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2
*
Department of Food Science and Human Nutrition and
Department of Health and Exercise Science, Colorado State University, Fort Collins, CO 80523
2To whom correspondence and reprint requests should be addressed. E-mail: davy{at}cahs.colostate.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • oat fiber • wheat fiber • arterial blood pressure • humans
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). In 1998, high blood pressure (BP)3
caused the deaths of 44,435 Americans (1
). Importantly, dietary and other lifestyle changes are recommended as the initial approach for the treatment of high-normal blood pressure (130–139/85–89) and Stage I hypertension (140–159/90–99) for individuals without target organ damage or clinical cardiovascular disease (2
).
Epidemiologic studies indicate that total dietary fiber intake is inversely related to arterial BP (3
–6
). Other studies (7
) have suggested that oat fiber intake in particular is inversely related to arterial blood pressure. However, previous clinical trials that have addressed the possibility that increased high fiber oat cereal consumption lowers BP have been inconsistent. To our knowledge, there are only three published studies (8
–10
) that address this issue. All of these studies were conducted in normotensive individuals. Of these, only one study (10
) reported a reduction in arterial BP in individuals consuming a hypocaloric diet high in oat fiber. These reductions in SBP were greater than observed in a control group that exhibited a similar weight loss. Therefore, it is unclear whether fiber-rich oat cereal, independent of weight loss, lowers BP in individuals with elevated arterial blood pressure. To test this hypothesis, we measured resting seated and supine BP and 24-h ambulatory BP in 36 middle-aged and older men before and after randomization to 12 wk of increased oat (n = 18) or wheat fiber (n = 18) consumption.
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SUBJECTS AND METHODS |
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TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Men aged 50–75 y with a body mass index (BMI) between 25 and 35 kg/m2 and elevated BP (systolic BP 130–159 mmHg and/or diastolic BP 85–99 mmHg) were recruited for this study from the surrounding community. Only subjects who were sedentary or minimally physically active (<two 30-min aerobic exercise sessions per wk) were included. Individuals were excluded if they reported or were observed to have any of the following: overt cardiovascular, metabolic, or pulmonary disease or use of any medications known to affect any of the dependent variables in this study. Subjects were excluded if they reported a high habitual intake of dietary fiber (e.g., >30g/d), as determined by the Block Screening Questionnaire for Fat and Fruit/Vegetable/Fiber Intake (11
). This study was approved by the Human Research Committee at Colorado State University. All subjects provided written informed consent before participation.
Experimental design.
After screening, subjects were randomly assigned to consume whole grain oat or wheat cereals containing 14 g/d of dietary fiber for 12 wk (Oat group, 60 g Quaker Oatmeal and 76 g Quaker Oat Bran ready-to-eat cold cereal, Quaker Oats Company, Barrington, IL; wheat group, 60 g Mother’s Whole Wheat Hot Natural Cereal, Quaker Oats Company and 81 g Frosted Mini-Wheats, Kellogg Company, Battle Creek, MI). The combination of oat cereals provided 5.5 g/d ß-glucan as determined by manufacturer’s analysis. The amount of cereal provided was matched between groups to achieve a similar amount of energy, macronutrients and dietary fiber according to the USDA Nutrient Database for Standard Reference, Release 12 (Table 1
). Subjects were instructed to consume the cereals daily at breakfast and as a snack while substituting and adjusting their habitual intake to the increased cereal consumption to maintain weight stability. Compliance was assessed by measuring the uneaten portion of cereal that was returned to the investigators on a biweekly (e.g., every other week) basis. The subjects were unaware of the amount of cereal provided in excess.
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Body weight was determined to the nearest 0.1 kg using a balance scale (Detecto, Webb City, MO) and height (cm) (without shoes) was measured using a wall-mounted stadiometer. Body weight was reassessed biweekly throughout the study. Skinfold thickness measurements (mm) were obtained at eight sites (chest, abdomen, triceps, suprailiac, midaxillary, subscapular, thigh and medial calf) using calipers (Lange, Cambridge Scientific Industries, Cambridge, MD). Waist circumference was measured (cm) at the level of the umbilicus using a Gulick II measuring tape (Country Technology, Gays Mills, WI). Anthropometric measurements were performed again during the last week of the intervention.
Dietary assessment.
Each subject kept a 4-d food intake record at baseline and during the final week of the study to assess dietary intake. Subjects were instructed by a research dietitian (B.D.) to record food intake accurately (e.g., portion sizes, food preparation methods or brand names of products) using two-dimensional food models. The dietitian reviewed all records with subjects upon completion for accuracy and sufficiency of detail. Subjects were asked to provide food labels for products used to determine appropriate substitutions when actual items consumed were not in the software database. Food intake records were analyzed using the Food Intake Analysis System (FIAS 3.98 nutrient analysis program, University of Texas School of Public Health, 1998). The FIAS database consists of the Primary Data Set (PDS) and the Survey Nutrient Data Base (Survey NDB) of the National Nutrient Data Bank, developed and maintained by the USDA.
Urinary measurements.
The 24-h urinary sodium, potassium and creatinine excretion was measured before and after the 12-wk intervention using a Vitros Model 250 analyzer (Ortho-Clinical Diagnostics, Johnson & Johnson, Rochester, NY). Assays for measurement of urinary sodium, potassium and creatinine were performed according to manufacturer’s instructions (Vitros Chemistry Products, Ortho-Clinical Diagnostics).
Blood pressure measurements.
Resting supine and seated BP measurements were obtained before and after the intervention using both a Hawksley Random Zero (RZ) sphygmomanometer (Hawksley Technology, London, UK) and an automated Dinamap XL vital signs monitor (model 9300, Johnson & Johnson Medical, Tampa, FL) to obtain independent and objective measurements. Before the intervention, BP measurements were obtained a minimum of three times over a 2-wk run-in period to ensure a stable baseline measurement. A stable BP was defined as three consecutive BP measurements on three separate occasions within 5 mmHg. On each occasion that BP was measured, care was taken to adhere to the identical protocol as follows: the subject was taken into a quiet room with the temperature kept at 23°C, where he rested quietly for exactly 5 min in a seated posture before the first BP measurement. Three measurements were obtained using the appropriate cuff size such that the bladder width was at least 40% of the subject’s arm circumference and bladder length at least 80% of the arm circumference. The rate of cuff deflation was 2–3 mmHg/s. The first and fifth Korotkoff sounds were used to determine systolic (SBP) and diastolic blood pressure (DBP), respectively. Each of the BP measurements was separated by a 2-min interval. Upon completion of the seated measurements, subjects were then placed in a supine position on a comfortable bed for 5 min, and supine blood pressure measurements were taken following the same protocol.
To eliminate interobserver bias from the measurements, BP measurements for each individual were taken by the same observer throughout the study, and the observer was unaware of subject group assignment. Subjects were unaware of all readings and were not informed of their BP values until completion of the study. The mean of the three measurements was used for statistical comparison. Seated BP measurements were obtained using the RZ at wk 4 and 8 of the interventions, and again at during the last week of the intervention (wk 12). Bimonthly Dinamap measurements were also obtained. All BP measurements were made early in the morning after a 12-h fast.
Ambulatory 24-h BP measurements were obtained using a noninvasive ambulatory BP monitor (model 90207, Spacelabs, Redlands, WA) at baseline and after the 12-wk intervention period. The cuff was programmed to inflate automatically every 15 min from 0600–2200 h and every 30 min from 2200–0600 h. For each subject, daytime was defined as the time from waking in the morning until going to bed in the evening. Nighttime was the remainder of the 24-h period. SBP and DBP variability were defined as the standard deviation of each individual’s 24-h SBP and DBP recordings. Nocturnal dip (%) was defined as the product of daytime minus nighttime BP, divided by the daytime BP, multiplied by 100. SBP and DBP load was calculated as the percentage of BP >140 mmHg and >90 mmHg, respectively, for 24-h, daytime and nighttime intervals.
Intravenous glucose tolerance test.
It has been hypothesized that if dietary fiber does reduce arterial BP, it may do so via an effect on blood insulin concentrations and/or insulin sensitivity, because insulin may increase sympathetic nervous system activity (12
). For this reason, an insulin-augmented frequently sampled intravenous glucose tolerance test was administered to these subjects at baseline and after the intervention. The test was performed in the supine position following an overnight, 12-h fast, after a 30-min relaxation period. An intravenous catheter was placed in each antecubital vein, one for the administration of insulin and glucose, and one for collecting blood samples. Two blood samples were obtained at time (t) = -10 and -5 min, with these two samples used for determination of the mean baseline insulin and glucose concentrations. A bolus of glucose (0.3 g/kg in a 50 g/L dextrose solution) was infused over a 90-s period at t = 0. At t = 20 min, a bolus of insulin (0.03 U/kg) was injected. Blood samples were obtained at t = 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 18, 22, 25, 30, 40, 50, 60, 70, 80, 90, 100, 120, 140, 160 and 180 min and immediately centrifuged at 4°C and analyzed for glucose concentrations by the glucose oxidase method using a glucose autoanalyzer (Yellow Springs Instruments, Yellow Springs, OH). A sample of plasma was stored at -20°C for later determination of insulin concentrations by the ELISA method (Diagnostic Systems Laboratories, Webster, TX). The MINMOD program (version 3.0, R. Bergman, University of Southern California) was used for determination of insulin sensitivity (SI). This model uses measurements of plasma glucose and insulin concentrations over the 3-h period to deduce in vivo whole-body insulin sensitivity (13
).
Statistical Analysis
Group, time and group by time interaction effects were assessed using repeated-measures ANOVA (General Linear Model, SPSS statistical software, SPSS 9.0 for Windows, Chicago, IL). Group characteristics at baseline were compared using independent t tests. Alpha was set a priori at P < 0.05. All data are expressed as means ± SEM.
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RESULTS |
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TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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. These characteristics at baseline did not differ between the oat and wheat cereal groups. Compliance with the intervention did not differ between groups (oats = 96 ± 4% vs. wheat = 95 ± 5%). One individual reported <88% compliance; exclusion of this individual from data analysis did not alter our findings. There were small but significant increases in body weight (
0.8 kg), BMI and the sum of skinfold thickness in both the oat and wheat groups over time (all P < 0.05).
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). No change in dietary intakes of sodium, potassium or calcium was observed. A significant main effect of time and a group by time interaction were found for magnesium intake, with a slightly greater increase noted in the oat group. However, 24-h urinary sodium, potassium or creatinine excretion did not change in either group as a result of the intervention (data not shown).
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). Bimonthly SBP and DBP determined with the Dinamap automated sphygmomanometer also did not change (data not shown) in either group as a result of the intervention.
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). There was a small, but significant, increase in nighttime DBP and MAP in both groups over time. However, no significant changes were noted in daytime or nighttime SBP, daytime DBP, 24-h or daytime mean arterial pressure (MAP), SBP or DBP variability, SBP or DBP nocturnal dip, or SBP and DBP load (data not shown).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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–7
). However, randomized controlled trials are required to confirm a causal relationship. Although the beneficial effect of increased oat consumption on blood lipids and lipoprotein concentrations has been well documented (8
,14
–16
), the effectiveness of increased oat consumption on arterial BP has yet to be confirmed.
The new finding of the present study is that increased high fiber oat or wheat cereal consumption does not lower casual or 24-h ambulatory BP in middle-aged and older men with high-normal BP or stage I hypertension. Conclusions and applications of previous intervention studies in this area are limited due to use of a normotensive population and intervention periods of only 4–6 wk (8
,9
). Our 12-wk intervention study used individuals with high-normal BP and Stage I hypertension, in whom dietary and lifestyle modifications are recommended as the initial treatment approach (2
). Additionally, other strengths of this study are an extended run-in baseline measurement period, meticulous measurements of seated and supine BP using two different instruments, BP measurements obtained at 2-wk intervals, the lack of awareness of both observers and participants regarding group assignment, and the use of the same observer for an individual participant on each occasion during the intervention to minimize interobserver bias.
Saltzman et al. (10
) recently reported a reduction in SBP in normotensive individuals following a 6-wk hypocaloric diet rich in oat fiber (7.2 g soluble fiber). These reductions in SBP were greater than observed in the hypocaloric lower fiber control group, which exhibited a similar weight loss. The intake of oat fiber in these study participants was 30% higher than the amount provided in our study. Possibly a higher level of soluble fiber from oats is necessary to lower BP. Alternatively, weight loss may be required to elicit a reduction in BP with oat fiber consumption (i.e., an interaction effect). Importantly, differences in baseline BP levels between the Saltzman study and ours, i.e., normotensives vs. individuals with high-normal BP to Stage 1 hypertension, is unlikely to explain the discrepancy because greater BP reductions would be expected in individuals with higher baseline BP.
We have consistently found in previous studies that Caucasian, African-American and Hispanic adults who ingest a plant-based diet rich in dietary fiber exhibit less hypertension than their nonvegetarian counterparts (17
–19
). We have suggested that this phenomenon of less hypertension in vegetarians is likely explained by multiple nutrients acting in concert, rather than a single factor such as dietary fiber. Results from the Dietary Approaches to Stop Hypertension (DASH) trial (20
) also suggest that a combination of dietary factors has a favorable effect on blood pressure. Specifically, a high fiber, low fat diet rich in fruits, vegetables, and low fat dairy products elicited a mean 5.5-mmHg reduction in resting casual SBP and 3.0-mmHg reduction in DBP compared with the control group. Body weight was maintained, and consumption of sodium and alcohol was similar in the groups. Similar reductions were noted in 24-h ambulatory BP. Both hypertensive and normotensive subjects experienced reductions in BP, with greater reductions evident among hypertensives. However, in contrast to the present study, the DASH trial was not designed to test the BP-lowering effect of particular foods, but rather to test the effectiveness of a specific dietary pattern in reducing BP.
There are some potential limitations of the present study that should be addressed. First, we observed a small but significant increase in average body weight (0.8 kg) in both groups over time. We do not believe this magnitude of average weight gain confounded the interpretation of our findings because weight gain was the same in the oat and wheat groups. However, it is possible that small reductions in BP in both groups might have been masked by weight gain with the respective interventions.
Second, we cannot exclude the possibility that our inability to detect the small reduction in arterial BP observed in both high fiber cereal groups was the result of a type II error. We estimated that 160 subjects per group would be necessary to detect a 2-mmHg reduction in arterial BP. Our samples size would have provided adequate power to detect a 6- to 7-mmHg reduction in arterial BP (i.e., that observed in the Saltzman study). Nonetheless, much larger clinical trials may be necessary to determine smaller, but clinically important reductions in arterial BP with fiber-rich cereal consumption.
Third, we cannot exclude the possibility that the compliance with our intervention was less than that determined by self-report. However, subjects were given cereal in excess of the amount needed and instructed to return uneaten portions to the study staff. The subjects were unaware of the amount of excess cereal provided. Therefore, we believe that under the conditions of the present study, the return of uneaten cereal is a reasonable estimate of compliance with our intervention.
In conclusion, our results indicate that 12 wk of increased fiber-rich oat or wheat cereal consumption has no effect on casual or 24-h ambulatory arterial BP in middle-aged and older men with mild-to-moderate hypertension. As suggested by findings from Saltzman et al. (10
) and Appel et al. (20
), either weight loss or a combination of dietary factors, rather than a single food, may be required to lower BP.
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FOOTNOTES |
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3 Abbreviations used: BMI, body mass index; BP, blood pressure; SBP, systolic blood pressure; DBP, diastolic blood pressure; MAP, mean arterial pressure; RZ, random zero; SI, insulin sensitivity.
Manuscript received 9 August 2001. Initial review completed 19 September 2001. Revision accepted 4 December 2001.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Saltzman, E., Das, S. K., Lichtenstein, A H., Dallal, G. .E, Corrales, A., Schaefer, E. J., Greenberg, A. S. & Roberts, S. B. (2001) An oat-containing hypocaloric diet reduces systolic BP and improves lipid profile beyond effects of weight loss in men and women. J Nutr 131:1465-1470.
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Scherrer, U. & Sartori, C. (1997) Insulin as a vascular and sympathoexcitatory hormone: implications for BP regulation, insulin sensitivity and cardiovascular morbidity. Circulation 96:4104-4113.
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14.
Anderson, J. W., Story, L., Sieling, B., Chen, W.J.C., Petro, M. S. & Story, J. (1984) Hypocholesterolemic effects of oat-bran or bean intake for hypercholesterolemic men. Am. J. Clin. Nutr. 40:1146-1155.
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Anderson, J. W., Spencer, D. B., Hamilton, C. C., Smith, S. F., Tietyen, J., Bryant, C. A. & Oeltgen, P. (1990) Oat-bran cereal lowers serum total and LDL cholesterol in hypercholesterolemic men. Am. J. Clin. Nutr. 52:495-499.
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Brown, L., Rosner, B., Willett, W. & Sacks, F. M. (1999) Cholesterol-lowering effects of dietary fiber: a meta-analysis. Am. J. Clin. Nutr. 69:30-42.
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Melby, C. L., Goldflies, D. G., Hyner, G. C. & Lyle, R. M. (1989) Relation between vegetarian/nonvegetarian diets and blood pressure in black and white adults. Am. J. Public Health 79:1283-1288.
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Melby, C. L., Toohey, M. L. & Cebrick, J. (1994) Blood pressure and blood lipids among vegetarian, semivegetarian, and nonvegetarian African-Americans. Am. J. Clin. Nutr. 59:103-109.
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Alexander, H., Lockwood, L., Harris, M. A. & Melby, C. L. (1999) Risk factors for cardiovascular disease and diabetes in two Hispanic populations with different dietary patterns. J. Am. Coll. Nutr. 18:127-134.
20.
Appel, L. J., Moore, T. J., Obarzanek, E., Vollmer, W., Svetkey, L., Sacks, F., Bray, G., Vogt, T., Cutler, J., Windhauser, M., Lin, P. & Karanja, N. (1997) A clinical trial of the effects of dietary patterns on blood pressure. N. Engl. J. Med. 336:1117-1124.
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