Editorial Comment
Fibre intake and blood pressure: more facts, more questions Pasquale Strazzullo a and Rosalba Giacco b
See original paper on page 897
T
he meta-analysis on the effects of increased dietary fibre on blood pressure (BP), published in this issue of the Journal of Hypertension [1], calls for renovated attention on one of the many aspects of our dietary habits deemed to be relevant for cardiovascular prevention. Although Evans’ work is not the first comprehensive assessment of the possible relationship between increased fibre intake and BP, the 10 years that have elapsed since the two previous systematic reviews [2,3] on the same subject have seen the publication of several new reports that allowed Evans et al. to carry a more in-depth evaluation and, in particular, to try and dissect out the effects of different types of fibre in addition to their overall impact on BP. One merit of the review is that it is exclusively based on the analysis of randomized controlled trials, most of which rated as good or very good quality, whereas previous evidence suggesting that larger habitual consumption of dietary fibre is beneficial for human health, by reducing the risk of obesity, type 2 diabetes, hypertension and their cardiovascular complications, has mainly come from observational studies [4–8]. The most novel content of Evans’ work is the evidence in favour of a particularly beneficial effect on BP of b-glucans, a variety of soluble dietary fibre abundant in oats and barley-rich foods, whereas the authors found little or no statistical evidence of the BP impact of other types of dietary fibre. A strength of the study is that in all the five trials included in the meta-analysis, b-glucans were provided as oat bran, oat meals or directly as oat b-glucan-soluble powder in substitution for wheat-based products used as control; thus, it seems unlikely that the observed effect of the intervention could be attributable to other concomitant dietary changes.
Journal of Hypertension 2015, 33:921–923 a Department of Clinical Medicine and Surgery, Excellence Centre for Hypertension, Federico II University of Naples Medical School, Naples and bInstitute of Food Science, National Research Council, Avellino, Italy
Correspondence to Pasquale Strazzullo, Professor of Internal Medicine, Department of Clinical Medicine and Surgery, Excellence Centre of Hypertension, Federico II University of Naples Medical School, Via S. Pansini 5, Naples 80131, Italy. Tel: +39 081 7463686; fax: +39 081 5466152; e-mail: [email protected] J Hypertens 33:921–923 Copyright ß 2015 Wolters Kluwer Health, Inc. All rights reserved. DOI:10.1097/HJH.0000000000000569
Journal of Hypertension
It also seems unlikely that body weight reduction induced by higher fibre intake during the intervention may have played a major role because in the three (out of five) studies reporting weight changes, the variations were actually minor. Furthermore, in the other studies included in the meta-analysis, which tested the effects of different fibres (pectins, mannans, etc.), similar or even greater body weight losses were not associated with the beneficial effects on BP. Three of the five studies featuring higher intake of oat meals and b-glucans recruited individuals with BP in the high–normal range or having grade 1 hypertension, which may have theoretically made it easier to appreciate a BPlowering effect of these substances; however, a favourable BP response was detected in only two of these three studies and, in addition, it was detected also in one of the two trials involving only normotensive individuals [1]. Indeed, two studies not included in Evans’ systematic review reported an impact of oat consumption on BP. In a pilot study by Keenan et al. [9], SBP and DBP were both reduced after whole oat cereal intake for 6 weeks. In another study on drug-treated hypertensive patients, the consumption of oat cereal meals lead to a significant reduction in their need for antihypertensive medication compared with the control group [10]. A puzzling question is of course through which possible mechanisms do b-glucans or other components of oat-rich products favour a decrease in BP. At least three potential mechanisms may be envisaged: one or more pathways related to an overall improvement in glucose metabolism, an anti-inflammatory and/or antioxidant effect and an effect mediated by a reduction in plasma lipids. As the first possibility, b-glucans are known to delay gastric emptying, increase the viscosity of gastrointestinal content and increase satiety. The increased viscosity of the gastrointestinal content reduces food transit time and slows glucose absorption, thus diminishing the postprandial glucose and insulin responses [11]. A similar mechanism was operating in the Study to Prevent Non-Insulin-Dependent Diabetes Mellitus, involving individuals with impaired glucose tolerance who were assigned to treatment with either the oligosaccharide acarbose or the placebo. Treatment with acarbose was shown to reduce postprandial glucose and insulin responses by slowing starch digestion and www.jhypertension.com
Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
921
Strazzullo and Giacco
delaying glucose absorption, and was associated with a significantly lower incidence of hypertension and evidence of carotid atherosclerosis over 3.3 years compared with control treatment [12]. Although there is no direct experimental evidence of the beneficial effects of b-glucan consumption on insulin resistance, lower postprandial insulin levels may conceivably affect BP, given the direct influence of hyperinsulinaemia on renal sodium reabsorption and sympathetic tone [13,14]. It is true that in addition to b-glucans, other viscous fibres also, for example, guar and mannan, used in some of the trials included in Evans’ meta-analysis, have shown a reduction in postprandial blood glucose response and insulin levels – albeit with some variation in the data reported; it remains obscure why no apparent effect on BP could be detected in those studies. Evidence from both experimental and clinical studies strongly suggests that one or more oat components may exert anti-inflammatory and antioxidant activity, which in turn may have an impact on endothelial function [15]. These include tocopherols, phenolic compounds (namely, the oat-specific polyphenols avenanthramides), phytic acids, sterols and flavonoids [16]. Of practical interest is that most of the antioxidant activities associated with oats seem to be resistant to heat treatment and thus likely to survive the commonly applied industrial processing treatments [17]. The phenolic components avenanthramides also exhibit anti-inflammatory properties through inhibition of nuclear factor kB signalling [18]. Only few clinical studies, however, have investigated the possible effect of oats on systemic inflammation. Some of them found no effect, possibly because of the use of chemical treatments susceptible to induce the loss of the active phenolic substances [19,20]. In a study in low-density lipoprotein (LDL) receptor-deficient mice fed oat bran, it was found that the levels of the inflammation-related plasma proteins fibrinogen and soluble vascular cell adhesion molecule-1 and the expression of the vascular cell adhesion molecule-1 in the aortic wall were consistently reduced [21]. In this study, however, a substantial reduction in plasma cholesterol also occurred as an effect of higher oat bran intake, a response capable to hamper the inflammatory activity per se [22]; this makes it difficult to discriminate between a direct or a secondary effect of oat consumption. Nevertheless, these results are in keeping with another in-vitro study on human endothelial cells showing that avenanthramides isolated from oat were able to reduce the interleukin-1-induced expression of several adhesion molecules, concomitantly reducing monocyte attachment to the cells and lowering interleukin secretion [23]. The oat phenolic compounds and flavonoids are also thought to exert an antioxidant action by scavenging reactive oxygen and nitrogen species and chelating transition minerals. In in-vitro studies, tocopherols were incorporated into low-density lipoproteins and protected them from oxidative agents [24]. Avenanthramides exert direct antioxidative effects and seem to be capable to stimulate the human body scavenging system in animals [25,26] and humans [27]. Both low-grade systemic inflammation and increased oxidative stress inhibit nitric oxide production and contribute to endothelial dysfunction. Thus, the reports in the 922
www.jhypertension.com
literature suggesting that oats could favourably influence endothelial function are not surprising [28,29]. Specifically, these studies support the concept that oat components, by affecting the expression of endothelial nitric oxide synthase (eNOS), could increase the production of nitric oxide, thereby contrasting the negative effects of certain postprandial events such as excess insulin release, production of oxygen-free radicals and generation of triglyceride-rich lipoprotein particles – all contributing to endothelial dysfunction and possibly resulting in vasoconstriction and increased BP [30]. In this respect, Katz et al. [29,31] reported that long-term elevated oat consumption effectively opposes the failure in endothelial function following a high-fat meal in both normal weight and overweight individuals. A third mechanism whereby oat and, in particular, b-glucan intake might improve BP levels may be traced to their LDL-cholesterol-lowering action, clearly demonstrated by Brown et al. [32] in their meta-analysis; nevertheless, it might be argued that this effect is common to other types of viscous fibres such as guar, pectin or psyllium, the consumption of which was not associated with lower BP levels in Evans’ study. A seminal work by Nickenig et al. [33] revealed a significant upregulation of the angiotensin-I receptor gene expression by LDL in vascular smooth muscle cells, ultimately leading to an elevated functional response of vascular smooth muscle cells to angiotensin II stimulation, which could conceivably affect BP [33]. On the contrary, treatment of hypercholesterolaemia with statins has been associated with small but significantly lower BP levels [34]. Moreover, in two specular intervention trials of the metabolic and vascular effects of changes in dietary fat intake in normal volunteers, changes in either direction of total and LDL-cholesterol were consistently paralleled by BP changes in the same direction [35,36]. In conclusion, based on the evidence available from the clinical and experimental studies so far available, it is not possible to definitely confirm or exclude any of the abovementioned potential mechanisms nor is it possible to discriminate if and to what extent the decrease in BP observed upon increased oat-based products intake is attributable to any one of the various oat compounds, including b-glucans. Evans et al. noted that, although advice on increasing fruit and vegetable consumption is included in the European Society of Hypertension/European Society of Cardiology (ESC/ESH) Guidelines for the Management of Arterial Hypertension [37], no advice is given for fibre consumption. A large habitual fibre intake has recognized merits in particular with regard to prevention of obesity and type 2 diabetes and, as such, is recommended in most nutritional guidelines, including the European Guidelines on Cardiovascular Disease Prevention in Clinical Practice [38]. Evans’ updated review of the evidence about fibre intake and BP does not bring further substantial support to larger fibre intake as an effective means for reducing BP. It nevertheless may focus the attention of scientists and health operators active in the nutritional field on the properties of b-glucans and, in general, of oat and possibly barley-rich foods. These foods are relatively popular in the framework of the Volume 33 Number 5 May 2015
Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
Fibre intake and blood pressure
so-called Western and Nordic diets whereas they are not prevalent within the Mediterranean dietary model. On practical grounds, although definitive conclusions on their specific value cannot be drawn, a reasonable increase in their consumption, where particularly low, promises to do more good than harm.
ACKNOWLEDGEMENTS Conflicts of interest There are no conflicts of interest.
REFERENCES 1. Evans CEL, Greenwood DC, Threapleton DE, Cleghorn CL, Nykjaer C, Woodhead CE, et al. Effects of dietary fiber type on blood pressure: a systematic review and meta-analysis of randomized controlled trials of healthy individuals. J Hypertens 2015; 33:897–911. 2. Whelton SP, Hyre AD, Pedersen B, Yi Y, Whelton PK, He J. Effect of dietary fiber intake on blood pressure: a meta-analysis of randomized, controlled clinical trials. J Hypertens 2005; 23:475–481. 3. Streppel MT1, Arends LR, van’t Veer P, Grobbee DE, Geleijnse JM. Dietary fiber and blood pressure: a meta-analysis of randomized placebo-controlled trials. Arch Intern Med 2005; 165:150–156. 4. Pol K, Christensen R, Bartels EM, Raben A, Tetens I, Kristensen M. Whole grain and body weight changes in apparently healthy adults: a systematic review and meta-analysis of randomized controlled studies. Am J Clin Nutr 2013; 98:872–884. 5. Montonen J, Knekt P, Ja¨rvinen R, Aromaa A, Reunanen A. Whole-grain and fiber intake and the incidence of type 2 diabetes. Am J Clin Nutr 2003; 77:622–629. 6. Wang L, Gaziano JM, Liu S, et al. Whole- and refined-grain intakes and the risk of hypertension in women. Am J Clin Nutr 2007; 86:472–479. 7. Larsson SC, Wolk A. Dietary fiber intake is inversely associated with stroke incidence in healthy Swedish adults. J Nutr 2014; 144:1952– 1955. 8. Threapleton DE, Greenwood DC, Evans CE, Cleghorn CL, Nykjaer C, Woodhead C, et al. Dietary fibre intake and risk of cardiovascular disease: systematic review and meta-analysis. BMJ 2013; 347:f6879. 9. Keenan JM, Pins JJ, Frazel C, Moran A, Turnquist L. Oat ingestion reduces systolic and diastolic blood pressure in patients with mild or borderline hypertension: a pilot trial. J Fam Pract 2002; 51:369. 10. Pins JJ, Geleva D, Keenan JM, Frazel C, O’Connor PJ, Cherney LM. Do whole-grain oat cereals reduce the need for antihypertensive medications and improve blood pressure control? J Fam Pract 2002; 51:353– 359. 11. Wang Q, Ellis PR. Oat b-glucan: physico-chemical characteristics in relation to its blood-glucose and cholesterol-lowering properties. Br J Nutr 2014; Suppl 2:S4–S13. 12. Chiasson JL, Josse RG, Gomis R, Hanefeld M, Karasik A, Laakso M, STOP-NIDDM Trial Research Group. Acarbose treatment and the risk of cardiovascular disease and hypertension in patients with impaired glucose tolerance: the STOP-NIDDM trial. JAMA 2003; 290:486–494. 13. Fonseca VA. Management of diabetes mellitus and insulin resistance in patients with cardiovascular disease. Am J Cardiol 2003; 92:50J–60J. 14. Scott CL. Diagnosis, prevention, and intervention for the metabolic syndrome. Am J Cardiol 2003; 92 (Suppl):35i–42i. 15. Andersson KE, Hellstrand P. Dietary oats and modulation of atherogenic pathways. Mol Nutr Food Res 2012; 56:1003–1013. 16. Peterson DM. Oat antioxidants. J Cereal Sci 2001; 33:115–129. 17. Handelman GJ1, Cao G, Walter MF, Nightingale ZD, Paul GL, Prior RL, Blumberg JB. Antioxidant capacity of oat (Avena sativa L.) extracts. 1. Inhibition of low-density lipoprotein oxidation and oxygen radical absorbance capacity. J Agric Food Chem 1999; 47:4888–4893. 18. Sur R, Nigam A, Grote D, Liebel F, Southall MD. Avenanthramides, polyphenols from oats, exhibit anti-inflammatory and antiitch activity. Arch Dermatol Res 2008; 300:569–574.
Journal of Hypertension
19. Theuwissen E, Plat J, Mensink RP. Consumption of oat beta-glucan with or without plant stanols did not influence inflammatory markers in hypercholesterolemic subjects. Mol Nutr Food Res 2009; 53:370– 376. 20. Queenan KM, Stewart ML, Smith KN, Thomas W, Fulcher RG, Slavin JL. Concentrated oat beta-glucan, a fermentable fiber, lowers serum cholesterol in hypercholesterolemic adults in a randomized controlled trial. Nutr J 2007; 6:6. 21. Andersson KE, Svedberg KA, Lindholm MW, Oste R, Hellstrand P. Oats (Avena sativa) reduce atherogenesis in LDL-receptor-deficient mice. Atherosclerosis 2010; 212:93–99. 22. Libby P, Ridker PM, Maseri A. Inflammation and atherosclerosis. Circulation 2002; 105:1135–1143. 23. Liu L, Zubik L, Collins FW, Marko M, Meydani M. The antiatherogenic potential of oat phenolic compounds. Atherosclerosis 2004; 175:39–49. 24. Singh U, Devaraj S, Jialal I. Vitamin E, oxidative stress, and inflammation. Annu Rev Nutr 2005; 25:151–174. 25. Ji LL, Lay D, Chung E, Fu Y, Peterson D. Effects of avenanthramides on oxidant generation and antioxidant enzyme activity in exercised rats. Nutr Res 2003; 23:1579–1590. 26. Ren Y, Yang X, Niu X, Liu S, Ren G. Chemical characterization of the avenanthramide-rich extract from oat and its effect on D-galactoseinduced oxidative stress in mice. J Agric Food Chem 2011; 59:206–211. 27. Chen CY, Milbury PE, Collins FW, Blumberg JB. Avenanthramides are bioavailable and have antioxidant activity in humans after acute consumption of an enriched mixture from oats. J Nutr 2007; 137:1375– 1382. 28. Nie L, Wise ML, Peterson DM, Meydani M. Avenanthramide, a polyphenol from oats, inhibits vascular smooth muscle cell proliferation and enhances nitric oxide production. Atherosclerosis 2006; 186:260– 266. 29. Katz DL, Nawaz H, Boukhalil J, Chan W, Ahmadi R, Giannamore V, Sarrel PM. Effects of oat and wheat cereals on endothelial responses. Prev Med 2001; 33:476–484. 30. Katz DL, Nawaz H, Boukhalil J, Giannamore V, Chan W, Ahmadi R, Sarrel PM. Acute effects of oats and vitamin E on endothelial responses to ingested fat. Am J Prev Med 2001; 20:124–129. 31. Katz DL, Evans MA, Chan W, Nawaz H, Comerford BP, Hoxley ML, et al. Oats, antioxidants and endothelial function in overweight, dyslipidemic adults. J Am Coll Nutr 2004; 23:397–403. 32. Brown L, Rosner B, Willett WW, Sacks FM. Cholesterol-lowering effects of dietary fiber: a meta-analysis. Am J Clin Nutr 1999; 69:30–42. 33. Nickenig G, Sachinidis A, Michaelsen F, Bo¨hm M, Seewald S, Vetter H. Upregulation of vascular angiotensin II receptor gene expression by low-density lipoprotein in vascular smooth muscle cells. Circulation 1997; 95:473–478. 34. Strazzullo P, Kerry SM, Barbato A, Versiero M, D’Elia L, Cappuccio FP. Do statins reduce blood pressure? A meta-analysis of randomized, controlled trials. Hypertension 2007; 49:792–798. 35. Strazzullo P, Ferro-Luzzi A, Siani A, Scaccini C, Sette S, Catasta G, Mancini M. Changing the Mediterranean diet: effects on blood pressure. J Hypertens 1986; 4:407–412. 36. Iacono JM, Puska P, Dougherty RM, Pietinen P, Vartiainen E, Leino U, et al. Effect of dietary fat on blood pressure in a rural Finnish population. Am J Clin Nutr 1983; 38:860–869. 37. Mancia G, Fagard R, Narkiewicz K, Redo´n J, Zanchetti A, Bo¨hm M, et al., Task Force Members. 2013 ESH/ESC Guidelines for the management of arterial hypertension: the Task Force for the management of arterial hypertension of the European Society of Hypertension (ESH) and of the European Society of Cardiology (ESC). J Hypertens 2013; 31:1281–1357. 38. Perk J, De Backer G, Gohlke H, Graham I, Reiner Z, Verschuren M, et al., European Association for Cardiovascular Prevention and Rehabilitation (EACPR); ESC Committee for Practice Guidelines (CPG). European Guidelines on cardiovascular disease prevention in clinical practice (version 2012). The Fifth Joint Task Force of the European Society of Cardiology and Other Societies on Cardiovascular Disease Prevention in Clinical Practice (constituted by representatives of nine societies and by invited experts). Eur Heart J 2012; 33:1635–1701.
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Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
923
Fibre intake and blood pressure: more facts, more questions Pasquale Strazzullo a and Rosalba Giacco b
See original paper on page 897
T
he meta-analysis on the effects of increased dietary fibre on blood pressure (BP), published in this issue of the Journal of Hypertension [1], calls for renovated attention on one of the many aspects of our dietary habits deemed to be relevant for cardiovascular prevention. Although Evans’ work is not the first comprehensive assessment of the possible relationship between increased fibre intake and BP, the 10 years that have elapsed since the two previous systematic reviews [2,3] on the same subject have seen the publication of several new reports that allowed Evans et al. to carry a more in-depth evaluation and, in particular, to try and dissect out the effects of different types of fibre in addition to their overall impact on BP. One merit of the review is that it is exclusively based on the analysis of randomized controlled trials, most of which rated as good or very good quality, whereas previous evidence suggesting that larger habitual consumption of dietary fibre is beneficial for human health, by reducing the risk of obesity, type 2 diabetes, hypertension and their cardiovascular complications, has mainly come from observational studies [4–8]. The most novel content of Evans’ work is the evidence in favour of a particularly beneficial effect on BP of b-glucans, a variety of soluble dietary fibre abundant in oats and barley-rich foods, whereas the authors found little or no statistical evidence of the BP impact of other types of dietary fibre. A strength of the study is that in all the five trials included in the meta-analysis, b-glucans were provided as oat bran, oat meals or directly as oat b-glucan-soluble powder in substitution for wheat-based products used as control; thus, it seems unlikely that the observed effect of the intervention could be attributable to other concomitant dietary changes.
Journal of Hypertension 2015, 33:921–923 a Department of Clinical Medicine and Surgery, Excellence Centre for Hypertension, Federico II University of Naples Medical School, Naples and bInstitute of Food Science, National Research Council, Avellino, Italy
Correspondence to Pasquale Strazzullo, Professor of Internal Medicine, Department of Clinical Medicine and Surgery, Excellence Centre of Hypertension, Federico II University of Naples Medical School, Via S. Pansini 5, Naples 80131, Italy. Tel: +39 081 7463686; fax: +39 081 5466152; e-mail: [email protected] J Hypertens 33:921–923 Copyright ß 2015 Wolters Kluwer Health, Inc. All rights reserved. DOI:10.1097/HJH.0000000000000569
Journal of Hypertension
It also seems unlikely that body weight reduction induced by higher fibre intake during the intervention may have played a major role because in the three (out of five) studies reporting weight changes, the variations were actually minor. Furthermore, in the other studies included in the meta-analysis, which tested the effects of different fibres (pectins, mannans, etc.), similar or even greater body weight losses were not associated with the beneficial effects on BP. Three of the five studies featuring higher intake of oat meals and b-glucans recruited individuals with BP in the high–normal range or having grade 1 hypertension, which may have theoretically made it easier to appreciate a BPlowering effect of these substances; however, a favourable BP response was detected in only two of these three studies and, in addition, it was detected also in one of the two trials involving only normotensive individuals [1]. Indeed, two studies not included in Evans’ systematic review reported an impact of oat consumption on BP. In a pilot study by Keenan et al. [9], SBP and DBP were both reduced after whole oat cereal intake for 6 weeks. In another study on drug-treated hypertensive patients, the consumption of oat cereal meals lead to a significant reduction in their need for antihypertensive medication compared with the control group [10]. A puzzling question is of course through which possible mechanisms do b-glucans or other components of oat-rich products favour a decrease in BP. At least three potential mechanisms may be envisaged: one or more pathways related to an overall improvement in glucose metabolism, an anti-inflammatory and/or antioxidant effect and an effect mediated by a reduction in plasma lipids. As the first possibility, b-glucans are known to delay gastric emptying, increase the viscosity of gastrointestinal content and increase satiety. The increased viscosity of the gastrointestinal content reduces food transit time and slows glucose absorption, thus diminishing the postprandial glucose and insulin responses [11]. A similar mechanism was operating in the Study to Prevent Non-Insulin-Dependent Diabetes Mellitus, involving individuals with impaired glucose tolerance who were assigned to treatment with either the oligosaccharide acarbose or the placebo. Treatment with acarbose was shown to reduce postprandial glucose and insulin responses by slowing starch digestion and www.jhypertension.com
Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
921
Strazzullo and Giacco
delaying glucose absorption, and was associated with a significantly lower incidence of hypertension and evidence of carotid atherosclerosis over 3.3 years compared with control treatment [12]. Although there is no direct experimental evidence of the beneficial effects of b-glucan consumption on insulin resistance, lower postprandial insulin levels may conceivably affect BP, given the direct influence of hyperinsulinaemia on renal sodium reabsorption and sympathetic tone [13,14]. It is true that in addition to b-glucans, other viscous fibres also, for example, guar and mannan, used in some of the trials included in Evans’ meta-analysis, have shown a reduction in postprandial blood glucose response and insulin levels – albeit with some variation in the data reported; it remains obscure why no apparent effect on BP could be detected in those studies. Evidence from both experimental and clinical studies strongly suggests that one or more oat components may exert anti-inflammatory and antioxidant activity, which in turn may have an impact on endothelial function [15]. These include tocopherols, phenolic compounds (namely, the oat-specific polyphenols avenanthramides), phytic acids, sterols and flavonoids [16]. Of practical interest is that most of the antioxidant activities associated with oats seem to be resistant to heat treatment and thus likely to survive the commonly applied industrial processing treatments [17]. The phenolic components avenanthramides also exhibit anti-inflammatory properties through inhibition of nuclear factor kB signalling [18]. Only few clinical studies, however, have investigated the possible effect of oats on systemic inflammation. Some of them found no effect, possibly because of the use of chemical treatments susceptible to induce the loss of the active phenolic substances [19,20]. In a study in low-density lipoprotein (LDL) receptor-deficient mice fed oat bran, it was found that the levels of the inflammation-related plasma proteins fibrinogen and soluble vascular cell adhesion molecule-1 and the expression of the vascular cell adhesion molecule-1 in the aortic wall were consistently reduced [21]. In this study, however, a substantial reduction in plasma cholesterol also occurred as an effect of higher oat bran intake, a response capable to hamper the inflammatory activity per se [22]; this makes it difficult to discriminate between a direct or a secondary effect of oat consumption. Nevertheless, these results are in keeping with another in-vitro study on human endothelial cells showing that avenanthramides isolated from oat were able to reduce the interleukin-1-induced expression of several adhesion molecules, concomitantly reducing monocyte attachment to the cells and lowering interleukin secretion [23]. The oat phenolic compounds and flavonoids are also thought to exert an antioxidant action by scavenging reactive oxygen and nitrogen species and chelating transition minerals. In in-vitro studies, tocopherols were incorporated into low-density lipoproteins and protected them from oxidative agents [24]. Avenanthramides exert direct antioxidative effects and seem to be capable to stimulate the human body scavenging system in animals [25,26] and humans [27]. Both low-grade systemic inflammation and increased oxidative stress inhibit nitric oxide production and contribute to endothelial dysfunction. Thus, the reports in the 922
www.jhypertension.com
literature suggesting that oats could favourably influence endothelial function are not surprising [28,29]. Specifically, these studies support the concept that oat components, by affecting the expression of endothelial nitric oxide synthase (eNOS), could increase the production of nitric oxide, thereby contrasting the negative effects of certain postprandial events such as excess insulin release, production of oxygen-free radicals and generation of triglyceride-rich lipoprotein particles – all contributing to endothelial dysfunction and possibly resulting in vasoconstriction and increased BP [30]. In this respect, Katz et al. [29,31] reported that long-term elevated oat consumption effectively opposes the failure in endothelial function following a high-fat meal in both normal weight and overweight individuals. A third mechanism whereby oat and, in particular, b-glucan intake might improve BP levels may be traced to their LDL-cholesterol-lowering action, clearly demonstrated by Brown et al. [32] in their meta-analysis; nevertheless, it might be argued that this effect is common to other types of viscous fibres such as guar, pectin or psyllium, the consumption of which was not associated with lower BP levels in Evans’ study. A seminal work by Nickenig et al. [33] revealed a significant upregulation of the angiotensin-I receptor gene expression by LDL in vascular smooth muscle cells, ultimately leading to an elevated functional response of vascular smooth muscle cells to angiotensin II stimulation, which could conceivably affect BP [33]. On the contrary, treatment of hypercholesterolaemia with statins has been associated with small but significantly lower BP levels [34]. Moreover, in two specular intervention trials of the metabolic and vascular effects of changes in dietary fat intake in normal volunteers, changes in either direction of total and LDL-cholesterol were consistently paralleled by BP changes in the same direction [35,36]. In conclusion, based on the evidence available from the clinical and experimental studies so far available, it is not possible to definitely confirm or exclude any of the abovementioned potential mechanisms nor is it possible to discriminate if and to what extent the decrease in BP observed upon increased oat-based products intake is attributable to any one of the various oat compounds, including b-glucans. Evans et al. noted that, although advice on increasing fruit and vegetable consumption is included in the European Society of Hypertension/European Society of Cardiology (ESC/ESH) Guidelines for the Management of Arterial Hypertension [37], no advice is given for fibre consumption. A large habitual fibre intake has recognized merits in particular with regard to prevention of obesity and type 2 diabetes and, as such, is recommended in most nutritional guidelines, including the European Guidelines on Cardiovascular Disease Prevention in Clinical Practice [38]. Evans’ updated review of the evidence about fibre intake and BP does not bring further substantial support to larger fibre intake as an effective means for reducing BP. It nevertheless may focus the attention of scientists and health operators active in the nutritional field on the properties of b-glucans and, in general, of oat and possibly barley-rich foods. These foods are relatively popular in the framework of the Volume 33 Number 5 May 2015
Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
Fibre intake and blood pressure
so-called Western and Nordic diets whereas they are not prevalent within the Mediterranean dietary model. On practical grounds, although definitive conclusions on their specific value cannot be drawn, a reasonable increase in their consumption, where particularly low, promises to do more good than harm.
ACKNOWLEDGEMENTS Conflicts of interest There are no conflicts of interest.
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