Mediterranean Diet for Primary Prevention of Cardiovascular Disease

The n e w e ng l a n d j o u r na l of m e dic i n e c or r e sp ondence Mediterranean Diet for Primary Prevention of Cardiovascular Disease To t...
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Mediterranean Diet for Primary Prevention of Cardiovascular Disease To the Editor: The interventional nutritional study by Estruch et al. (April 4 issue)1 is limited by low primary composite cardiovascular outcome rates (3.8% and 3.4% in the intervention groups vs. 4.4% in the control group) with minor absolute risk differences (range, 0.6 to 1%). Given these small margins of risk, there were at least four statistically significant differences in baseline characteristics between the groups, which could contribute substantially to these minor absolute differences in risk. There were significantly higher percentages of women (+5.7%), obese persons (+4.7%), diuretic use (+3.5%), and oral hypoglycemic use (+3.2%) in the control group than in the intervention group assigned to a Mediterranean diet supplemented with nuts. The implication of these baseline discrepancies may be reflected in the Kaplan–Meier survival curves for the primary composite cardiovascular outcome. The initial differences in risk

between the intervention and control groups, starting at time zero, when no apparent difference in risk would be expected, particularly for a dietary intervention,2 may have contributed substantially to the overall minor cumulative absolute differences in risk. Eran Kopel, M.D., M.P.H. Yechezkel Sidi, M.D. Shaye Kivity, M.D. Chaim Sheba Medical Center Tel Hashomer, Israel [email protected] No potential conflict of interest relevant to this letter was reported. This letter was updated on March 13, 2014, at NEJM.org. 1. Estruch R, Ros E, Salas-Salvadó J, et al. Primary prevention

of cardiovascular disease with a Mediterranean diet. N Engl J Med 2013;368:1279-90. 2. Cook NR, Cutler JA, Obarzanek E, et al. Long term effects of dietary sodium reduction on cardiovascular disease outcomes: observational follow-up of the Trials of Hypertension Prevention (TOHP). BMJ 2007;334:885-8. DOI: 10.1056/NEJMc1306659

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To the Editor: In the PREDIMED trial (Prevención con Dieta Mediterránea), a modified Mediterranean diet supplemented with extra-virgin olive oil or nuts was reported to have major cardiovascular benefits. There was a specific benefit on the primary end point in the subgroup with dyslipidemia and the subgroup with hypertension. Therefore, the reduction in levels of lowdensity lipoprotein (LDL) cholesterol and in blood pressure could help to explain the outcome benefits. Extra-virgin olive oil, even in much smaller amounts (30 ml per day) than that used in the study (1 liter per week), delays gastric emptying and decreases postprandial hyperglycemia.1 Extravirgin olive oil is high in monounsaturated fatty acids, which reduce blood pressure and, in persons with type 2 diabetes, reduce glycated hemo-

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globin levels.2 Eating tree nuts, such as almonds, reduces LDL cholesterol levels by 3 to 19%.3 Mechanistically, it is therefore highly relevant to know the initial and final LDL cholesterol and bloodpressure levels in the two treatment groups. Alternatively, the apparent benefits of the modified Mediterranean diets, which were high in olive oil or nuts, could reflect the adverse qualities of the control diet, which was low in these beneficial foods. Is this interpretation not possible? Lionel H. Opie, M.D., D.Phil. Hatter Institute for Cardiovascular Research in Africa Cape Town, South Africa [email protected] No potential conflict of interest relevant to this letter was reported. 1. Gentilcore D, Chaikomin R, Jones KL, et al. Effects of fat on

gastric emptying of and the glycemic, insulin, and incretin responses to a carbohydrate meal in type 2 diabetes. J Clin Endocrinol Metab 2006;91:2062-7. 2. Schwingshackl L, Hoffmann G. Monounsaturated fatty acids and risk of cardiovascular disease: synopsis of the evidence available from systematic reviews and meta-analyses. Nutrients 2012;4:1989-2007. 3. Berryman CE, Preston AG, Karmally W, Deckelbaum RJ, Kris-Etherton PM. Effects of almond consumption on the reduc-

tion of LDL-cholesterol: a discussion of potential mechanisms and future research directions. Nutr Rev 2011;69:171-85. DOI: 10.1056/NEJMc1306659

To the Editor: The PREDIMED trial reported that a Mediterranean diet supplemented with extra-virgin olive oil or nuts reduced cardiovascular risk. Since the study was conducted in Spain, many participants were already following Mediterranean-type diets, which means that the achieved dietary changes were modest. Are the results too good to be true? The Global Burden of Diseases, Nutrition, and Chronic Diseases Expert (NUTRICODE) Group systematically reviewed evidence for causal effects of dietary factors on chronic diseases and obtained estimates of quantitative effects, largely based on observational cohorts.1-3 The reductions in risk achieved in PREDIMED, a study of dietary patterns, are consistent with the predicted benefits calculated from observational data on individual dietary components reported by the NUTRICODE Group (Table 1). Fewer cardiovascular events were apparent

Table 1. Observed and Predicted Effects in the Mediterranean-Diet Groups of Supplementation with Extra-Virgin Olive Oil or Nuts on Myocardial Infarction. Dietary Factor

PREDIMED Change Achieved with Mediterranean Diet with EVOO

NUTRICODE

Change Achieved with Mediterranean Diet with Nuts

Estimated Effect on MI (RR)*

(g/day)

Predicted Predicted Predicted Effect on Effect on Effect on Serving MI with MI with MI for Groups Size EVVO (RR) Nuts (RR) Combined (RR) (g/day)

Nuts

3.3

21.0

0.89

28.35

0.987

0.917

0.951

Vegetables

1.8

10.0

0.93

100

0.999

0.993

0.996

Legumes

2.4

2.4

0.93

100

0.998

0.998

0.998

Fruits

6.3

12.5

0.91

100

0.994

0.988

0.991

Marine n–3 fatty acids

0.11

0.12

0.94†

 0.100

0.934

0.928

0.931

EVOO (% energy)

5.0

1.1

0.9

5.00

0.891

0.975

0.932

Predicted overall effect by NUTRICODE

0.814

0.813

0.814

Observed effect in PREDIMED

0.80

0.74

0.77

* In reviewing the reported dietary changes in the PREDIMED (Prevención con Dieta Mediterránea) trial, the NUTRICODE (Global Burden of Diseases, Nutrition, and Chronic Diseases [GBDNG] Expert) Group1–3 found evidence of causal effects on myocardial infarction (MI) with the consumption of fruits, vegetables, nuts, and marine n–3 fatty acids. For most factors, the magnitudes of effect were quantified with meta-analyses of multivariable-adjusted observational cohorts. Our analyses did not identify sufficient studies to confirm additional effects of legumes or extra-virgin olive oil (EVOO), key dietary changes in PREDIMED. To enable comparison, the potential effects of legumes were imputed from those of vegetables and the potential effects of EVOO from those of polyunsaturated vegetable fats. Given the smaller numbers of available prospective studies, the effects of key dietary factors (e.g., nuts) on stroke could not be quantified, making it difficult to estimate the observed effects as compared with the predicted effects on stroke in PREDIMED. RR denotes relative risk. † In calculating the estimated effect of dietary n–3 fatty acids on MI, we assumed that half of MIs were fatal, since the NUTRICODE estimated risk reduction for marine n–3 fatty acids was specific for fatal MI, not nonfatal MI.

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within 1 year, which is consistent with rapid changes in risk factors when dietary quality is altered in controlled feeding studies and in populations when nutrition trends shift.4 These benefits — in the PREDIMED trial, long-term observational cohorts, and short-term feeding interventions — are largely independent of changes in adiposity (i.e., they are primarily due to dietary composition). The consistency of the results of the PREDIMED trial with prior evidence confirms the crucial importance of dietary quality — focused on healthful foods and dietary patterns rather than single nutrients — for cardiovascular disease. Dariush Mozaffarian, M.D., Dr.P.H. Brigham and Women’s Hospital Boston, MA [email protected] Dr. Mozaffarian reports receiving research grants from GlaxoSmithKline, Sigma Tau Pharmaceuticals, and Pronova Biopharma; travel reimbursement or honoraria from Bunge, the Pollock Institute, Quaker Oats, and the Life Sciences Research Organization; consulting fees from Foodminds and Nutrition Impact; and royalties from UpToDate; and reports being a member of the Unilever North America Scientific Advisory Board. No other potential conflict of interest relevant to this letter was reported.

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than a chance increase in the rate of stroke among controls, the protective mechanisms need to be explained. A possible lack of adherence after initial compliance was addressed and ruled out. In contrast with the findings by Estruch et al., a large secondary prevention trial has shown that the protection conferred by a Mediterranean diet was maintained for up to 4 years.1 It would be shortsighted not to recognize the potential public health benefits of a Mediterranean diet; unfortunately, the current study’s apparent limitations do not provide further support for the case. Rudolf Hoermann, M.D., Ph.D. Mathis Grossmann, M.D., Ph.D. University of Melbourne Heidelberg, VIC, Australia [email protected] No potential conflict of interest relevant to this letter was reported. 1. de Lorgeril M, Salen P, Martin JL, Monjaud I, Delaye J, Mam-

elle N. Mediterranean diet, traditional risk factors, and the rate of cardiovascular complications after myocardial infarction: final report of the Lyon Diet Heart Study. Circulation 1999;99:77985. DOI: 10.1056/NEJMc1306659

1. Micha R, Kalantarian S, Wirojratana P, et al. Estimating the

global and regional burden of suboptimal nutrition on chronic disease: methods and inputs to the analysis. Eur J Clin Nutr 2012;66:119-29. 2. Khatibzadeh S, Micha R, Afshin A, Rao M, Yakoob MY, Mozaffarian D. Major dietary risk factors for chronic diseases: a systematic review of the current evidence for causal effects and effect sizes. Circulation 2012;125:Suppl:AP060. abstract. 3. Lim SS, Vos T, Flaxman AD, et al. A comparative risk assessment of burden of disease and injury attributable to 67 risk factors and risk factor clusters in 21 regions, 1990-2010: a systematic analysis for the Global Burden of Disease Study 2010. Lancet 2012;380:2224-60. [Erratum, Lancet 2013;381:1276.] 4. Mozaffarian D, Appel LJ, Van Horn L. Components of a cardioprotective diet: new insights. Circulation 2011;123:2870-91. DOI: 10.1056/NEJMc1306659

To the Editor: Estruch et al. conclude that a Mediterranean diet reduced the incidence of major cardiovascular events. However, this reduction was attributable only to a single event class — stroke — and the difference in event rates was apparent only early in follow-up. When events occurring in the first year were excluded in a supplementary sensitivity analysis, this difference was no longer significant. Remarkably, the trial was terminated after a median follow-up of just 4.8 years. Assuming that this early difference was due to a protective effect of the diet, rather

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To the Editor: Estruch et al. report that the 2425 participants with type 2 diabetes in the two groups assigned to the Mediterranean diet had the same rate of reduction in cardiovascular events as the nondiabetic population. In September 2012, the last patient in our own interventional trial,1 which compared the effects of a Mediterranean diet (108 participants) with those of a low-fat diet (107 participants) in a population with newly diagnosed type 2 diabetes, reached the primary end point (the need for an antihyperglycemic drug), with a follow-up of 8 years. The mean (±SD) values for the macronutrient composition of the Mediterranean diet (43.7±6.1% carbohydrate, 18.5±2.7% protein, and 37.8±5.1% total fat) were similar to those in the study by Estruch et al. The rate of regression in the intima– media thickness of the carotid artery2 was higher and the rate of progression lower in the Mediterranean-diet group as compared with the low-fat diet group; moreover, adiponectin levels were higher and the homeostasis model assessment of insulin resistance was lower (indicating higher insulin sensitivity) in the Mediterraneandiet group (Fig. 1). In addition to reducing the

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B Progression of Intima–Media Thickness

100

100

75

75 P=0.03

Percent

Percent

A Regression of Intima–Media Thickness

50

25

0

P=0.009 50

25

Mediterranean Diet

C Adiponectin 12

0

Low-Fat Diet

Mediterranean Diet

Low-Fat Diet

D HOMA Assessment of Insulin Resistance 8

P

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