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International Journal of Obesity (2000) 24, 1303±1309 ß 2000 Macmillan Publishers Ltd All rights reserved 0307±0565/00 $15.00 www.nature.com/ijo

Different patterns of brisk walking are equally effective in decreasing postprandial lipaemia MH Murphy1, AM Nevill2 and AE Hardman3* 1

Sport and Exercise Sciences, University of Ulster at Jordanstown, Northern Ireland, UK; 2School of Sport, Performing Arts and Leisure, University of Wolverhampton, Wolverhampton, UK; and 3Human Muscle Metabolism Research Group, Loughborough University, Leicestershire, UK

OBJECTIVE: To compare the effects of different patterns of brisk walking on day-long plasma triacylglycerol concentrations in sedentary adults. DESIGN: A three-trial, repeated measures design in which subjects were studied in the fasted state and throughout a day during which they consumed three standardized, mixed meals. On different occasions, subjects undertook no exercise (control), walked briskly for 10 min before each meal (short walks) or walked briskly for 30 min before breakfast (long walk). SUBJECTS: Seven postmenopausal sedentary women and three sedentary men aged between 34 and 66 y, with body mass index between 24 and 35 kg=m2. MEASUREMENTS: Plasma concentrations of triacylglycerol, non-esteri®ed fatty acids, glucose and insulin, metabolic rate and whole-body substrate oxidation in the fasted state and at hourly intervals for 3 h after each meal. RESULTS: Postprandial plasma triacylglycerol concentrations were lower (P 0.009) during the walking trials than during the control trial (average values: control 2.08 0.28 mmol=l; short walks 1.83 0.22 mmol=l; long walk 1.84 0.22 mmol=l (mean s.e.) but did not differ between the two patterns of walking. The difference between control and walking trials increased as successive meals were consumed (interaction of trialmeal P 0.03). Plasma triacylglycerol concentration increased during the 3 h after breakfast, changed little after lunch and decreased after the evening meal (interaction of mealtime P 0.001). When both walking trials were treated as one condition, walking increased postprandial fat oxidation (average values: control, 0.066 0.009 g=min; walking 0.074 0.008 g=min; P < 0.01). CONCLUSIONS: Thirty minutes of brisk walking, undertaken in one session or accumulated throughout a day, reduces postprandial plasma triacylglycerol concentrations and increases fat oxidation. International Journal of Obesity (2000) 24, 1303±1309 Keywords: accumulated brisk walking; postprandial TAG; intermittent; continuous

Introduction Recommendations from expert bodies regarding desirable levels of physical activity encompass the principle that frequent short exercise sessions confer health bene®ts similar to those acquired through less frequent but longer sessions of equivalent total duration. For example, recent guidelines advise that adults should accumulate 30 min of moderate intensity activity on most, preferably all, days of the week,1 through sessions of at least 10 min in duration.2 The rationale for this advice rests largely on epidemiological evidence for an inverse association between the total energy expended in physical activity and disease risk,3 ± 5 because some of the activities associated with favour-

*Correspondence: AE Hardman, Department of PE, Sports Science and Recreation Management, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK. E-mail: a.e.hardman.ac.uk Received 10 December 1999; revised 24 March 2000; accepted 22 May 2000

able health outcomes, for example walking, climbing stairs and gardening, seem likely to have been performed at least partly on an intermittent basis.3,6 However, whilst there are reports that frequent short sessions of exercise can, over a period of months, improve ®tness and decrease body fatness,7 ± 9 scienti®c evidence for the health bene®ts of such a regimen is scanty. The purpose of the present study was to compare the short-term effects of two different patterns of exercise on one aspect of cardiovascular disease risk, namely postprandial plasma triacylglycerol (TAG) concentrations. Case ± control studies,10 as well as epidemiological studies,11 suggest that high postprandial plasma concentrations of TAG are an important risk marker for cardiovascular disease. According to the `TAG intolerance hypothesis',12 impaired TAG transport in and uptake from plasma leads to increased susceptibility for the rapid progression of atherosclerosis because repeated episodes of exaggerated postprandial lipaemia represent a daily, repeated atherogenic in¯uence on other lipoprotein species.13

Brisk walking patterns and postprandial lipaemia MH Murphy et al

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Previous studies have demonstrated that exercise reduces postprandial lipaemia. For example, exercise for 90 min at 60% of maximal oxygen uptake (V O2max) taken one afternoon reduced the plasma TAG response to a high-fat meal consumed the following morning by around 20% in middle-aged men14 and women.15 The magnitude of the TAGlowering effect of exercise seems to be determined by the associated increase in energy expenditure16 and=or energy de®cit, 17 but there is no information for sessions as short as the 30 min per day currently recommended. The present study therefore compared the effects on postprandial plasma TAG concentrations of two different patterns of brisk walking in sedentary adults. Walking was performed either as one 30 min session or in three 10 min sessions taken at intervals throughout the day.

Methods Subjects

Ten subjects (three men and seven postmenopausal women) were recruited from the local community through advertisements. Exclusion criteria were: arterial blood pressure > 150 mmHg systolic or > 95 mmHg diastolic; or body mass index > 35 kg=m2; or use of medication known to in¯uence fat or carbohydrate metabolism. Some of the subjects' physical characteristics are shown in Table 1. With the exception of one man who was borderline hyperlipidaemic (total cholesterol 6.25 mmol=l, fasting TAG 2.35 mmol=l), subjects were normolipidaemic. All were sedentary and none had engaged in more than 20 min of continuous exercise per week during the 6 weeks prior to the study. Preliminary procedures

Height and weight were determined using standard methods. After subjects were habituated to treadmill walking, V O2max was determined using an incremental uphill walking test at a constant speed selected on an individual basis (1.25 ± 2.0 m=s). Criteria for attainment of a maximal test were: heart rate within 10 beat=min of predicted maximal value for age; respira tory exchange ratio > 1.11; increase in VO2 of less than 5% after the last increase in treadmill gradient; and rating of perceived exertion18 of 19 or 20. In six tests all criteria were ful®lled and in four tests three Table 1 Some physical characteristics of the subjects Women (n 7), mean (range) Age (y) Body mass (kg) Body mass index (kg=m2) VO2max (ml=kg=min) International Journal of Obesity

55.0 69.7 27.2 28.8

(49 ± 66) (55.5 ± 85.6) (23.9 ± 33.2) (23.4 ± 34.8)

Men (n 3), mean (range) 46.0 98.6 29.7 40.3

(34 ± 52) (79.5 ± 118.0) (25.1 ± 34.5) (26.3 ± 48.3)

were ful®lled. In a second treadmill test, the relationship between treadmill speed and oxygen uptake was determined to identify the speed which elicited 60% of V O2max for each individual. Design

Subjects undertook three trials, with intervals of at least 7 days, in a repeated measures, randomized design. The study was approved by the University of Ulster Research Ethical Committee and all subjects gave their written consent, after a full explanation of the requirements and associated risks. During each day-long trial, subjects consumed three meals, ie breakfast (20% of energy intake), lunch (50% of energy intake) and an early evening meal (30% of energy intake). During one trial (short walks), subjects walked for 10 min immediately prior to each meal; during another (long walk), they walked for 30 min before breakfast; and on the other they performed no exercise (control). Other than these walks, subjects rested or worked quietly. Subjects weighed and recorded all food and drink consumed during the day preceding their ®rst trial and, for purposes of standardization, replicated this prior to subsequent trials. They agreed to refrain from drinking alcohol the day before each trial. Protocol

The study protocol is shown diagrammatically in Figure 1. Subjects reported to the laboratory at 0730 after an overnight fast. After 15 min of supine rest a 5 min expired air sample was obtained. A cannula was placed in a forearm or antecubital vein and a baseline blood sample was obtained. Starting a few minutes later, subjects then walked on the treadmill for either 10 min (short walks trial) or 30 min (long walk trial) or rested (control trial) before consuming breakfast at about 8:30 am. In the short walks trial, subjects also walked for 10 min immediately before lunch (12:30 pm) and for 10 min immediately before their early evening meal (4:30 pm). Expired air samples were collected during walking for measurement of V O2 and V CO2 (long walk 5 ± 10, 15 ± 20 and 25 ± 30 min; short walks 5 ± 10 min). In addition, expired air and blood samples were obtained at rest at hourly intervals for 3 h following each meal. At each observation point, subjects were supine for 10 min prior to expired air collections and remained supine until the blood sample had been obtained. Meals were given according to subjects' body mass (details in legend to Figure 1) and consumed within 20 min. Breakfast comprised cereal with milk, yoghurt, bread and boiled egg (30 9 g fat, 58 17 g carbohydrate, 28 8 g protein and 2.40 0.73 MJ; mean s.d.). Lunch consisted of pasta with minced beef in a creamy tomato sauce, bread and butter, chocolate biscuits, peanuts and orange juice (76 22 g fat, 144 43 g carbohydrate, 70 21 g pro-


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Figure 1 Study protocol. Meals were given per kg body mass: breakfast, 0.37 g fat, 0.71 g carbohydrate, 0.34 g protein; lunch, 0.93 g fat, 1.77 g carbohydrate, 0.86 g protein; and early evening meal, 0.56 g fat, 1.06 g carbohydrate, 0.51 g protein.

tein and 6.14 1.82 MJ). The evening meal comprised chicken, tomato, bread and butter, potato crisps and orange juice (46 5 g fat, 86 26 g carbohydrate, 41 12 g protein and 3.68 1.09 MJ). Over the day, 47% of energy intake was derived from fat, 35% from carbohydrate and 18% from protein. Water was consumed ad libitum during each subject's ®rst trial and the volume recorded so that this consumption could be replicated during subsequent trials. Analytical methods

Oxygen uptake and carbon dioxide production were determined using an integrated gas analysis system (Metabolic Cart, Quinton Instruments Co., Bothell, WA, US) and heart rate using short-range telemetry (SportTester, Polar Electro, Tampere, Finland). Blood samples (10 ml on each occasion) were collected into pre-cooled EDTA tubes and separated within 15 min. Portions of plasma were stored at 7 20 C for later determination using enzymatic, colourimetric methods (all Randox NI Ltd, Crumlin, Northern Ireland) of concentrations of TAG, nonesteri®ed fatty acids (NEFA), glucose and, for samples obtained in the fasted state, total and high density lipoprotein (HDL) cholesterol. Plasma insulin was determined by radioimmunassay (Amersham Pharmaceuticals, Little Chalfont, Bucks, UK). All samples from each subject were analysed in the same batch. Accuracy and precision were monitored using quality control sera (Randox NI Ltd, Crumlin, Northern Ireland). Within-batch coef®cients of variation were 1.2% for TAG, 1.5% for NEFA, 0.7% for glucose, 1.2% for total cholesterol, 0.9% for HDL cholesterol and 4.8% for insulin. Haemoglobin and haematocrit were measured at baseline and at the end of the observation period to monitor changes in plasma volume.19 Calculations and statistics

Substrate oxidation and energy expenditure were estimated using indirect calorimetry. Plasma concen-

trations measured in the fasted state were compared between trials using a repeated measures ANOVA. Since there were no signi®cant differences between trials for any variable examined, these baseline values were omitted and concentrations at the nine postprandial time points were compared using a (333) ANOVA with repeated measures, all three factors within subjects. The three factors were trial (control, short walks, long walk), meal (breakfast, lunch, evening meal) and time (1, 2 or 3 hrs after ingestion of the meal). To con®rm that omitting baseline values from subsequent analysis was justi®ed, the comparative statistical analysis was repeated for the change for each subject at each time point from their baseline level. This did not alter any of the ®ndings. ANOVA was performed using the statistical software MINITAB (version 12). The residuals were saved and examined for normality using the Anderson ± Darling test. In all cases, the residuals were found to be normally distributed. For variables where no differences between trials were uncovered by the initial comparative analysis, and where the responses in both walking trials were observed to be close, the two data sets were treated as a single condition and the analysis was repeated to compare control with walking trials. This comparison was employed for the dependent variables respiratory exchange ratio and estimates of fat oxidation. Unless otherwise stated, the values reported are mean s.e. A 5% level of con®dence was accepted as signi®cant.

Results Brisk walking

Subjects walked at 1.64 0.71 m=s, with an oxygen uptake which represented 59.6 1.2% (short walks) and 60.4 0.9% (long walk) of V O2max. The respective average heart rates (n 8) were 119 4 and International Journal of Obesity


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121 3 beat=min. There were no signi®cant differ ences in VO2 or heart rate between trials. Total fat oxidation during 30 min of brisk walking was lower (P 0.009) during the short walks (6.7 0.9 g) than during the long walk (8.8 1.2 g). Plasma concentrations

Changes in plasma volume during the day were small (7 1.7 1.2%, 7 2.1 1.5%, 7 1.9 1.4% for control, short walks and long walks, respectively) and did not differ between trials so no adjustments were made to measured concentrations. In the fasting state there were no signi®cant differences between trials in plasma concentrations of any of the constituents measured (Table 2). There was considerable interindividual variation in the postprandial TAG response but the highest values (range 1.20 ± 3.92 mmol=l) were consistently observed 2 h after lunch. Average postprandial plasma TAG concentrations (Figure 2) were signi®cantly higher in the control trial than in either of the walking trials (control 2.08 0.28 mmol=l; short walks 1.83 0.22 mmol=l; long walk 1.84 0.22 mmol=l; main effect of trial, P < 0.01) but there was no difference in the effect of the different patterns of walking. There was a signi®cant trialmeal effect (P 0.03); inspection of Figure 2 shows that this re¯ects the trend for the difference in plasma TAG Table 2 Plasma concentrations measured in the fasting state for control, short walks and long walk trials Control TAG (mmol=l) Total cholesterol (mmol=l) HDL cholesterol (mmol=l) NEFA (mmol=l) Glucose (mmol=l) Insulin (mU=ml)

1.04 6.08 1.16 0.55 5.29 9.5

(0.09) (0.30) (0.10) (0.09) (0.28) (1.5)

Short walks Long walk 1.09 5.95 1.18 0.48 5.01 11.0

(0.13) (0.37) (0.10) (0.13) (0.16) (1.8)

1.05 6.18 1.19 0.43 5.10 11.1

(0.12) (0.29) (0.12) (0.04) (0.18) (1.4)

Values are mean (s.e.) for 10 subjects (seven women). TAG, triacylglycerol; HDL, high density lipoprotein; NEFA, nonesteri®ed fatty acids. No signi®cant differences between trials.

between control and walking trials to be greater after lunch than after breakfast and greater after the evening meal than after lunch. There was also a mealtime effect (P 0.001), indicating that the change in plasma TAG concentration during the 3 hrs after a meal differed between breakfast, lunch and dinner; inspection of Figure 2 shows that plasma TAG increased after breakfast, changed little after lunch and decreased after the evening meal. There were no signi®cant differences between trials in the postprandial responses of plasma NEFA, glucose or insulin (Figure 3). Metabolic rate and substrate oxidation

At baseline, in the fasting state there were no differences between trials in the metabolic rate (control 6.2 0.3 kJ=min; short walks 6.0 0.2 kJ=min; long walk 5.8 0.1 kJ=min) or in the respiratory exchange ratio (control 0.84 0.02; short walks 0.88 0.03; long walks 0.85 0.02). Postprandial metabolic rate did not differ between trials (average values: control, 6.3 0.2 kJ=min; short walks 6.6 0.4 kJ=min; long walk 6.3 0.3 kJ=min). When data from the two walking trials were treated as a single condition, the respiratory exchange ratio was signi®cantly lower during the walking trials than in the control trial (average postprandial values: control 0.85 0.02; walking 0.83 0.02; P 0.005; Figure 4) and fat oxidation was higher (average postprandial values: control 0.066 0.009 g=min, walking 0.074 0.008 g=min, P < 0.01).

Discussion Our ®ndings show that moderate exercise reduces the postprandial lipaemia associated with the consumption of three ordinary meals, as previously demonstrated for single, high-fat meals.20,21 Just 30 min of

Figure 2 Plasma triacylglycerol (TAG) concentrations in the fasted and postprandial state for three trials. Mean and s.e. for seven women and three men. For details of protocol, see Figure 1. Main effect of trial, P 0.009; trialmeal effect, P 0.03; mealtime effect, P 0.001. International Journal of Obesity


Figure 3 Plasma concentrations of insulin (top panel), glucose (middle panel) and non-esteri®ed fatty acids (NEFA) (bottom panel) in the fasted and postprandial states. Mean and s.e. for seven women and three men. For details of protocol, see Figure 1. No signi®cant differences between trials.

Figure 4 Respiratory exchange ratio values (R) in the fasted and postprandial state. Mean and s.e. for seven women and three men. For details of protocol, see Figure 1. Average postprandial values were signi®cantly lower during walking trials (data from short and long walks trials treated as a single condition) than during the control trial (P 0.005).

brisk walking at 60% of VO2max decreased plasma TAG concentrations. Moreover, the magnitude of this decrease was similar whether this amount of walking was performed in one session or accumulated through

three 10 min sessions. These ®ndings strengthen the case for promoting walking as part of a preventive strategy against cardiovascular disease and add to the limited available evidence that health bene®ts achieved through multiple short exercise sessions may be similar to those achieved through fewer, longer sessions.22,23 Plasma TAG concentrations were highest in the early afternoon, some 2 h after lunch, as previously reported,24 and decreased following the evening meal, despite the consumption of a further 34 g of fat. These changes accord well with established knowledge of the co-ordinated regulatory responses to the consumption of mixed meals. Maximal TAG extraction is not seen until 4 ± 5 h after eating,25 because peak activity of lipoprotein lipase (LPL) in adipose tissue, the most important site of TAG removal, is not achieved until 3 ± 4 h of insulin stimulation. Moreover, insulin-stimulated processes become `primed' by previous insulin stimulation, so that the effect of subsequent meals is to reinforce the pattern of TAG storage in adipose tissue. Thus, when three meals are consumed during the day, adipose tissue LPL activity is highest in the late afternoon and early evening.26 The reduction in TAG concentrations with walking became apparent only during the late afternoon and early evening, more than 5 h after breakfast. This coincides with the period of maximal TAG extraction and suggests that the lower concentrations during the walking trials may be attributable mainly to an effect on TAG clearance. The rate of postprandial uptake of TAG into skeletal muscle has been reported to be only 30% of that into adipose tissue,27 but uptake into muscle could have been enhanced by walking, increasing whole-body clearance. Two mechanisms may be implicated. First, contraction-induced increases to muscle LPL activity and, second, increases in muscle blood ¯ow. Over a day of rest during which subjects consume three meals there is little change to skeletal muscle LPL activity,26 probably because insulin inhibits LPL activity in this tissue.28 However, exercise leads to an increase in muscle LPL activity which apparently counteracts its downregulation by insulin.29 This increase is not seen immediately after exercise but is delayed by 4 ± 8 h,29,30 a time-scale which would tie in with the fact that the difference in TAG concentrations between trials was evident during the later part of our observation period. Increased skeletal muscle blood ¯ow during exercise might also enhance TAG clearance by increasing the exposure of LPL to TAG-rich lipoproteins, but fatty acids from TAG-rich lipoproteins are not thought to be an important substrate for muscle metabolism during exercise.31 Muscle blood ¯ow remains high for a short time after exercise,32 however, and TAG uptake into muscle during these periods may have been higher than when subjects were resting. There was no evidence of lower TAG concentrations immediately after the long walk, however, when such an

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effect would likely be greatest, and no evidence for three discrete periods of lower TAG concentrations during the short walks trial. Limitations to this reasoning, however, are the hour-long interval between blood samples and the fact that plasma TAG concentrations depend on appearance rates of TAG-rich lipoproteins, as well as on clearance rates. It seems unlikely that the appearance rate of chylomicrons would be lower during the walking trials because blood ¯ow to the splanchnic organs is apparently not compromised during moderate exercise.33 Furthermore, an effect of this kind would have delayed peak TAG concentrations during the walking trials Ð something we did not observe. There could be decreased hepatic VLDL-TAG secretion during the walking trials but plasma concentrations of NEFA and insulin, both important in¯uences on this,34 did not differ between trials. The ®nding that the reduction in lipaemia with walking was strikingly similar with both patterns of walking is dif®cult to explain. Our design did not include a trial with a single 10 min walk before breakfast, so we cannot exclude the possibility that any pre-prandial exercise reduces subsequent lipaemia. Intuitively, though, it seems unlikely that just 10 min of walking would result in a discernable decrease in plasma TAG hours later. Our earlier studies, showing that exercise taken some 18 h before a fatty meal reduces subsequent postprandial lipaemia, suggest that the duration35 and=or the energy expenditure16 of exercise determine the magnitude of its effects in this regard. Compared with the control trial, an additional 5 g of fat was oxidized over the 11 h observation period, decreasing fat storage by 4 ± 5%. Fat balance is increasingly regarded as the important determinant of energy balance and so our ®ndings suggest that regular walking may have the potential to help weight regulation. In summary, 30 min of brisk walking, even when accumulated during the day through three short sessions, reduced postprandial plasma TAG concentrations. Our subjects were sedentary and mostly overweight, with blood lipid pro®les representative of people in Northern Ireland,36 a population with a high incidence of cardiovascular disease. The experimental meals they consumed re¯ected the habitual, rather high-fat, diet of that population.37 For all these reasons it seems our ®ndings are relevant for individuals at risk of cardiovascular disease. Acknowledgements

The authors wish to thank Dr A Mardhavan and Dr S Friers from Whiteabbey Hospital for assistance with venous cannulation, Dr M O'Hare and Ms B McKibben from the Institute of Clinical Science at the Queens University of Belfast for performing insulin analyses, Dr John Brown from the University of International Journal of Obesity

Ulster at Jordanstown for help with plasma lipid analyses and Mr T Ross for help with data collection. References

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