Diet quality in nutrition research is frequently characterized by how closely the dietary pattern adheres to either a “Mediterranean” or a “Western” pattern. The Mediterranean diet is characterized as one that is rich in olive oil, fruit and vegetables, legumes, nuts, and seafood; moderate in alcohol intake; and low in intakes of red meats and saturated fats. This is in contrast to the “Western” diet typified by the consumption of sugar-sweetened soft drinks, processed meats, and refined grains. Although broadly understood to be “healthy,” the high oil content of the Mediterranean diet places it in a position of conflict with current public health dietary recommendations. Current UK guidelines advise that oils and spreads be used “in small amounts” (1). In the United States, although the 2015–2020 Dietary Guidelines includes a section on a “healthy Mediterranean-style eating pattern,” this document recommends consuming <2 tablespoons of oils per day (2). This is in contrast to the higher reported habitual intakes of monounsaturated and polyunsaturated oils among those in Mediterranean regions (3). The Mediterranean diet is associated with a reduced risk of mortality, cardiovascular diseases, cancer, neurodegenerative diseases, and diabetes (4). The increasing incidence of obesity and metabolic syndrome among Western populations places a significant burden upon both individual health and health care systems. There is therefore significant interest in identifying whether dietary interventions can be an effective tool in reducing noncommunicable disease risk or optimizing health status. As any specific bioactive nutrients or components within the Mediterranean diet are identified, there may be the potential to reduce mortality or disease risk through targeted dietary interventions or recommendations. It would be of particular value if the simple addition of target foods could confer benefits among those following an otherwise “Western” diet, particularly within established at-risk groups, such as those with features of metabolic syndrome. As a result, studies of walnuts and walnut oil have been conducted to assess whether short- or long-term regular consumption might confer health benefits. Studies to date have reported beneficial effects with the potential to mitigate the incidence of obesity and metabolic syndrome. The acute consumption of walnuts has been shown to significantly increase postmeal satiety (5), with evidence that these effects may be exerted through changes to postprandial gut hormones (6) or adipokines (7). Longer-term dietary interventions have also altered markers of cardiovascular disease risk, such as favorable changes in blood lipid profiles (8, 9), apolipoprotein B (10), and measures of endothelial function (11, 12). However, it is clear that the design of such studies and the characteristics of the cohort assessed significantly influence the results observed, with null findings among studies providing lower daily nut intakes (13) or in interventions conducted in obese participants (14, 15). Walnuts contain a number of nutrients and components that may underpin their observed health effects. Walnuts are a rich source of PUFAs and MUFAs, but are also a source of protein, phytochemicals, fiber, and minerals. Each of these features may confer distinct outcomes, or there may be additive or synergistic health effects arising from the multiple components. For example, the indigestible fibers contained within walnuts may have a role in influencing the composition of the gut microbiota. Observational studies have identified that broader dietary patterns influence the composition of the gut microbiota, with clear differences between those following a Western diet typified by the intake of animal protein and saturated fats when compared with those following a mainly vegetarian dietary pattern that is richer in carbohydrates (16). Given that there are significant differences between the gut microbiota of individuals with obesity or metabolic syndrome and that of healthy controls (17), the question arises as to whether modifying the pattern of foods within an individual's diet can induce changes in the microbiome and thereby confer health benefits. Nutrition intervention studies that aim to influence the microbiome have tended to focus on foods with a known direct effect on the gut microbiota, such as probiotics or prebiotics, typically provided as supplements or within fortified foods. In the current issue of the Journal, Holscher et al. (18) have investigated the effect of walnut consumption on both serum lipid profiles and the gut microbiota. In this study, participants were provided with 42 g of walnut pieces/d over a 3-wk period, and serum and fecal samples were collected before and after treatment. This intervention led to significant changes in the abundance of bacterial genera, including Faecalibacterium and Bifidobacteria and associated reductions in LDL cholesterol. It is of particular interest that the healthy population studied in this US cohort could be described as a profile of a presymptomatic “at-risk” Western population, with an average age of 53 y and a BMI (kg/m2) of 29, but with blood pressure measurements and plasma lipid concentrations within normal ranges. These observations are supported by available evidence that dietary interventions targeting the gut microbiota can alter blood lipid profiles, with several meta-analyses indicating that probiotics can induce significant reductions in LDL cholesterol (19, 20). One mechanism by which the gut microbiome may influence blood cholesterol concentrations is via the action of gut bacteria upon bile acids (21). The work by Holscher et al. (18) has identified that walnut consumption resulted in lower concentrations of toxic secondary bile acids and that these changes were correlated to the changes observed in the gut microbiota. This, therefore, provides a plausible mechanism by which the consumption of walnuts may provide benefits to blood lipid profiles beyond those that can be attributed to the increased consumption of unsaturated oils within the nut. The integration of microbiome analysis within nutrition science research will be fundamental to ensuring our full understanding of the complex and synergistic effects that foods or dietary patterns can have on human health. This work by Holscher et al. (18) provides a fascinating example of a study design that interrogates both the direct and indirect health effects arising from foods consumed. Further studies will be required for a full exploration of the translational value of such findings in improving individual health or as a potential public health message that could mitigate the broader burden of noncommunicable diseases, such as cardiovascular disease and metabolic syndrome. Ultimately, ensuring that any review of dietary advice given in public health messaging around the consumption of oils or foods rich in polyunsaturated oils is informed by the scientific evidence will depend on the effective and timely communication of nutrition science to policymakers. Acknowledgments The sole author had responsibility for all parts of the manuscript. Notes Supported by HOST Therabiomics Ltd. Author disclosures: CEC discloses current research funding from HOST Therabiomics. References 1. Public Health England in association with the Welsh Government, Food Standards Scotland and the Food Standards Agency in Northern Ireland . The Eatwell guide . Public Health England ; 2016 . Contract No.: 9 August 2016. [Accessed May 3, 2018]. Available from: https://www.gov.uk/government/publications/the-eatwell-guide . 2. U.S. Department of Health and Human Services and U.S. Department of Agriculture . 2015-2020 Dietary Guidelines for Americans . 8th ed . 2015 . [Accessed May 3, 2018]. Available from: http://health.gov/dietaryguidelines/2015/guidelines/ . 3. Serra-Majem L , Ngo de la Cruz J , Ribas L , Tur JA . Olive oil and the Mediterranean diet: beyond the rhetoric . Eur J Clin Nutr 2003 ; 57 : S2 – 7 . Google Scholar CrossRef Search ADS PubMed 4. 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An acute intake of a walnut-enriched meal improves postprandial adiponectin response in healthy young adults . Nutr Res 2013 ; 33 : 1012 – 8 . Google Scholar CrossRef Search ADS PubMed 8. Zibaeenezhad MJ , Farhadi P , Attar A , Mosleh A , Amirmoezi F , Azimi A . Effects of walnut oil on lipid profiles in hyperlipidemic type 2 diabetic patients: a randomized, double-blind, placebo-controlled trial . Nutr Diabetes 2017 ; 7 : e259 . Google Scholar CrossRef Search ADS PubMed 9. Torabian S , Haddad E , Cordero-MacIntyre Z , Tanzman J , Fernandez ML , Sabate J . Long-term walnut supplementation without dietary advice induces favorable serum lipid changes in free-living individuals . Eur J Clin Nutr 2010 ; 64 : 274 – 9 . Google Scholar CrossRef Search ADS PubMed 10. Wu L , Piotrowski K , Rau T , Waldmann E , Broedl UC , Demmelmair H , Kolezko B , Stark RG , Nagel JM , Mantzoros CS et al. Walnut-enriched diet reduces fasting non-HDL-cholesterol and apolipoprotein B in healthy Caucasian subjects: a randomized controlled cross-over clinical trial . Metabolism 2014 ; 63 : 382 – 91 . Google Scholar CrossRef Search ADS PubMed 11. Ros E , Núñez I , Pérez-Heras A , Serra M , Gilabert R , Casals E , Deulofeu R . A walnut diet improves endothelial function in hypercholesterolemic subjects: a randomized crossover trial . Circulation 2004 ; 109 : 1609 – 14 . Google Scholar CrossRef Search ADS PubMed 12. Ma Y , Njike VY , Millet J , Dutta S , Doughty K , Treu JA , Katz DL . Effects of walnut consumption on endothelial function in type 2 diabetic subjects: a randomized controlled crossover trial . Diabetes Care 2010 ; 33 : 227 – 32 . Google Scholar CrossRef Search ADS PubMed 13. Din JN , Aftab SM , Jubb AW , Carnegy FH , Lyall K , Sarma J , Newby DE , Flapan AD . Effect of moderate walnut consumption on lipid profile, arterial stiffness and platelet activation in humans . 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Google Scholar CrossRef Search ADS PubMed 21. Ridlon JM , Kang DJ , Hylemon PB , Bajaj JS . Bile acids and the gut microbiome . Curr Opin Gastroenterol . 2014 ; 30 : 332 – 8 . Google Scholar CrossRef Search ADS PubMed © 2018 American Society for Nutrition. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)
Journal of Nutrition – Oxford University Press
Published: Jun 7, 2018
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