Nutrition, Metabolism & Cardiovascular Diseases (2015) 25, 364e369
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Nutrition, Metabolism & Cardiovascular Diseases journal homepage: www.elsevier.com/locate/nmcd
Effects of non-fat dairy products added to the routine diet on vascular function: A randomized controlled crossover trial D.R. Machin, W. Park, M. Alkatan, M. Mouton, H. Tanaka* Cardiovascular Aging Research Laboratory, Department of Kinesiology and Health Education, The University of Texas at Austin, Austin, TX 78712, USA Received 19 November 2014; received in revised form 2 January 2015; accepted 20 January 2015 Available online 31 January 2015
KEYWORDS Milk; Dietary intervention; Arterial stiffness; Prehypertension; Lifestyle modification
Abstract Background and aims: High consumption of low- and non-fat dairy products is associated with reduced risk of high blood pressure (BP) and central arterial stiffness. However, interventional studies to determine if the addition of non-fat dairy products to the diet is capable of reducing central BP and improving vascular function are lacking. The aim of this study was to determine if the solitary addition of non-fat dairy products to the normal routine diet would reduce central BP and improve vascular function in middle-aged and older adults with elevated BP. Methods and results: Using a randomized, crossover intervention study design, forty-nine adults (44% men, 53 2 years, 170 2 cm, 88 3 kg; mean SEM) with elevated BP (134 1/ 81 1 mm Hg) underwent a High Dairy condition (þ4 servings/day of conventional non-fat dairy products) and No Dairy condition (þ4 servings/day fruit products) in which all dairy products were removed. Both dietary conditions lasted 4 weeks with a 2-week washout before crossing over into the alternate condition. The High Dairy condition produced reductions in central systolic BP (3 1 mm Hg) and carotid-femoral pulse wave velocity (0.5 0.1 m/sec), with a concomitant increase in brachial flow-mediated dilation (þ1.1 0.4%) and cardiovagal baroreflex sensitivity (þ5 1 ms/mm Hg) (P < 0.05 for all vs. baseline). In the No Dairy condition, brachial flow-mediated dilation was reduced (1.0 0.1%, P < 0.05 vs. baseline). Conclusions: The solitary manipulation of conventional dairy products in the normal routine diet modulates levels of central BP and vascular function in middle-aged and older adults with elevated BP. Clinical trial registration: clinicaltrials.gov Identifier: NCT01577030. ª 2015 Elsevier B.V. All rights reserved.
Introduction This rise in arterial blood pressure (BP), particularly systolic BP, with age is a major contributor to age-associated increases in risk of cardiovascular disease (CVD) [1]. The physiological mechanisms by which BP increases with age
* Corresponding author. Department of Kinesiology and Health Education, The University of Texas at Austin, 2109 San Jacinto Blvd, D3700, Austin, TX 78712, USA. Tel.: þ1 512 232 4801; fax: þ1 512 471 0946. E-mail address: [email protected] (H. Tanaka). http://dx.doi.org/10.1016/j.numecd.2015.01.005 0939-4753/ª 2015 Elsevier B.V. All rights reserved.
are not known. However, structural and functional alterations of the walls of large arteries, including arterial stiffening, have been implicated [2]. Increases in arterial stiffness are associated with reductions in arterial endothelial function and baroreflex sensitivity (BRS), an important short-term mechanism for regulating arterial BP [3e5], further contributing to elevated CVD risk. To reduce CVD incidence, it is of paramount importance to identify treatments that can lower BP and improve vascular function. In most individuals at risk of CVD, the initial treatment approach is the adoption of healthy
Dairy consumption and vascular function
lifestyle modifications [6]. Indeed, lifestyle changes such as regular exercise and salt restriction have reported reductions in systolic BP and improvements in vascular function in middle-aged and older adults [7,8]. We and others have reported that the consumption of a diet high in low- and non-fat dairy products are associated with the reductions in BP [9e12]. Information concerning the physiological mechanisms by which dairy consumption lowers arterial BP is extremely limited [13,14]. It is plausible to hypothesize that improved vascular function in general and de-stiffening of large arteries in particular may play an important role in producing the hypotensive effects of dairy products [7,8]. Currently, it is not known if the addition of conventional non-fat dairy products to the routine diet is capable of improving vascular function and whether such improvements are associated with reductions in BP, particularly central BP [15]. Accordingly, the primary aims of the present study were to determine if the solitary addition of conventional non-fat dairy products to the routine diet would reduce central BP and improve vascular function in middle-aged and older adults with elevated BP. We hypothesized that the addition of non-fat dairy products would reduce central BP and be accompanied by improvements in vascular function. To maximally differentiate the dietary dairy intake, all dairy products were removed from the routine diet in the control condition. Methods Study population All subject had seated brachial systolic BP 120e159 mm Hg with a diastolic BP < 100 mm Hg. Detailed description of subjects and inclusion criteria have been described previously [12]. Five subjects were taking antihypertensive medications, and dosages and routines were maintained throughout the study. All subjects underwent a 2-week “run-in period” that included 3 screening visits to ensure that all subjects demonstrated stable BP within the appropriate range. The University of Texas at Austin Institutional Review Board reviewed and approved the study. All volunteers gave their written informed consent before participation. Experimental design This study is part of a randomized trial with two 4-week dietary conditions and a washout period of 2 weeks (Fig. 1), reported in detail previously [12]. Briefly, the two dietary conditions were: High Dairy condition, in which subjects added four daily servings of non-fat dairy products in addition to their baseline dietary dairy intake and No Dairy condition, in which all dairy products were removed from the diet and four daily servings of fruit products were added. Each dietary condition had its own baseline and endpoint. All measurements were taken at the same time of day and after having abstained from food, alcohol, caffeine, and exercise for at least 12 h. In
365
premenopausal women, measurements were performed during the early follicular phase of the menstrual cycle.
Central arterial stiffness and BP After 10 min of supine rest, brachial and ankle BP, carotid and femoral pressure waveforms, and heart rate were simultaneously measured by an automated vascular testing device (Colin VP-2000, Omron Healthcare, Kyoto, Japan) [16]. Carotid-femoral pulse wave velocity (cfPWV) was calculated as travel distance divided by the transit time, as described previously [17]. Analog output of carotid pressure waveforms was recorded (WinDaq 2000, Dataq Instruments, Akron, OH) for determination of central systolic BP and PP, as described previously [18]. Day-to-day coefficients of variation are 3.2 0.2% for cfPWV in our laboratory [19].
Cardiovagal BRS After 10 min in the seated upright position, cardiovagal BRS was determined using the Valsalva maneuver [18] by having subjects forcibly exhale against a closed airway, while ReR interval and beat-by-beat BP (Pilot 9200, Colin Medical Instruments, San Antonio, TX) were measured continuously. Data were analyzed during the phase IV overshoot (WinDaq 2000, Dataq Instruments, Akron, OH) [4,20]. The coefficient of variation for repeated assessment of cardiovagal BRS was 10 2% [20]. Flow-mediated dilation (FMD) Brachial artery FMD was assessed after 10 min of supine rest using a Doppler ultrasound machine equipped with a high-resolution linear array transducer (Philips iE33 Ultrasound System, Bothel, WA) positioned 5e10 cm proximal to the antecubital fossa [21]. After baseline arterial diameter determination, a pneumatic cuff positioned 3e5 cm distal to the antecubital fossa was inflated to >100 mm Hg systolic BP for 5 min (E20 Rapid Cuff Inflator, D.E. Hokanson, Bellevue, WA). To capture peak diameter, brachial artery diameter was monitored until 3 min past cuff deflation. Ultrasound-derived diameter data were analyzed using automated image analysis software (Brachial Analyzer, Coralville, IA).
Dietary analyses Subjects were given detailed instructions on how to keep weekly 3-day dietary records (i.e., two weekdays and one weekend day) by the research bionutritionist, described previously [12]. Study compliance during each dietary condition was assessed by having each subject complete daily dietary surveys that indicated consumption of the food products provided and any dairy products consumed in their normal diet.
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Figure 1 Overview of randomized, controlled crossover trial design.
Statistical analyses All data were analyzed by the same investigator who was blinded to dietary conditions. A 2-way (condition time) and 3-way (sex condition time) mixed model ANOVA with repeated measures was used to evaluate the effect of condition time interaction using SPSS software (IBM, Chicago, IL). Where a significant interaction was found, paired-samples t-tests were conducted. Bivariate correlations were used to determine relations between dependent variables. Statistical significance was set at P < 0.05 for all analyses. Data are presented as mean SEM.
Results Forty-nine adults (44% men, 53 2 years [range 30e76 years]) with elevated BP (134 1/81 1 mm Hg) were studied. Subject compliance was 97 and 96% in the High Dairy and No Dairy conditions based on the daily dietary survey. There were no sex-related differences in these responses to the dietary conditions or in any of the responses described. Elimination/addition of subjects with antihypertensive medications did not affect the overall results. In the High Dairy condition, total dairy and non-fat dairy intake increased significantly from 1.2 0.1 and 0.3 0.1 to 4.7 0.1 and 4.0 0.1 servings/day (P < 0.05). In the No Dairy condition, total dairy and non-fat dairy intake decreased significantly from 1.4 0.1 and 0.4 0.1 to 0.0 0.0 and 0.0 0.0 servings/day (P < 0.05). Detailed information on dietary intake has been published elsewhere [12]. As shown in Table 1, there were significant reductions in supine brachial systolic BP, brachial pulse pressure (PP), and ankle PP after the High Dairy condition (P < 0.05). Additionally, brachial PP increased significantly after the No Dairy condition (P < 0.05). There were no changes in diastolic BP after either condition. Compared to baseline and between endpoints, cardiovagal BRS was significantly higher after the High Dairy condition (P < 0.05) (Table 1). As depicted in Fig. 2, central systolic BP decreased significantly after the high Dairy condition (P < 0.05). While no such changes were observed after the No Dairy condition, there remained a difference in central systolic
BP between endpoints (P < 0.05). Central PP was reduced after the High Dairy condition, but increased after the No Dairy condition (P < 0.05). Additionally, there were differences in central PP between endpoints (P < 0.05). Compared to baseline and between endpoints, cfPWV was significantly reduced after the high dairy condition (P < 0.05) (Fig. 3A). As shown in Fig. 4, compared to baseline, FMD was significantly higher after the High Dairy condition, but lower after the No Dairy condition (P < 0.05). This trend of changes in FMD was also observed with the absolute change in brachial artery diameter in both High Dairy and No Dairy conditions (Table 1) (P < 0.05). Additionally, there were differences in relative and absolute FMD between endpoints (P < 0.05). Changes in cfPWV were related to changes in central systolic BP (r Z 0.55; P < 0.05) (Fig. 3B), as well as to changes in brachial systolic BP (r Z 0.55), ankle systolic BP (r Z 0.57), and mean BP (r Z 0.58) (all P < 0.05; data not shown). Among the measures of vascular function, changes in cfPWV was associated with changes in cardiovagal BRS (r Z 0.27; P < 0.05), but changes in FMD were not related to cfPWV or cardiovagal BRS (r Z 0.05 and 0.17; P > 0.05). There were no relations between changes in BP and FMD or cardiovagal BRS.
Discussion We found that the addition conventional non-fat dairy products to the routine diet decreased central systolic BP and PP in middle-aged and older adults with elevated BP. The reductions in central BP were related to the corresponding changes in cfPWV and cardiovagal BRS and were accompanied by improved endothelial function, as measured by FMD. When all dairy products were removed from the routine diet, central PP increased and endothelial function decreased significantly. Taken together, these findings indicate that the solitary manipulation of dairy products, particularly non-fat dairy products, can modulate central BP and vascular function in middle-aged and older adults with elevated BP. Previous studies have demonstrated reductions in peripheral BP after consumption of a diet high in low- and non-fat dairy products [9e12,22]. In recent years, “central
Dairy consumption and vascular function
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Table 1 Changes in selected subject characteristics. Variables
No dairy Baseline
Height, cm Body mass, kg Heart rate, bpm Supine brachial systolic BP, mm Hg Supine brachial diastolic BP, mm Hg Supine brachial PP, mm Hg Ankle systolic BP, mm Hg Ankle diastolic BP, mm Hg Ankle PP, mm Hg Ankle-brachial index Carotid augmentation index, % (n Z 43) Cardiovagal BRS, ms/mm Hg (n Z 44) Baseline brachial artery diameter, mm (n Z 47) Peak brachial artery diameter, mm (n Z 47) Absolute FMD, mm (n Z 47)
170 87.8 64 134 80 54 152 80 72 1.12 18 14 3.83 3.96 0.13
2 3 1 1 1 1 2 1 2 0.01 3 1 0.13 0.14 0.01
High dairy Endpoint
Baseline
e 87.7 63 135 80 56 152 79 73 1.11 17 13 3.90 3.99 0.09
170 88.1 63 135 80 55 152 79 74 1.11 18 15 3.94 4.06 0.12
3 1 2 1 1* 2 1 2 0.01 3 1 0.13 0.13 0.02*
2 3 1 1 1 1 2 1 2 0.01 3 1 0.13 0.14 0.02
Endpoint e 88.3 64 132 78 54 149 77 71 1.11 15 20 3.92 4.09 0.16
3 1 2*y 1 1* 2 1 2* 0.01 3 2*y 0.14 0.14 0.02*y
Values are means SEM (n Z 49 unless stated otherwise). All significant differences were preceded by a significant condition time interaction. *P < 0.05 vs. Baseline; yP < 0.05 vs. Endpoint in No Dairy condition.
Figure 2 Central artery systolic blood pressure (A) and pulse pressure (B) before and after each dietary condition (n Z 49). *P < 0.05 vs. Baseline. yP < 0.05 vs. Endpoint in No Dairy condition. Values are means SEM.
BP” has gathered a lot of attention as it is more strongly associated with vascular disease and outcome than “peripheral BP” [15,23]. Indeed, reductions in central BP decrease the risk of future cardiovascular events to a greater extent than reductions in peripheral BP alone [24]. The present study is the first to observe reductions in central systolic BP and PP after the addition of non-fat dairy products to the diet. Moreover, we found that changes in central systolic BP were related to corresponding changes in central arterial stiffness. Indeed, individuals who consume a greater number of daily servings of dairy products have lower BP and central arterial stiffness [13]. Additionally, retinal damage, which is frequently associated with central arterial stiffening [25], is higher in those with lower dairy product consumption [26]. While there appears to be a link between dairy product consumption, BP, and vascular function, interventional studies to investigate these relations are very limited and no studies have examined the effects of the solitary addition of non-fat dairy products to the diet on vascular function. In the present study, we observed significant reductions in central artery stiffness, as measured by cfPWV after the High Dairy condition. Additionally, reduction in cfPWV were inversely related to improvements in cardiovagal BRS. The arterial baroreflex is associated with increased risk of ventricular tachyarrhythmias and sudden cardiac death [27]. The arterial baroreceptors are located in the carotid arteries and aorta. Decreases in compliance of carotid arteries and aorta lead to corresponding reductions in the ability of these reflexogenic regions to transduce signals, resulting in decreases in cardiovagal BRS [18]. Taken together, these results indicate that the solitary addition of conventional non-fat dairy products to the normal routine diet produce significant reductions not only in peripheral [12], but also in central BP and arterial stiffness, and that these changes appear to be interrelated.
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Figure 3 Carotid-femoral pulse wave velocity before and after each dietary condition (A). Relation of DcfPWV and Dcentral systolic blood pressure in both conditions (B). *P < 0.05 vs. Baseline. yP < 0.05 vs. Endpoint in No Dairy condition. Values are means SEM.
Figure 4 Flow-mediated dilation before and after each dietary condition (n Z 47). *P < 0.05 vs. Baseline. yP < 0.05 vs. Endpoint in No Dairy condition. Values are means SEM.
D.R. Machin et al.
As a marker of vascular endothelial function [28], FMD has a strong inverse association with CVD and cardiovascular event risks [29], and was shown to be increased significantly after the High Dairy condition. Interestingly, when dairy products were removed endothelial function decreased significantly. Although, changes in endothelial function appeared to follow the same pattern as PP, there was no significant relation between the two measurements. While physiological mechanisms for improved endothelial function remains unclear, it has been suggested that minerals contained in dairy products (i.e., calcium, potassium, magnesium) may aid in the restoration of endothelial cell membrane function resulting in decreased vasoconstrictor response and improve vasodilation [30]. Moreover, milk proteins may also contribute to improved endothelial function, as we have previously shown that supplementation with lactotripeptides improves FMD [14]. Regardless, the solitary dietary dairy product manipulation appears to modulate levels of endothelial function in middle-aged and older adults. We observed a significant increase in central PP and a significant reduction in endothelial function after the no dairy condition. These results indicate that the baseline dairy consumption may have been providing hypotensive and vasoprotective effects in these subjects. This is a reasonable hypothesis, as a recent cross-sectional study demonstrated an inverse relation between dairy product consumption and PP and arterial stiffness [13], supporting the notion that the hypotensive and vasoprotective effects may be present at the fairly low level of dairy product consumption observed in our subjects at baseline. A limitation of the present study was that we were unable to determine which non-fat dairy product was more effective in reducing BP and improving vascular function. Individuals in the study had the choice of non-fat yogurt, milk, and/or cheese, in addition to other dairy products consumed as part of their normal routine diet. We are unable to determine which ingredients of dairy products (milk protein, calcium, potassium, milk peptide, etc.) were responsible for hypotensive effects of dairy products. The same is true in regard to the seemingly hypertensive effects of fruit products, potential mechanisms for these have been discussed previously [12]. Future studies should include measurements of urinary markers that might be related to changes in BP and vascular function. Lastly, we cannot exclude the possibility that the influence of non-fat dairy products on arterial stiffness was an epiphenomenon of changes in mean BP. Lifestyle modifications, including dietary changes, are the first line approach for treating elevated BP. The present findings indicate that simply adding four servings of conventional non-fat dairy products to the routine diet are effective in reducing central (carotid artery) systolic BP and PP in middle-aged and older adults. Additionally, the addition of non-fat dairy products reduced cfPWV and improved cardiovagal BRS and endothelial function, as measured by FMD. On the contrary, removal of dairy products increased central PP and reduced endothelial function. Unlike other lifestyle modifications that are
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difficult to achieve high compliance and adherence, dairy products can be easily incorporated into the daily routine to gain hypotensive and vasoprotective benefits. Sources of funding This study was supported by a research grant from the Dairy Research Institute.
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[13]
[14]
[15] [16]
Disclaimers None. Acknowledgments None. References [1] Lewington S, Clarke R, Qizilbash N, Peto R, Collins R, Prospective Studies C. Age-specific relevance of usual blood pressure to vascular mortality: a meta-analysis of individual data for one million adults in 61 prospective studies. Lancet 2002;360: 1903e13. [2] O’Rourke MF, Hashimoto J. Mechanical factors in arterial aging: a clinical perspective. J Am Coll Cardiol 2007;50:1e13. [3] La Rovere MT, Bigger Jr JT, Marcus FI, Mortara A, Schwartz PJ. Baroreflex sensitivity and heart-rate variability in prediction of total cardiac mortality after myocardial infarction. ATRAMI (autonomic tone and reflexes after myocardial infarction) investigators. Lancet 1998;351:478e84. [4] Monahan KD, Tanaka H, Dinenno FA, Seals DR. Central arterial compliance is associated with age- and habitual exercise-related differences in cardiovagal baroreflex sensitivity. Circulation 2001;104:1627e32. [5] Seals DR, Jablonski KL, Donato AJ. Aging and vascular endothelial function in humans. Clin Sci (Lond) 2011;120:357e75. [6] Chobanian AV, Bakris GL, Black HR, Cushman WC, Green LA, Izzo Jr JL, et al. Seventh report of the Joint National Committee on prevention, detection, evaluation, and treatment of high blood pressure. Hypertension 2003;42:1206e52. [7] Nualnim N, Parkhurst K, Dhindsa M, Tarumi T, Vavrek J, Tanaka H. Effects of swimming training on blood pressure and vascular function in adults >50 years of age. Am J Cardiol 2012;109: 1005e10. [8] Gates PE, Tanaka H, Hiatt WR, Seals DR. Dietary sodium restriction rapidly improves large elastic artery compliance in older adults with systolic hypertension. Hypertension 2004;44:35e41. [9] Appel LJ, Moore TJ, Obarzanek E, Vollmer WM, Svetkey LP, Sacks FM, et al. A clinical trial of the effects of dietary patterns on blood pressure. DASH collaborative research group. N Engl J Med 1997;336:1117e24. [10] van Meijl LE, Mensink RP. Low-fat dairy consumption reduces systolic blood pressure, but does not improve other metabolic risk parameters in overweight and obese subjects. Nutr Metab Cardiovasc Dis 2011;21:355e61. [11] Buonopane GJ, Kilara A, Smith JS, McCarthy RD. Effect of skim milk supplementation on blood cholesterol concentration, blood pressure, and triglycerides in a free-living human population. J Am Coll Nutr 1992;11:56e67. [12] Machin DR, Park W, Alkatan M, Mouton M, Tanaka H. Hypotensive effects of solitary addition of conventional nonfat dairy products
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to the routine diet: a randomized controlled trial. Am J Clin Nutr 2014;100:80e7. Crichton GE, Elias MF, Dore GA, Abhayaratna WP, Robbins MA. Relations between dairy food intake and arterial stiffness: pulse wave velocity and pulse pressure. Hypertension 2012;59:1044e51. Yoshizawa M, Maeda S, Miyaki A, Misono M, Choi Y, Shimojo N, et al. Additive beneficial effects of lactotripeptides intake with regular exercise on endothelium-dependent dilatation in postmenopausal women. Am J Hypertens 2010;23:368e72. Safar M, Jankowski P. Central blood pressure and hypertension: role in cardiovascular risk assessment. Clin Sci 2009;116:273e82. Cortez-Cooper MY, Supak JA, Tanaka H. A new device for automatic measurements of arterial stiffness and ankle-brachial index. Am J Cardiol 2003;91. 1519e22, A9. Sugawara J, Hayashi K, Yokoi T, Tanaka H. Age-associated elongation of the ascending aorta in adults. JACC Cardiovasc Imaging 2008;1:739e48. Monahan KD, Dinenno FA, Seals DR, Clevenger CM, Desouza CA, Tanaka H. Age-associated changes in cardiovagal baroreflex sensitivity are related to central arterial compliance. Am J Physiol Heart Circ Physiol 2001;281:H284e9. Sugawara J, Hayashi K, Tanaka H. Distal shift of arterial pressure wave reflection sites with aging. Hypertension 2010;56:920e5. Monahan KD, Dinenno FA, Tanaka H, Clevenger CM, DeSouza CA, Seals DR. Regular aerobic exercise modulates age-associated declines in cardiovagal baroreflex sensitivity in healthy men. J Physiol 2000;529(Pt 1):263e71. Corretti MC, Anderson TJ, Benjamin EJ, Celermajer D, Charbonneau F, Creager MA, et al. Guidelines for the ultrasound assessment of endothelial-dependent flow-mediated vasodilation of the brachial artery: a report of the International Brachial Artery Reactivity Task Force. J Am Coll Cardiol 2002;39:257e65. Svetkey LP, Simons-Morton D, Vollmer WM, Appel LJ, Conlin PR, Ryan DH, et al. Effects of dietary patterns on blood pressure: subgroup analysis of the dietary approaches to stop hypertension (DASH) randomized clinical trial. Arch Intern Med 1999;159: 285e93. Wang K-L, Cheng H-M, Chuang S-Y, Spurgeon HA, Ting C-T, Lakatta EG, et al. Central or peripheral systolic or pulse pressure: which best relates to target-organs and future mortality? J Hypertens 2009;27:461. Williams B, Lacy PS, Thom SM, Cruickshank K, Stanton A, Collier D, et al. Differential impact of blood pressureelowering drugs on central aortic pressure and clinical outcomes principal results of the conduit artery function evaluation (CAFE) Study. Circulation 2006;113:1213e25. Liao D, Wong TY, Klein R, Jones D, Hubbard L, Sharrett AR. Relationship between carotid artery stiffness and retinal arteriolar narrowing in healthy middle-aged persons. Stroke 2004;35: 837e42. Gopinath B, Flood VM, Wang JJ, Burlutsky G, Mitchell P. Lower dairy products and calcium intake is associated with adverse retinal vascular changes in older adults. Nutr Metab Cardiovasc Dis 2014;24:155e61. Billman GE, Schwartz PJ, Stone HL. Baroreceptor reflex control of heart rate: a predictor of sudden cardiac death. Circulation 1982; 66:874e80. Celermajer DS, Sorensen KE, Gooch VM, Sullivan ID, Lloyd JK, Deanfield JE, et al. Non-invasive detection of endothelial dysfunction in children and adults at risk of atherosclerosis. Lancet 1992;340:1111e5. Green DJ, Jones H, Thijssen D, Cable NT, Atkinson G. Flow-mediated dilation and cardiovascular event prediction: does nitric oxide matter? Hypertension 2011;57:363e9. Vapaatalo H, Mervaala E. Clinically important factors influencing endothelial function. Med Sci Monit 2001;7:1075e85.
Available online at www.sciencedirect.com
Nutrition, Metabolism & Cardiovascular Diseases journal homepage: www.elsevier.com/locate/nmcd
Effects of non-fat dairy products added to the routine diet on vascular function: A randomized controlled crossover trial D.R. Machin, W. Park, M. Alkatan, M. Mouton, H. Tanaka* Cardiovascular Aging Research Laboratory, Department of Kinesiology and Health Education, The University of Texas at Austin, Austin, TX 78712, USA Received 19 November 2014; received in revised form 2 January 2015; accepted 20 January 2015 Available online 31 January 2015
KEYWORDS Milk; Dietary intervention; Arterial stiffness; Prehypertension; Lifestyle modification
Abstract Background and aims: High consumption of low- and non-fat dairy products is associated with reduced risk of high blood pressure (BP) and central arterial stiffness. However, interventional studies to determine if the addition of non-fat dairy products to the diet is capable of reducing central BP and improving vascular function are lacking. The aim of this study was to determine if the solitary addition of non-fat dairy products to the normal routine diet would reduce central BP and improve vascular function in middle-aged and older adults with elevated BP. Methods and results: Using a randomized, crossover intervention study design, forty-nine adults (44% men, 53 2 years, 170 2 cm, 88 3 kg; mean SEM) with elevated BP (134 1/ 81 1 mm Hg) underwent a High Dairy condition (þ4 servings/day of conventional non-fat dairy products) and No Dairy condition (þ4 servings/day fruit products) in which all dairy products were removed. Both dietary conditions lasted 4 weeks with a 2-week washout before crossing over into the alternate condition. The High Dairy condition produced reductions in central systolic BP (3 1 mm Hg) and carotid-femoral pulse wave velocity (0.5 0.1 m/sec), with a concomitant increase in brachial flow-mediated dilation (þ1.1 0.4%) and cardiovagal baroreflex sensitivity (þ5 1 ms/mm Hg) (P < 0.05 for all vs. baseline). In the No Dairy condition, brachial flow-mediated dilation was reduced (1.0 0.1%, P < 0.05 vs. baseline). Conclusions: The solitary manipulation of conventional dairy products in the normal routine diet modulates levels of central BP and vascular function in middle-aged and older adults with elevated BP. Clinical trial registration: clinicaltrials.gov Identifier: NCT01577030. ª 2015 Elsevier B.V. All rights reserved.
Introduction This rise in arterial blood pressure (BP), particularly systolic BP, with age is a major contributor to age-associated increases in risk of cardiovascular disease (CVD) [1]. The physiological mechanisms by which BP increases with age
* Corresponding author. Department of Kinesiology and Health Education, The University of Texas at Austin, 2109 San Jacinto Blvd, D3700, Austin, TX 78712, USA. Tel.: þ1 512 232 4801; fax: þ1 512 471 0946. E-mail address: [email protected] (H. Tanaka). http://dx.doi.org/10.1016/j.numecd.2015.01.005 0939-4753/ª 2015 Elsevier B.V. All rights reserved.
are not known. However, structural and functional alterations of the walls of large arteries, including arterial stiffening, have been implicated [2]. Increases in arterial stiffness are associated with reductions in arterial endothelial function and baroreflex sensitivity (BRS), an important short-term mechanism for regulating arterial BP [3e5], further contributing to elevated CVD risk. To reduce CVD incidence, it is of paramount importance to identify treatments that can lower BP and improve vascular function. In most individuals at risk of CVD, the initial treatment approach is the adoption of healthy
Dairy consumption and vascular function
lifestyle modifications [6]. Indeed, lifestyle changes such as regular exercise and salt restriction have reported reductions in systolic BP and improvements in vascular function in middle-aged and older adults [7,8]. We and others have reported that the consumption of a diet high in low- and non-fat dairy products are associated with the reductions in BP [9e12]. Information concerning the physiological mechanisms by which dairy consumption lowers arterial BP is extremely limited [13,14]. It is plausible to hypothesize that improved vascular function in general and de-stiffening of large arteries in particular may play an important role in producing the hypotensive effects of dairy products [7,8]. Currently, it is not known if the addition of conventional non-fat dairy products to the routine diet is capable of improving vascular function and whether such improvements are associated with reductions in BP, particularly central BP [15]. Accordingly, the primary aims of the present study were to determine if the solitary addition of conventional non-fat dairy products to the routine diet would reduce central BP and improve vascular function in middle-aged and older adults with elevated BP. We hypothesized that the addition of non-fat dairy products would reduce central BP and be accompanied by improvements in vascular function. To maximally differentiate the dietary dairy intake, all dairy products were removed from the routine diet in the control condition. Methods Study population All subject had seated brachial systolic BP 120e159 mm Hg with a diastolic BP < 100 mm Hg. Detailed description of subjects and inclusion criteria have been described previously [12]. Five subjects were taking antihypertensive medications, and dosages and routines were maintained throughout the study. All subjects underwent a 2-week “run-in period” that included 3 screening visits to ensure that all subjects demonstrated stable BP within the appropriate range. The University of Texas at Austin Institutional Review Board reviewed and approved the study. All volunteers gave their written informed consent before participation. Experimental design This study is part of a randomized trial with two 4-week dietary conditions and a washout period of 2 weeks (Fig. 1), reported in detail previously [12]. Briefly, the two dietary conditions were: High Dairy condition, in which subjects added four daily servings of non-fat dairy products in addition to their baseline dietary dairy intake and No Dairy condition, in which all dairy products were removed from the diet and four daily servings of fruit products were added. Each dietary condition had its own baseline and endpoint. All measurements were taken at the same time of day and after having abstained from food, alcohol, caffeine, and exercise for at least 12 h. In
365
premenopausal women, measurements were performed during the early follicular phase of the menstrual cycle.
Central arterial stiffness and BP After 10 min of supine rest, brachial and ankle BP, carotid and femoral pressure waveforms, and heart rate were simultaneously measured by an automated vascular testing device (Colin VP-2000, Omron Healthcare, Kyoto, Japan) [16]. Carotid-femoral pulse wave velocity (cfPWV) was calculated as travel distance divided by the transit time, as described previously [17]. Analog output of carotid pressure waveforms was recorded (WinDaq 2000, Dataq Instruments, Akron, OH) for determination of central systolic BP and PP, as described previously [18]. Day-to-day coefficients of variation are 3.2 0.2% for cfPWV in our laboratory [19].
Cardiovagal BRS After 10 min in the seated upright position, cardiovagal BRS was determined using the Valsalva maneuver [18] by having subjects forcibly exhale against a closed airway, while ReR interval and beat-by-beat BP (Pilot 9200, Colin Medical Instruments, San Antonio, TX) were measured continuously. Data were analyzed during the phase IV overshoot (WinDaq 2000, Dataq Instruments, Akron, OH) [4,20]. The coefficient of variation for repeated assessment of cardiovagal BRS was 10 2% [20]. Flow-mediated dilation (FMD) Brachial artery FMD was assessed after 10 min of supine rest using a Doppler ultrasound machine equipped with a high-resolution linear array transducer (Philips iE33 Ultrasound System, Bothel, WA) positioned 5e10 cm proximal to the antecubital fossa [21]. After baseline arterial diameter determination, a pneumatic cuff positioned 3e5 cm distal to the antecubital fossa was inflated to >100 mm Hg systolic BP for 5 min (E20 Rapid Cuff Inflator, D.E. Hokanson, Bellevue, WA). To capture peak diameter, brachial artery diameter was monitored until 3 min past cuff deflation. Ultrasound-derived diameter data were analyzed using automated image analysis software (Brachial Analyzer, Coralville, IA).
Dietary analyses Subjects were given detailed instructions on how to keep weekly 3-day dietary records (i.e., two weekdays and one weekend day) by the research bionutritionist, described previously [12]. Study compliance during each dietary condition was assessed by having each subject complete daily dietary surveys that indicated consumption of the food products provided and any dairy products consumed in their normal diet.
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Figure 1 Overview of randomized, controlled crossover trial design.
Statistical analyses All data were analyzed by the same investigator who was blinded to dietary conditions. A 2-way (condition time) and 3-way (sex condition time) mixed model ANOVA with repeated measures was used to evaluate the effect of condition time interaction using SPSS software (IBM, Chicago, IL). Where a significant interaction was found, paired-samples t-tests were conducted. Bivariate correlations were used to determine relations between dependent variables. Statistical significance was set at P < 0.05 for all analyses. Data are presented as mean SEM.
Results Forty-nine adults (44% men, 53 2 years [range 30e76 years]) with elevated BP (134 1/81 1 mm Hg) were studied. Subject compliance was 97 and 96% in the High Dairy and No Dairy conditions based on the daily dietary survey. There were no sex-related differences in these responses to the dietary conditions or in any of the responses described. Elimination/addition of subjects with antihypertensive medications did not affect the overall results. In the High Dairy condition, total dairy and non-fat dairy intake increased significantly from 1.2 0.1 and 0.3 0.1 to 4.7 0.1 and 4.0 0.1 servings/day (P < 0.05). In the No Dairy condition, total dairy and non-fat dairy intake decreased significantly from 1.4 0.1 and 0.4 0.1 to 0.0 0.0 and 0.0 0.0 servings/day (P < 0.05). Detailed information on dietary intake has been published elsewhere [12]. As shown in Table 1, there were significant reductions in supine brachial systolic BP, brachial pulse pressure (PP), and ankle PP after the High Dairy condition (P < 0.05). Additionally, brachial PP increased significantly after the No Dairy condition (P < 0.05). There were no changes in diastolic BP after either condition. Compared to baseline and between endpoints, cardiovagal BRS was significantly higher after the High Dairy condition (P < 0.05) (Table 1). As depicted in Fig. 2, central systolic BP decreased significantly after the high Dairy condition (P < 0.05). While no such changes were observed after the No Dairy condition, there remained a difference in central systolic
BP between endpoints (P < 0.05). Central PP was reduced after the High Dairy condition, but increased after the No Dairy condition (P < 0.05). Additionally, there were differences in central PP between endpoints (P < 0.05). Compared to baseline and between endpoints, cfPWV was significantly reduced after the high dairy condition (P < 0.05) (Fig. 3A). As shown in Fig. 4, compared to baseline, FMD was significantly higher after the High Dairy condition, but lower after the No Dairy condition (P < 0.05). This trend of changes in FMD was also observed with the absolute change in brachial artery diameter in both High Dairy and No Dairy conditions (Table 1) (P < 0.05). Additionally, there were differences in relative and absolute FMD between endpoints (P < 0.05). Changes in cfPWV were related to changes in central systolic BP (r Z 0.55; P < 0.05) (Fig. 3B), as well as to changes in brachial systolic BP (r Z 0.55), ankle systolic BP (r Z 0.57), and mean BP (r Z 0.58) (all P < 0.05; data not shown). Among the measures of vascular function, changes in cfPWV was associated with changes in cardiovagal BRS (r Z 0.27; P < 0.05), but changes in FMD were not related to cfPWV or cardiovagal BRS (r Z 0.05 and 0.17; P > 0.05). There were no relations between changes in BP and FMD or cardiovagal BRS.
Discussion We found that the addition conventional non-fat dairy products to the routine diet decreased central systolic BP and PP in middle-aged and older adults with elevated BP. The reductions in central BP were related to the corresponding changes in cfPWV and cardiovagal BRS and were accompanied by improved endothelial function, as measured by FMD. When all dairy products were removed from the routine diet, central PP increased and endothelial function decreased significantly. Taken together, these findings indicate that the solitary manipulation of dairy products, particularly non-fat dairy products, can modulate central BP and vascular function in middle-aged and older adults with elevated BP. Previous studies have demonstrated reductions in peripheral BP after consumption of a diet high in low- and non-fat dairy products [9e12,22]. In recent years, “central
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Table 1 Changes in selected subject characteristics. Variables
No dairy Baseline
Height, cm Body mass, kg Heart rate, bpm Supine brachial systolic BP, mm Hg Supine brachial diastolic BP, mm Hg Supine brachial PP, mm Hg Ankle systolic BP, mm Hg Ankle diastolic BP, mm Hg Ankle PP, mm Hg Ankle-brachial index Carotid augmentation index, % (n Z 43) Cardiovagal BRS, ms/mm Hg (n Z 44) Baseline brachial artery diameter, mm (n Z 47) Peak brachial artery diameter, mm (n Z 47) Absolute FMD, mm (n Z 47)
170 87.8 64 134 80 54 152 80 72 1.12 18 14 3.83 3.96 0.13
2 3 1 1 1 1 2 1 2 0.01 3 1 0.13 0.14 0.01
High dairy Endpoint
Baseline
e 87.7 63 135 80 56 152 79 73 1.11 17 13 3.90 3.99 0.09
170 88.1 63 135 80 55 152 79 74 1.11 18 15 3.94 4.06 0.12
3 1 2 1 1* 2 1 2 0.01 3 1 0.13 0.13 0.02*
2 3 1 1 1 1 2 1 2 0.01 3 1 0.13 0.14 0.02
Endpoint e 88.3 64 132 78 54 149 77 71 1.11 15 20 3.92 4.09 0.16
3 1 2*y 1 1* 2 1 2* 0.01 3 2*y 0.14 0.14 0.02*y
Values are means SEM (n Z 49 unless stated otherwise). All significant differences were preceded by a significant condition time interaction. *P < 0.05 vs. Baseline; yP < 0.05 vs. Endpoint in No Dairy condition.
Figure 2 Central artery systolic blood pressure (A) and pulse pressure (B) before and after each dietary condition (n Z 49). *P < 0.05 vs. Baseline. yP < 0.05 vs. Endpoint in No Dairy condition. Values are means SEM.
BP” has gathered a lot of attention as it is more strongly associated with vascular disease and outcome than “peripheral BP” [15,23]. Indeed, reductions in central BP decrease the risk of future cardiovascular events to a greater extent than reductions in peripheral BP alone [24]. The present study is the first to observe reductions in central systolic BP and PP after the addition of non-fat dairy products to the diet. Moreover, we found that changes in central systolic BP were related to corresponding changes in central arterial stiffness. Indeed, individuals who consume a greater number of daily servings of dairy products have lower BP and central arterial stiffness [13]. Additionally, retinal damage, which is frequently associated with central arterial stiffening [25], is higher in those with lower dairy product consumption [26]. While there appears to be a link between dairy product consumption, BP, and vascular function, interventional studies to investigate these relations are very limited and no studies have examined the effects of the solitary addition of non-fat dairy products to the diet on vascular function. In the present study, we observed significant reductions in central artery stiffness, as measured by cfPWV after the High Dairy condition. Additionally, reduction in cfPWV were inversely related to improvements in cardiovagal BRS. The arterial baroreflex is associated with increased risk of ventricular tachyarrhythmias and sudden cardiac death [27]. The arterial baroreceptors are located in the carotid arteries and aorta. Decreases in compliance of carotid arteries and aorta lead to corresponding reductions in the ability of these reflexogenic regions to transduce signals, resulting in decreases in cardiovagal BRS [18]. Taken together, these results indicate that the solitary addition of conventional non-fat dairy products to the normal routine diet produce significant reductions not only in peripheral [12], but also in central BP and arterial stiffness, and that these changes appear to be interrelated.
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Figure 3 Carotid-femoral pulse wave velocity before and after each dietary condition (A). Relation of DcfPWV and Dcentral systolic blood pressure in both conditions (B). *P < 0.05 vs. Baseline. yP < 0.05 vs. Endpoint in No Dairy condition. Values are means SEM.
Figure 4 Flow-mediated dilation before and after each dietary condition (n Z 47). *P < 0.05 vs. Baseline. yP < 0.05 vs. Endpoint in No Dairy condition. Values are means SEM.
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As a marker of vascular endothelial function [28], FMD has a strong inverse association with CVD and cardiovascular event risks [29], and was shown to be increased significantly after the High Dairy condition. Interestingly, when dairy products were removed endothelial function decreased significantly. Although, changes in endothelial function appeared to follow the same pattern as PP, there was no significant relation between the two measurements. While physiological mechanisms for improved endothelial function remains unclear, it has been suggested that minerals contained in dairy products (i.e., calcium, potassium, magnesium) may aid in the restoration of endothelial cell membrane function resulting in decreased vasoconstrictor response and improve vasodilation [30]. Moreover, milk proteins may also contribute to improved endothelial function, as we have previously shown that supplementation with lactotripeptides improves FMD [14]. Regardless, the solitary dietary dairy product manipulation appears to modulate levels of endothelial function in middle-aged and older adults. We observed a significant increase in central PP and a significant reduction in endothelial function after the no dairy condition. These results indicate that the baseline dairy consumption may have been providing hypotensive and vasoprotective effects in these subjects. This is a reasonable hypothesis, as a recent cross-sectional study demonstrated an inverse relation between dairy product consumption and PP and arterial stiffness [13], supporting the notion that the hypotensive and vasoprotective effects may be present at the fairly low level of dairy product consumption observed in our subjects at baseline. A limitation of the present study was that we were unable to determine which non-fat dairy product was more effective in reducing BP and improving vascular function. Individuals in the study had the choice of non-fat yogurt, milk, and/or cheese, in addition to other dairy products consumed as part of their normal routine diet. We are unable to determine which ingredients of dairy products (milk protein, calcium, potassium, milk peptide, etc.) were responsible for hypotensive effects of dairy products. The same is true in regard to the seemingly hypertensive effects of fruit products, potential mechanisms for these have been discussed previously [12]. Future studies should include measurements of urinary markers that might be related to changes in BP and vascular function. Lastly, we cannot exclude the possibility that the influence of non-fat dairy products on arterial stiffness was an epiphenomenon of changes in mean BP. Lifestyle modifications, including dietary changes, are the first line approach for treating elevated BP. The present findings indicate that simply adding four servings of conventional non-fat dairy products to the routine diet are effective in reducing central (carotid artery) systolic BP and PP in middle-aged and older adults. Additionally, the addition of non-fat dairy products reduced cfPWV and improved cardiovagal BRS and endothelial function, as measured by FMD. On the contrary, removal of dairy products increased central PP and reduced endothelial function. Unlike other lifestyle modifications that are
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difficult to achieve high compliance and adherence, dairy products can be easily incorporated into the daily routine to gain hypotensive and vasoprotective benefits. Sources of funding This study was supported by a research grant from the Dairy Research Institute.
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