Cochrane meta-anaylisis about Diet & Cardiovascular Disease

A meta-analysis is a statistical method in which results of individual studies are mathematically combined in order to improve the reliability of the results. This technique presents enormous advantages over classical literature reviews. Meta-analyses also present weaknesses and they are fundamentally limited by the quality of the underlying studies. The Cochrane Collaboration is an international network that, using the meta-analysis method, analyzes randomized clinical trials (RCT) available on various subjects, and synthesizes them into regularly updated systematic reviews. The information found in Cochrane is considered by many to be the Gold Standard to evaluate drug therapy for treating disease.Additionally, meta-analysis of several small studies does not predict the results of a single large study, especially in a field like medicine where results are truly unpredictable [1]. It is debatable whether or not the Cochrane methodology is adequate for the nutrition questions in medical practice. It has been argued that more of the evidence base in nutrition is observational and a strict rule for excluding all non-RCT evidence would prevent many relevant studies from being included in the analysis [2]. Likewise, some argue that randomization would not be important for dietary experiments in which all participants are exposed to the same experimental diet [3]. Cochrane authors rebutted these criticisms by stating that cohort or non-randomized trials suffer the problems of bias and confounding [4]. Confounding arises because of clustering of dietary behaviors with other ‘healthy’ lifestyle attributes – those who eat less saturated fat or salt are more likely to be wealthier, better educated, have better social support, be non-smokers, take sufficient physical activity and eat plenty of fruit, vegetables, nuts, whole grains, low-fat dairy food, etc. Adjusting for a handful of these components does not guarantee complete control of such confounding. All the above can be summarized by saying that Cochrane meta-analysis are, undoubtedly, the most prestigious systematic reviews on the medical field. Here we sumarize some information compiled by Cochrane guys to debunk some diet myths and learn the truth about eating healthfully. However, caution must be exercised (as always) when drawing inferences from the results of these studies.

Reduced dietary salt for the prevention of cardiovascular disease (January,2011)
This review was aimed to 1)assess the long term effects of reducing dietary salt on mortality and cardiovascular morbidity and 2) investigate whether blood pressure reduction is an explanatory factor on mortality and cardiovascular outcomes.
Seven studies (including 6,489 participants) met the inclusion criteria – three in normotensives (n=3518), two in hypertensives (n=758), one in a mixed population of normo- and hypertensives (n=1981) and one in heart failure (n=232) with end of trial follow-up of seven to 36 months and longest observational follow up (after trial end) to 12.7 yrs. Relative risks for all cause mortality in normotensives and hypertensives showed no strong evidence of any effect of salt reduction. Cardiovascular morbidity in people with normal blood pressure or raised blood pressure at baseline also showed no strong evidence of benefit. Salt restriction increased the risk of all-cause death in those with congestive heart failure. The authors concluded that there is still insufficient power to exclude clinically important effects of reduced dietary salt on mortality or cardiovascular morbidity in normotensive or hypertensive populations. Further evidence is needed to confirm whether restriction of sodium is harmful for people with heart failure.(+)
Dietary advice for reducing cardiovascular risk (January, 2009)
Thirty-eight trials comparing dietary advice with no advice were included in the review. Dietary advice reduced total serum cholesterol and LDL cholesterol after 3-24 months. Mean HDL cholesterol levels and triglyceride levels were unchanged. Dietary advice reduced blood pressure and 24-hour urinary sodium excretion. Three trials reported small increases in plasma antioxidants. Compared to no advice, dietary advice increased fruit and vegetable intake by 1.25 servings/day. Dietary fiber intake increased with advice while total dietary fat ,as a percentage of total energy intake, fell by 4.49 % and saturated fat intake fell by 2.36 %.
Dietary advice did appear to be effective in bringing about modest beneficial changes in diet and cardiovascular risk factors over approximately 10 months but the trials did not manage to answer the question of whether the beneficial changes in cardiovascular risk factors resulted in a reduced incidence of heart disease, stroke, or heart attack. (+)
Reduced or modified dietary fat for preventing cardiovascular disease (July, 2011)
This updated review suggested that modifying dietary fat (not reducing fat from the diet) decreased the risk of cardiovascular events in trials of, at least, two years duration and in studies of men (not of women). However, there were no significant effects of dietary fat changes on total mortality (RR 0.98, 95% CI 0.93 to 1.04, 71,790 participants) or cardiovascular mortality (RR 0.94, 95% CI 0.85 to 1.04, 65,978 participants). (+)
Omega 3 fatty acids for prevention and treatment of cardiovascular disease (January, 2009)
Forty eight randomised controlled trials (36,913 participants) and 41 cohort analyses were included. Pooled trial results did not show a reduction in the risk of total mortality or combined cardiovascular events in those taking additional omega 3 fats
Restricting analysis to trials increasing fish-based omega 3 fats, or those increasing short chain omega 3s, did not suggest significant effects on mortality or cardiovascular events in either group. Subgroup analysis by dietary advice or supplementation, baseline risk of CVD or omega 3 dose suggested no clear effects of these factors on primary outcomes. (+)
Multiple risk factor interventions for primary prevention of coronary heart disease (January, 2011)
This review is an update of all relevant randomised trials that have evaluated an intervention that aimed to reduce more than one risk factor (high cholesterol, excessive salt intake, high blood pressure, excess weight, a high-fat diet, smoking, diabetes and a sedentary lifestyle) in people without evidence of cardiovascular disease. The findings are from 55 trials of between six months and 12 years duration conducted in several countries over the course of four decades. Multiple risk factor intervention does result in small reductions in risk factors including blood pressure, cholesterol and smoking. Contrary to expectations, multiple risk factor interventions had little or no impact on the risk of coronary heart disease mortality or morbidity. This could be because these small risk factor changes were not maintained in the long term. Alternatively, the small reductions in risk factors may be caused by biases in some of the studies. The methods of attempting behavior change in the general population are limited and do not appear to be effective.(+)

[1] LeLorier J et al. Discrepancies between Meta-Analyses and Subsequent Large Randomized, Controlled Trials. N Engl J Med (1997); 337:536-542 (Link)
[2] Shrier I et al. Should Meta-Analyses of Interventions Include Observational Studies in Addition to Randomized Controlled Trials? A Critical Examination of Underlying Principles. Am. J. Epidemiol. (2007) 166 (10): 1203-1209. (PDF)
[3] Truswell AS. Some problems with Cochrane reviews of diet and chronic disease. European journal of clinical nutrition (2005) 59 Suppl 1 (s1) p. S150-4 (PDF)
[4] Hooper L et al. Cochrane reviews on dietary advice for reducing intakes of fat and salt. European Journal of Clinical Nutrition (2006) 60, 926–928. (PDF)

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