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1 From the Centre for Physical Activity and Nutrition, School of Exercise and Nutrition Sciences, Deakin University, Burwood Highway, Burwood, Australia
2 Supported by the Dairy Research and Development Corporation. 3 Address reprint requests to CA Nowson, School of Exercise and Nutrition Sciences, Deakin University, 221 Burwood Highway, Burwood, VIC 3125 Australia. E-mail: nowson{at}deakin.edu.au.
ABSTRACT
Background: Weight loss reduces blood pressure, and the Dietary Approaches to Stop Hypertension (DASH) diet has also been shown to lower blood pressure.
Objective: Our goal was to assess the effect on blood pressure of 2 weight-reduction diets: a low-fat diet (LF diet) and a moderate-sodium, high-potassium, high-calcium, low-fat DASH diet (WELL diet).
Design: After baseline measurements, 63 men were randomly assigned to either the WELL or the LF diet for 12 wk, and both diet groups undertook 0.5 h of moderate physical activity on most days of the week.
Results: Fifty-four men completed the study. Their mean (±SD) age was 47.9 ± 9.3 y (WELL diet, n = 27; LF diet, n = 27), and their mean baseline home systolic and diastolic blood pressures were 129.4 ± 11.3 and 80.6 ± 8.6 mm Hg, respectively. Body weight decreased by 4.9 ± 0.6 kg (±SEM) in the WELL group and by 4.6 ± 0.6 kg in the LF group (P < 0.001 for both). There was a greater decrease in blood pressure in the WELL group than in the LF group [between-group difference (week 12 –baseline) in both SBP (5.5 ± 1.9 mm Hg; P = 0.006) and DBP (4.4 ± 1.2 mm Hg; P = 0.001)].
Conclusions: For a comparable 5-kg weight loss, a diet high in low-fat dairy products, vegetables, and fruit (the WELL diet) resulted in a greater decrease in blood pressure than did the LF diet. This dietary approach to achieving weight reduction may confer an additional benefit in reducing blood pressure in those who are overweight.
Key Words: Weight loss • blood pressure • diet • potassium • calcium
INTRODUCTION
Hypertension is an important public health issue and contributes to the incidence of stroke and coronary heart disease (1). The prevalence of hypertension in Australia was recently shown to be 29% (2). Furthermore, hypertension accounts for 6.1% of the total problems managed in general practice (3). Education pertaining to nutrition and weight accounted for 10% of all nonpharmacologic treatments provided by general practitioners and was one of the 3 most common forms of advice (3). Around the world, the incidence of overweight and obesity has increased (4). The prevalence of obesity in Australia has more than doubled in the past 20 y, and almost 60% of adults have been estimated to be overweight or obese (5). There is a direct positive relation between overweight and hypertension, such that it has been estimated that the control of obesity may eliminate 48% of the hypertension in whites (6). Dietary sodium increases blood pressure (BP), whereas dietary potassium lowers the risk of hypertension and stroke. In a controlled intervention study, a multifaceted dietary approach (DASH: Dietary Approaches to Stop Hypertension) that included a diet high in fruit, vegetables, and low-fat dairy products was shown to result in large decreases in BP (11 mm Hg systolic and 5 mm Hg diastolic pressure in hypertensive persons and 5 mm Hg systolic and 3 mm Hg diastolic in normotensive persons) (7). Therefore, the aim of the present study was to determine the effect on BP of a DASH-type weight-loss diet (WELL diet) and to compare this with usual low-fat dietary advice (LF diet) in free-living individuals who selected and prepared their own food.
SUBJECTS AND METHODS
Subjects
Subjects were recruited through newspaper articles advertising the study and at BP measurement sessions provided in workplaces and at the study center. Subjects were eligible if they were male, aged > 25 y, and had a seated office BP of 120 mm Hg systolic blood pressure (SBP) or 80 mm Hg diastolic blood pressure (DBP) at their first visit (mean of the last 3 of 4 measurements taken at 1-min intervals). Subjects who were taking antihypertensive medication were included, provided they were willing to maintain their current medication level. Subjects were excluded if they had experienced a cardiovascular event in the past 6 mo, had insulin-dependent diabetes, were taking medications such as warfarin or phenytoin, ate their main meal outside the home more than twice per week, drank >30 standard (10 g alcohol) alcoholic drinks per week, were planning to change smoking habits, or were unwilling to cease taking dietary supplements (including vitamins). Subjects were included if they had a body mass index (BMI; in kg/m2) between 25 and 35. All subjects provided written informed consent before starting the study, which was approved by the Deakin University Human Research Ethics Committee.
Two hundred twenty persons responded to advertisements, and 165 of these were sent a screening questionnaire and invited to attend further screening. Ninety-four men attended one screening appointment, and 63 who met the entry criteria and wished to participate undertook baseline home BP measurements for 2 wk and were then randomly assigned to either the LF or the WELL diet.
Study design
Subjects were seen twice at baseline, and commenced a 12-wk intervention study and were seen at weeks 2, 4, 8, and 12. Phone contact was made with the subjects at weeks 6 and 10. Clinical BP, height, and weight were measured at baseline. Subjects monitored their home BP daily for 2 wk before being randomly assigned (stratified by antihypertensive medication use) to 1 of the 2 diets. Randomization was performed by the chief investigator (CAN) with the use of a random number generator in blocks of 8 (EXCEL 2000; Microsoft Corporation, Redmond, WA).
Anthropometry and blood pressure measurement
Height was measured with a wall-mounted stadiometer. Body weight was measured at each visit on a digital scale while the subjects wore light clothing and no shoes. Waist circumference was measured with a fiberglass tape measure anteriorly halfway between the lowest lateral portion of the ribcage and the iliac crest. Home BP was measured on the left arm with the use of an automated BP monitor (AND UA-767-PC; A&D Co Ltd, Tokyo, Japan). Subjects were trained to correctly apply the cuff and were instructed to take their BP measurements alone, at the same time of day, after 5 min of rest in a quiet room and to take 3 measurements with 1 min in between (the mean of the last 2 measurements taken each day was used in the analysis). BP measurement data were downloaded directly to a computer by the study staff at the end of each fortnight.
Biochemical indexes
Fasting (10 h overnight) blood samples were collected at baseline and at the end of the study. Serum total cholesterol, HDL cholesterol, and triacylglycerol were measured on the Hitachi 704 analyzer by using enzymatic reagents (Boehringer, Mannheim, Germany). LDL cholesterol was calculated by use of the Friedewald equation (8).
Dietary assessment
Subjects completed a 24-h dietary record each fortnight on the day before their visit with study staff. Trained research personnel checked this record. Dietary information was entered into a dietary analysis program (FOODWORKS, Professional Edition, version 3.02; Xyris Software, New York, NY) to calculate daily nutrient intakes. The mean of two 24-h records at baseline and the mean of four 24-h records at weeks 2, 4, 8, and 12 of the intervention were used in the analysis. A food-frequency questionnaire was completed at baseline and at week 12 to assess usual intake of fruit, vegetables, and dairy products.
Lifestyle intervention
Subjects were assisted with setting goals for exercise and diet. At each visit, a trained dietitian set dietary and exercise goals (3 goals per visit, including 1 for exercise and 1 for diet). Recipes, educational materials (diet and exercise), and tips to encourage compliance were provided to all subjects.
Dietary instruction
Dietary counseling was overseen by the coordinating dietitian (CM) and was provided by trained research staff. The WELL diet was based on our previous OZDASH diet (9), which had been modified from the US DASH diet (7). This diet included advice to consume 4 servings of fruit or fruit juice [1 serving = 1 medium piece of fruit (100 g) or fruit juice (200 mL)], 4 servings of vegetables [1 serving = 0.5 cup cooked vegetables (50 g), 1 cup salad vegetables, or 1 medium potato] and 3 servings of nonfat dairy products [1 serving = milk (200 mL), yogurt (200 g), or cottage or ricotta cheese (0.5 cup)] per day. Fish (1 serving = 120 g cooked) was to be consumed 3 times per week, legumes (1 serving = 1 cup cooked) at least once per week, and unsalted nuts and seeds (1 serving = 30 g) 4 times per week. Red meat was restricted to no more than 2 servings (1 serving = 90–100 g cooked) per week and fat to a maximum of 4 servings (4 teaspoons) per day. Subjects were advised to avoid butter, added salt (table or cooking), and obviously salty foods and to use lower-salt (<380 mg Na per 100 g) mono- or polyunsaturated margarine. Those in the WELL group received a detailed dietary information booklet, recipes, and simple advice (tips).
The LF group was advised to limit their intake of high-energy foods and drinks, reduce their saturated fat intake, choose mainly plant-based foods, consume nonfat or reduced-fat milk and yogurt, limit their cheese and ice cream intake to twice per week, select lean meat, and avoid frying foods in fat. No specific targets were set. The "Healthy Weight Guide" booklet by the National Heart Foundation of Australia (2002) was provided, together with the same recipes and tips as received by the WELL group. A maximum of 4 caffeine-containing drinks per day (eg, cola drinks, coffee, and tea) and 4 standard (10-g alcohol) alcoholic drinks per week were permitted for both diet groups.
Several incentives were included. Both groups received measuring cups and spoons and individual feedback on their daily intake of fruit and vegetables. Those in the WELL group also received individual feedback compared with the specified diet targets (fruit, vegetables, and dairy). A dairy product of their choice [eg, 200-mL tub of nonfat yogurt or a packet of reduced-fat (<15% fat) cheese slices] was offered once during the study to all subjects. Subjects could also participate in a drawing to win a double movie pass, and feedback on the group's progression regarding targets and weight loss was provided graphically throughout the study.
The main difference between the LF diet and the WELL diet was that the WELL diet had specified targets for fruit, vegetable, and dairy intake, whereas the LF diet provided general guidelines focusing on increasing fruit and vegetable intake and reducing fat intake, particularly saturated fat.
Physical activity
All subjects were required to participate in moderate-intensity exercise for 30 min on all or most days of the week. The "Be Active Every Day" booklet by the National Heart Foundation (1999) was given to each participant, and individual exercise goals were set at each visit. Information was provided on calculating maximum heart rate [220 –age (y)], and subjects were advised to increase their heart rate to 60–79% of their maximum heart rate to reach a moderate level of exercise intensity and to maintain this for 30 min for each session. The amount of walking was monitored by using the CHAMPS questionnaire (10) at baseline (for 4 wk) and the average hours per week calculated for the intervention period.
Statistical analysis
Data were analyzed by using SPSS for WINDOWS (version 11.0; SPSS Inc, Chicago, IL) to calculate the descriptive statistics and perform the regression analysis. Mean home BP readings were calculated for each 2-wk period. Unpaired Student's t tests were used to evaluate the difference between the LF and WELL diets in the changes between baseline and the last visit. P values of 0.05 were considered to be significant. Additionally, the effect of the diet intervention was assessed by using two-factor repeated-measures analysis of variance (diet x time) with covariates of baseline weight and BP included in specific analysis where indicated. Baseline values were used as covariates in GLM univariate analysis of variance to calculate the adjusted mean changes to test for the difference between groups for the dietary data only.
RESULTS
Nine subjects dropped out before completing the study (4 in the LF group and 5 in the WELL group); the subjects who dropped out did not differ significantly from the rest of the group with respect to age or BMI. Eight found it too difficult to comply with the study demands, and one moved interstate.
Of the 54 men who completed the study, 18 were taking antihypertensive medications (9 WELL, 9 LF). Subjects in the WELL group who were receiving antihypertensive therapy included 4 receiving single therapy [1 taking an angiotensin-converting enzyme (ACE) inhibitor, 1 taking a calcium-channel blocker, and 2 taking angiotensin II receptor antagonists (AT1)] and 5 receiving combination therapy (AT1 + ß-blocker, n = 1; AT1 + diuretic, n = 2; AT1 + calcium-channel blocker, n = 1; ACE inhibitor + diuretic, n = 1). Subjects in the LF group who were receiving antihypertensive therapy included 5 receiving single therapy (2 taking an ACE inhibitor, 1 taking an AT1, and 2 taking calcium-channel blockers) and 4 receiving combination therapy (ACE inhibitor + diuretic, n = 1; AT1 + diuretic, n = 1; ACE inhibitor + diuretic, n = 1; AT1 + diuretic + ß-blocker, n = 1).
At baseline, there were no significant differences in dietary intakes of fruit, vegetables, or calcium-containing dairy products between groups; however, those randomly assigned to the LF group were heavier, were taller, and had a greater waist measurement and had a BMI of 31 compared with 30 for the WELL group (Table 1 and Table 2).
View this table:
TABLE 1. . Baseline characteristics of the study groups1
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TABLE 2. . Intervention outcomes at baseline and 12 wk by randomized group assignment1
Effects of the intervention
The amount of time spent walking increased in both groups over the intervention period, with no significant difference between the groups: WELL group increased to 4.4 ± 0.7 (
View this table:
TABLE 3. Intervention nutrient outcomes at baseline and 12 wk by randomized group assignment1
After adjustment for baseline dietary intake, the 24-h dietary records indicated that the reductions in dietary fat (g/d), saturated fat (g/d), percent of energy from fat, percent of energy from saturated fat, and sodium (mg/d) were greater in the WELL group than in the LF group, and the increases in the percent of energy from protein, percent of energy from carbohydrate, potassium (mg/d), calcium (mg/d), magnesium (mg/d), and phosphorus (mg/d) were greater in the WELL group than in the LF group (Table 3).
Weight and blood pressure changes
Weight decreased significantly in both groups by 5.0 kg (P < 0.001 for both), with subjects in the WELL group losing 6% of body weight (P < 0.001) and those in the LF group losing 5% (P < 0.001; Table 2). The rate of weight loss was not significantly different between the diet groups throughout the study. In the first 2 wk, weight loss was 1.2 ± 0.2 kg in the WELL group and 1.5 ± 0.2 kg in the LF group, and the effect did not differ significantly between diet groups (two-factor ANOVA: time x diet effect, NS).
The greatest decrease in BP in both groups was seen after 4 wk of intervention (Figure 1). There was a greater decrease in the WELL group than in the LF group [between-group difference (week 12 –baseline) in both SBP (5.5 ± 1.9 mm Hg; P = 0.006) and DBP (4.4 ± 1.2 mm Hg; P = 0.001)]. Pulse rate also fell by 3.8 ± 1.6 beats/min more in the WELL group (P = 0.023; Table 2).
FIGURE 1.. Mean (±SEM) systolic (SBP) and diastolic (DBP) blood pressure over 12 wk of intervention in the LF () and WELL () diet groups. WELL, DASH-type weight-loss diet (moderate sodium, high potassium, high calcium, low fat, with less red meat and more fish); LF, low-fat diet. Two-factor repeated-measures ANOVA: time x diet effect, P = 0.006 for SBP, P = 0.001 for DBP; n = 54 (unadjusted).
The percentage decrease in SBP was 5.5 ± 1.0% in the WELL group compared with 1.4 ± 0.9% in the LF group. The percentage decrease in DBP was 6.4 ± 1.1% in the WELL group compared with 1.0 ± 1.0% in the LF group. The significance of the difference in the percentage change between groups was P = 0.005 (SBP) and P = 0.001 (DBP).
After adjustment for baseline BP and body weight, the difference in the decrease in SBP and DBP between groups remained [SBP: 5.2 ± 1.8 mm Hg (P = 0.006); DBP: 4.8 ± 1.3 mm Hg (P = 0.001)]. Overall, there was a significant effect of diet on BP (SBP, P = 0.006; DBP, P = 0.001; n = 54; repeated measures, two-factor ANOVA: time x diet effect). Adding baseline body weight as a covariate to the model did not significantly affect the results (SBP, P = 0.001; DBP, P = 0.001; n = 54; repeated-measures, two-factor ANOVA: time x diet effect). Additionally, a model that included baseline BP and body weight as covariates did not significantly affect the results (SBP, P = 0.003; DBP, P = 0.001; n = 54; repeated-measures, two-factor ANOVA: time x diet effect). The pulse rate was lower in the WELL group than in the LF group (P = 0.031; n = 54; repeated-measures, two-factor ANOVA: time x diet effect).
Serum total cholesterol, HDL cholesterol, and LDL cholesterol decreased significantly in both groups by the end of the study; however, there was no significant change in triacylglycerol in the LF group and a tendency for a decrease in the WELL group of 0.3 ± 0.1 mmol/L (P = 0.051; Table 2).
Regression analysis indicated that initial weight was not related to changes in SBP and DBP. However, the percentage weight loss was related to the percentage change in SBP and DBP [SBP: R2 = 0.16, ß (±SE) = 0.65 ± 0.20, P = 0.003; DBP: R2 = 0.16, ß = 0.73 ± 0.23, P = 0.003]; a 10% change in weight was associated with a 7% decrease in both SBP and DBP. Univariate linear regression analysis indicated that the increase in total dairy product intake was associated with the decrease in DBP (R2 = 0.118, ß = 0.959 ± 0.364, P = 0.011) and the increase in vegetable intake was associated with the decrease in DBP (R2 = 0.071, ß = 0.968 ± 0.487, P = 0.052).
DISCUSSION
The present study investigated the effects on home BP of 2 dietary interventions—one based on the DASH dietary pattern and the other a usual low-fat diet—combined with increased physical activity to achieve weight loss. We found that the subjects in both diet groups achieved a weight loss of 5–6% of body weight over 3 mo. Those in the WELL group, however, had greater decreases in SBP and DBP of 5 and 4 mm Hg, respectively. The groups were well matched at baseline for BP and for the number of subjects taking antihypertensive medication (33% in each group), although BMI was initially one unit higher in the LF group than in the WELL group. This difference, however, is unlikely to have contributed to the increased effectiveness of the WELL diet with respect to BP, because there was no significant difference in percentage weight loss between the groups.
The reason for the greater decrease in BP with the WELL diet is not clear. We found no significant difference between the 2 groups in the change in blood lipids, although those in the WELL diet group did appear to have a greater reduction in total fat and particularly saturated fat intake. Our power to detect a difference in blood lipids, however, was low because of the limited number of subjects.
Some of the dietary differences between the WELL and the LF diet may explain some of the improved BP-lowering effect of the WELL diet, specifically, the increase in dietary potassium, which has been shown to lower BP by 3 mm Hg systolic and 2 mm Hg diastolic (11). Dietary calcium and magnesium have also been weakly associated with lower BP in population studies (12, 13), although evidence for a BP-lowering effect in controlled intervention studies is not consistent (14). It appears, however, that a diet combining these nutrient changes—eg, lower sodium and saturated fat and higher potassium, calcium, magnesium, and phosphorus—within a diet and physical activity pattern that induces negative energy balance achieves a greater reduction in BP than does a low-fat diet.
The food-frequency questionnaire indicated a difference between groups at the end of the study in dairy intake only and not in fruit and vegetable intakes. This likely reflects the insensitivity of a food-frequency questionnaire in picking up relatively small changes in food intake. It may also indicate that the increase in dairy products, when combined with more vegetables, is a significant factor with respect to BP reduction, particularly because the linear regression analysis indicated that the change in total dairy intake together with the change in vegetable intake was univariately associated with the reduction in DBP.
Although dietary records are not a good measure of actual sodium intake, after adjustment for baseline sodium intake, there was a significant reduction in dietary sodium in the WELL group only. It is therefore likely that those following the WELL diet did have a lower intake of sodium. Subjects were advised to avoid added salt and obviously salty foods, and we found in a previous study (9) that this type of dietary advice can result in a decrease in 24-h urinary sodium of 30 mmol in a weight-stable situation. In the present study, in which there was a reduction in energy intake, the reduction in sodium is likely to have been greater, but without having data on 24-h urine collections we cannot confirm this finding.
The decreases in BP in the present study were somewhat greater than in other studies, particularly the 5–mm Hg decrease in DBP. A meta analysis of weight loss and BP indicated an average decrease in SBP and BDP of 4 mm Hg in studies with energy restriction with or without exercise (15), whereas we found decreases of 8 and 5 mm Hg in SBP and DBP, respectively. Our results contrast with those of the recent large, multicenter Premier study (16). In that study, untreated subjects with mild hypertension who were randomly assigned to the DASH intervention achieved decreases of 11 mm Hg in SBP and 7 mm Hg in DBP with a 5-kg weight loss over 6 mo, and these decreases were not significantly different from those seen in the established care group, who had a similar weight loss.
Ours is the first study to assess the effect of a weight-loss intervention on home BP measurements rather than on investigator-measured BP. Some of the difference in BP response could be attributed to the different method of BP assessment. Home BP, measured at the same time of day, under the same conditions, shows reduced variability. Home BP measurement is now emerging as a preferred method of measuring BP (17), because it has been shown to share some of the advantages of ambulatory BP, that is, to have no "white coat" effect (18), to be more reproducible (19, 20), and to be more predictive of the presence and progression of organ damage than are office or clinic values (21). Because all our subjects used BP monitors for which their data were downloaded directly to a computer by the study staff, there was no possibility for errors in subject recording.
The results of the present study clearly show that targeted dietary advice (ie, to include 4 servings each of fruit and vegetables per day, 3 servings of nonfat dairy products per day, and 3 servings of fish and 1 serving of legumes per week and to avoid butter and added salt) combined with advice to walk 0.5 h on most days of the week resulted in a 5% weight loss, an 8–mm Hg decrease in SBP, and a 5–mm Hg decrease in DBP over 3 mo. In addition, the study showed that a lifestyle intervention that can be successfully implemented by obese or overweight free-living individuals results in a greater decrease in BP than does the usual, general dietary advice to reduce fat intake. The reason for the increased efficacy of this diet over the low-fat diet with respect to BP is not clear, but may be related to the increases in potassium and calcium intakes.
ACKNOWLEDGMENTS
CAN and AW were responsible for the conception and overall supervision of the study. CAN was responsible for the drafting of the manuscript, critical revision of the manuscript for intellectual content, and final approval of the manuscript. CM, MKJ, SJG, and AB interviewed all participants, administered dietary counseling, and with AW contributed to drafting of the manuscript. SJG was responsible for biochemical processing and analysis. None of the authors had any conflicts of interest.
REFERENCES