous exercise (CON-EX) for 20-60 min has been recommended previously, but this exercise duration was cited as a barrier to exercise participation (18). Accumulating multiple 10-min exercise bouts each day that total at least 30 min in moderate-intensity lifestyle activity or intermittent exercise (INT-EX) was speculated to benefit people with busy lifestyles or low fitness (18,27). Previous INT-EX training (TRAIN) research indicated improved fitness (8,17) and variable lipid and lipoprotein responses (6,8,17,28). Despite previous reports of lipoprotein profile improvement (6,8,17), Durstine et al. (7) suggest that little is known about the effects of INT-EX TRAIN on lipids and the lipoprotein profile.
Previous research on TRAIN has varied much (3,6,8,10,12,21,24,28), and Halle et al. (10) reported an improved lipoprotein profile after 4 wk in a combined intervention of a daily diet of 1000 kcal with a weekly exercise expenditure of 2200 kcal. Pate et al. (18) recommended a weekly minimum of expenditure of 1200 kcal in INT-EX, which also is a caloric expenditure reported to decrease fasting total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C) (12,23), and increase high-density lipoprotein cholesterol (HDL-C) (3,12,24). With 14 d of detraining following TRAIN, Mankowitz et al. (16) reported decreased HDL-C and increased postprandial lipemia (PPL), an independent risk factor for coronary heart disease (CHD) (19).
Patsch et al. (19) reported that susceptibility for atherosclerotic development increases when TG remain elevated for prolonged durations. Altena et al. (2) demonstrated that 30 min of INT-EX significantly attenuated PPL in acute exercise accumulation, which might suggest that CON-EX and INT-EX affect lipids and lipoproteins differently with TRAIN. Halbert et al. (9) suggested that the best exercise method for improving the lipoprotein profile was still unclear, and that the lipoprotein profile would undergo little if any change with exercise in normolipidemic populations. The effect of INT-EX in short-term TRAIN on PPL and lipoprotein subfractions has not been addressed for untrained, normolipidemic populations to prevent dyslipidemia.
Using an exercise volume of 1200 kcal·wk−1 (18), the importance of CHD prevention for normolipidemic subjects is the foundation for this research rather than studying the aged (28), obese (6), or diseased (10). Evidence exists that larger exercise volumes can improve the lipoprotein profile of diabetic subjects in 4 wk (10). This study, therefore, compared fasting lipoprotein subfractions and PPL before and after 4 wk of either CON-EX or INT-EX TRAIN in untrained, normolipidemic subjects.
METHODS
Subjects.
A total of 18 untrained, nonobese normolipidemic men (N = 7) and women (N = 11) ages 18-45 yr volunteered for the study (1). Subjects were alternatively assigned to either the CON-EX (N = 10) or INT-EX (N = 8) group as they were screened for the study. Subjects were excluded if they were participating in a regular exercise program consisting of more than 2 d·wk−1 and more than 20 continuous minutes per session, had known cardiovascular or metabolic disease, were currently smoking, were taking lipid-altering medications, were ingesting cold-water fish more than 3 d·wk−1, or were known to be pregnant. All female subjects were eumenorrheic, and all but two females reported use of oral birth control. Before participation, subjects signed informed consent documents approved by the health sciences IRB at the University of Missouri-Columbia, Columbia, Missouri.
Body composition.
Body composition was assessed using the sum of three skinfold measurements specific for males (chest, abdomen, thigh) and females (triceps, supraillium, thigh) (1). Body mass index and waist:hip ratio were also measured and calculated using known methods (1). All body measurements were made by a single assessor blinded at post-TRAIN.
Maximal oxygen uptake testing.
Subjects performed an exhaustive treadmill test to determine fitness levels at before and after TRAIN. This protocol has been described previously (2,22,29). Briefly, after a 2-min walking warm-up, treadmill speed began at 3.5 mph for women and 4.0 mph for men for 2 min, and was increased by 0.5 mph each until 6.0 and 6.5 mph were reached for women and men, respectively. After maximal speed was achieved, grade was increased 2% each minute until exhaustion. Metabolic data were collected using open-circuit spirometry. The highest 30-s V̇O2 during the test was determined as being the subject's V̇O2max.
Dietary habits.
Subjects were instructed to consume foods reflective of typical eating choices, which were recorded in a 3-d food intake record consisting of two weekdays and one weekend day. Diet records were analyzed using the Food Processor software, version 7.8 (ESHA, Salem OR).
Experimental design.
Lipoproteins and the postprandial response were measured before and after 4 wk of TRAIN. Subjects were free living throughout both trials, and trials were scheduled on days with similar routines (e.g., work schedule, low-intensity ambulation). Blood samples of women were not collected during menses or the 5 d before predicted menses (4,20). During trials, subjects were asked about their daily activities and were reminded to maintain their typical daily routine and minimal ambulation.
Trial preparations.
At 48 h before high-fat meal (HFM) trials, subjects were instructed to continue with their typical routines and to abstain from both moderate and vigorous activity. Subject activity levels were not directly monitored or recorded for 48 h preceding or during PPL trials. At 24 h before the pre-TRAIN HFM trial, each subject consumed meals that were indicative of their typical eating habits with no dietary restrictions. Foods were recorded as they were eaten. This diet was repeated before the post-TRAIN HFM trial in respect to portion sizes, preparation, and number of times food was consumed. The pretrial diet included a 12-h overnight dietary fast, where subjects were instructed to consume only water and restrict activity. Subjects were instructed not to ingest alcohol in the 48h before HFM trials, and to abstain from caffeine 24 h before and during the HFM trials. Laboratory staff contacted subjects daily by telephone and e-mail to remind them of the activity-control period, repeated diet, and dietary fast in the 72 h before both trials.
High-fat meal.
For the HFM trials, the HFM was a shake composed of heavy whipping cream and specialty ice cream previously reported by our laboratory (2,29). Caloric composition was based on body weight and contained 1.5 g of fat, 0.05 g of protein, and 0.4 g of carbohydrate per kilogram of body weight, and total fat content was 88% of calories. This test meal was well tolerated by all subjects. On morning arrival at the laboratory, a fasting (0 h) blood sample was collected, and the subject was given the HFM, which was ingested within 10 min. After consuming the HFM, subjects returned to the laboratory every 2 h for blood sampling, which occurred for a total of 8 h. During both HFM trials, subjects were instructed to consume only water. Fluid intake was not monitored.
Exercise training sessions.
The TRAIN session was 5 d·wk−1 for a total of 4 wk. Laboratory staff monitored all TRAIN sessions, and daily TRAIN times were flexible to accommodate subject schedules. All subjects completed 20 exercise sessions in 4 wk. TRAIN was extended up to 5 d beyond the 4-wk protocol to accommodate menstrual cycle irregularities.
The TRAIN program consisted of jogging on a level treadmill for 30 min at a speed that elicited 75% HRmax, which was closely associated with 60% individual pre-TRAIN V̇O2max. Treadmill speed was titrated to maintain 75% HRmax after the warm-up period. Both CON-EX and INT-EX protocols were conducted as previously reported (2). CON-EX sessions began with a 9-min warm-up that started with walking at 2.5 mph with incremental speed increases each minute. Subjects began jogging at the sixth minute of warm-up. INT-EX sessions were performed in three 10-min bouts. INT-EX warm-up was 3 min and began with walking at 2.5 mph, with incremental speed increases every 30 s. Jogging began at the third minute of warm-up. The three INT-EX bouts were separated by two 20-min rest periods, during which subjects were seated and allowed to quietly read and do paperwork. Both CON-EX and INT-EX totaled 30 min at 75% HRmax. HR was measured with wireless telemetry.
Blood collection and preparation.
Blood samples were collected from an antecubital vein after subjects were seated in a semisupine position for 3-5 min. Samples were collected into 10-mL tubes containing EDTA, and were separated by refrigerated centrifugation at 4°C for 15 min at 2000 × g. Plasma was transferred to labeled cryogenic vials and stored at −80°C until analyzed. To eliminate interassay variability, all samples from a single subject were analyzed together for each assay, and all samples were analyzed in triplicate.
Plasma triglyceride analysis.
Triglyceride was measured enzymatically using a diagnostic kit (Infinity TG Reagent #344, Sigma, St. Louis, MO). Mean coefficient of variation (cv) for this assay was 1.3%. TG concentration was measured in all samples during the postprandial period at 0, 2, 4, 6, and 8 h to determine the PPL response. Quantified using the trapezoidal rule, total area under the curve (AUCT) was calculated using the actual value of all TG responses, and AUCI was calculated using the difference between all TG responses from the 0-h (pre-HFM) sample (2,29). TG peak responses were quantified as total peak (PeakT) being the highest TG concentration, and incremental peak (PeakI) being the difference between the highest TG during the postprandial period and the 0-h sample.
Total cholesterol and high-density lipoprotein cholesterol analysis.
Fasting TC was analyzed using a diagnostic kit (Infinity Cholesterol Reagent #402, Sigma, St. Louis, MO). Mean cv for this assay was 0.9%. Fasting plasma HDL-C and subfractions were determined using a modified heparin-MnCl2-dextran sulfate method (5). HDLTotal-C and HDL3-C were determined with the same procedure as TC, and HDL2-C was calculated. Mean cv were 1.6, 1.4, and 0.6% for HDLTotal-C, HDL2-C, and HDL3-C, respectively.
Low-density lipoprotein subfractions and particle size.
Low-density lipoprotein subfractions and particle size were quantitated using the Lipoprint system (Quantimetrix, Redondo Beach, CA) using a similar procedure as previously reported (13). Briefly, plasma was mixed with Sudan Black B dye to stain lipoproteins. Stained lipoproteins were photopolymerized and electrophoresed in a precast 3% polyacrylamide gel tubes. Densitometry was performed at 610 nm using a Helena EDC system (Helena Labs) in a specialty holding device. Raw data generated by the densitometer and the electrophoretic scan patterns of each individual subject were analyzed by a computer program that calculated individual lipoprotein subfractions as related to the measured TC of each subject at pre- and post-TRAIN. During electrophoresis, lipoprotein subfractions migrate respective of particle density and size (14). From least dense to most dense, lipoproteins gained from this procedure were very-low-density lipoprotein cholesterol (VLDL-C), total intermediate-density lipoprotein cholesterol (IDL-C), and LDL-C subfractions 1, 2, and 3. LDL particle size was determined by a procedure as a range of 220-280Å (14). Intraassay coefficient of variability was 3-10% for the various LDL-C subfractions.
Statistical analysis.
Pre-TRAIN CON-EX and INT-EX group differences were analyzed with an independent t-test for subject characteristics and all plasma variables. The effect of TRAIN was analyzed with a two-way (group × time) ANOVA with repeated measures. Significance was set at P < 0.05 each variable.
RESULTS
Subject age in years and height did not change during the study. Subject age in years was 25.0 ± 1.6 (mean ± SE), 29.0 ± 2.2, and 22.0 ± 2.4, respective of combined groups, CON-EX, and INT-EX. Subject height was 1.7 ± 0.1 m with groups combined and respective of group assignment. No group differences were discovered at pre-TRAIN for subject characteristics (Table 1) and plasma variables (Table 2). Caloric composition and macronutrients among pre- and post-TRAIN 3-d dietary records and the 1-d dietary records were not significantly different (data not shown).
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TABLE 1: Pre- and post-TRAIN subject characteristics.
TABLE 2: Pre-TRAIN postprandial and fasting triglycerides and fasting and lipoproteins.
Compared with pre-TRAIN variables, no difference was discovered with post-TRAIN fasting TG and VLDL-C, and groups were not different (Fig. 1). Pre- and post-TRAIN postprandial variables are reported in Table 3. Post-TRAIN measurements of AUCT, AUCI, peakT, and peakI were not different compared with pre-TRAIN values, and TRAIN methods were not different. At post-TRAIN, CON-EX decreased, and INT-EX increased all PPL variables nonsignificantly.
TABLE 3: Pre- and post-TRAIN postprandial variables.
FIGURE 1: TC significantly decreased at post-TRAIN with groups combined. Pre- and post-TRAIN values for VLDL-C and TG were not different. CON-EX and INT-EX groups were not different at post-TRAIN. * Significant post-TRAIN differences compared with pre-TRAIN values for the COMBINED group. Values are reported as means ± SE of value changes between pre- and post-TRAIN (mg•dL−1).
Post-TRAIN fasting TC significantly decreased 7.8% with groups combined. CON-EX and INT-EX lowered fasting TC 4.7 and 11.3%, respectively, at post-TRAIN and not different between groups (Fig. 1). HDL-C and its subfractions are reported in Figure 2. No group differences were discovered for HDL-C and its subfractions at post-TRAIN. With groups combined, HDLTotal-C, HDL2-C, and HDL3-C increased 2.9, 2.0, and 0.5%, respectively at post-TRAIN. Only HDL2-C increased significantly at post-TRAIN. CON-EX increased HDLTotal-C and HDL2-C at post-TRAIN by 2.0 and 1.2%, respectively, and decreased HDL3-C 1.4%. INT-EX increased HDLTotal-C, HDL2-C, and HDL3-C respectively by 3.7, 2.8, and 2.4% at post-TRAIN. The TC:HDL ratio significantly decreased 15.9% at post-TRAIN with groups combined from 3.7 ± 0.4 to 3.1 ± 0.3, and CON-EX and INT-EX were not different. CON-EX lowered the TC:HDL ratio 13.4% to 3.6 ± 0.4, and INT-EX decreased 19.1% to 2.6 ± 0.4 at post-TRAIN.
FIGURE 2: HDLTotal-C and HDL3-C remained unchanged at post-TRAIN compared with pre-TRAIN values. HDL2-C significantly increased at post-TRAIN compared with pre-TRAIN values. Groups were not significantly different at post-TRAIN. * Significant post-TRAIN difference compared with pre-TRAIN values for the COMBINED group. Values are reported as means ± SE of value changes between pre- and post-TRAIN (mg•dL−1).
With groups combined, total intermediate-density lipoprotein cholesterol (IDL-C) decreased 6.7% to 32.6 ± 2.4 mg·dL−1 at post-TRAIN. No difference was discovered between groups, and total IDL-C was decreased to 36.8 ± 3.1 mg·dL−1 and 28.4 ± 3.5 mg·dL−1, respectively, for CON-EX and INT-EX. Post-TRAIN LDL-C and its subfractions are depicted in Figure 3. LDLTotalC significantly decreased 8.4% at post-TRAIN with groups combined. Both CON-EX and INT-EX decreased LDLTotal-C by 2.9 and 11.2%, respectively, but was not different between groups. With groups combined, LDL1-C, LDL2-C, and LDL3-C decreased 4.1, 20.6, and 51.7%, respectively, and were not different compared with pre-TRAIN values. LDL mean particle size significantly increased at post-TRAIN to 272.5 ± 1.0Å with groups combined. At post-TRAIN, CON-EX increased LDL particle size to 271.9.±1.3Å, and INT-EX increased LDL particle size to 273.0 ± 1.5Å. Post-TRAIN LDL particle size was not different between groups.
FIGURE 3: Post-TRAIN LDLTotal-C was significantly different than pre-TRAIN values, but was not different between CON-EX and INT-EX groups. TRAIN did not affect LDL1-C, LDL2-C, or LDL3-C subfractions and were not different between TRAIN groups. * Significant post-TRAIN difference compared with pre-TRAIN values for the COMBINED group. Values are reported as means ± SE of value changes between pre- and post-TRAIN (mg•dL−1).
DISCUSSION
The present study compared the lipoprotein profile after normolipidemic subjects completed 4 wk of exercise TRAIN consisting of either CON-EX or INT-EX for 30 min each day. The intention of the INT-EX recommendations were to prevent chronic disease and promote physical activity (18), yet Durstine et al. (7) questioned the ability of INT-EX to affect the lipoprotein profile compared with CON-EX. Based on the present results, CON-EX and INT-EX were similar in lipoprotein profile improvements, which could reduce dependence on statin drug prescriptions. Similar to the current results, Woolf-May et al. (28) and others (6,8,17) reported that INT-EX increased fitness and decreased LDL-C (28) after TRAIN. The improved lipoprotein profile without decreased body weight is supported by previous research (15,24). A lipoprotein profile shift from smaller atherogenic subfractions to larger particles in 4 wk TRAIN has been reported by others (10), as evidenced further by increased mean LDL particle size at post-TRAIN with groups combined. Our finding of increased HDL2-C is supported by previous research (6,8). The present results substantiate TC affected by the sum of all lipoprotein subfractions, even when individual subfractions do not change significantly.
The shifted lipoprotein profile and increased HDL2-C could be the result of enhanced reverse cholesterol transport (RCT). Thomas et al. (22) and others (30) reported that RCT did not change after acute exercise, but Wilund et al. (27) reported that RCT could be enhanced with TRAIN. They speculated that RCT changes from TRAIN would increase HDL-C, as discovered in the present study. It remains unclear whether CON-EX and INT-EX can differently affect RCT. To our knowledge, a comparison of CON-EX and INT-EX on LDL particle size has not yet been reported, and the present findings indicate similarities between exercise methods for preventing dyslipidemia through decreased TC, and increased HDL2-C and LDL particle size.
Caloric expenditure of a single exercise session has been promoted as a primary contributor to acute lipid and lipoprotein changes (25), and Pate et al. (18) emphasized a daily expenditure of at least 200 kcal in INT-EX. Based on pre-TRAIN exercise trials (previously reported), approximately 245 kcal were expended per exercise session, and approximately 1225 kcal·wk−1, and was not different between TRAIN groups (2). In the current study, the lipoprotein profile changed with a lower weekly caloric expenditure than Halle et al. (10), who reported an exercise volume of 2200 kcal·wk−1 coupled with a 1000 kcal·d−1 diet. Previous research has shown that a similar weekly exercise volume can decrease TC (21,26) and LDLTotal-C with concurrent increases in TG and HDLTotal-C (21).
Pate et al. (18) suggest that short exercise bouts accumulate. The INT-EX protocol implemented in this study used 1:2 exercise:rest ratio of 10 min of jogging followed by 20 min of seated rest (2), which is different from previous INT-EX research (6,8,28). Previously, our group reported an acute INT-EX bout of this ratio attenuated PPL compared with a control trial, but acute CON-EX of equal caloric volume did not (2). Others (11) have also reported no PPL changes after TRAIN. The current findings emphasize the importance of caloric expenditure through exercise rather than the exercise method.
CONCLUSION
Although the results of the current study are limited by not having a control group, the findings suggest that moderate-intensity CON-EX and INT-EX TRAIN similarly affect the lipoprotein profile of untrained, normolipidemic men and women. Both CON-EX and INT-EX shifted the lipoprotein profile and LDL particle size from smaller particles to larger, less atherogenic subfractions, but PPL was unaffected at post-TRAIN. Additional research is needed to assess the mechanisms of the exercise and rest timing and RCT.
This study was supported by NIH T32 Postdoctoral Training Grant, University of Missouri-Columbia Foods for the 21st Century, University of Missouri-Columbia Alumni Grant, and a grant from the Elizabeth Hegarty Foundation. The authors would like to thank Dr. Bryan K. Smith for his assistance with reviewing this manuscript, Dr. Steven W. Edwards for statistical insight, and Dr. Ying Liu and R. Scott Rector for assistance with subject preparation and analytical technique advice.
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