Milk and dairy consumption and incidence of cardiovascular diseases and all-cause mortality: dose-response meta-analysis of prospective cohort studies

Abstract

Background: The consumption of dairy products may influence the risk of cardiovascular disease (CVD) and total mortality, but conflicting findings have been reported.

Objective: The objective was to examine the associations of milk, total dairy products, and high- and low-fat dairy intakes with the risk of CVD [including coronary heart disease (CHD) and stroke] and total mortality.

Design: PubMed, EMBASE, and SCOPUS were searched for articles published up to February 2010. Of > 5000 titles evaluated, 17 met the inclusion criteria, all of which were original prospective cohort studies. Random-effects meta-analyses were performed with summarized dose-response data. Milk as the main dairy product was pooled in these analyses.

Results: In 17 prospective studies, there were 2283 CVD, 4391 CHD, 15,554 stroke, and 23,949 mortality cases. A modest inverse association was found between milk intake and risk of overall CVD [4 studies; relative risk (RR): 0.94 per 200 mL/d; 95% CI: 0.89, 0.99]. Milk intake was not associated with risk of CHD (6 studies; RR: 1.00; 95% CI: 0.96, 1.04), stroke (6 studies; RR: 0.87; 95% CI: 0.72, 1.05), or total mortality (8 studies; RR per 200 mL/d: 0.99; 95% CI: 0.95, 1.03). Limited studies of the association of total dairy products and of total high-fat and total low-fat dairy products (per 200 g/d) with CHD showed no significant associations.

Conclusion: This dose-response meta-analysis of prospective studies indicates that milk intake is not associated with total mortality but may be inversely associated with overall CVD risk; however, these findings are based on limited numbers.

FIGURE 1.

Flow chart of the meta-analysis of dairy product consumption and incident cardiovascular disease (CVD) and all-cause mortality. The numbers of studies retrieved from 3 electronic databases [PubMed (http://www.ncbi.nlm.nih.gov/pubmed), EMBASE (http://www.embase.com), and SCOPUS (http://www.scopus.com)] are shown. Each box contains the number of studies found. The steps from title to abstract to full-text screening are indicated, as are the inclusion and exclusion criteria. Hand searches are indicated separately from the electronic search.

FIGURE 2.

Relation between milk (per 200 mL/d) and cardiovascular disease: dose-response meta-analyses of 4 prospective cohort studies (n = 13,518, no. of cases = 2283). Shown are author names, reference number, year of publication, country of study, and the size of the association per study expressed in squares (size of square indicates weight of the study to the overall meta-analysis); the horizontal lines indicate 95% CIs. The last 2 columns contain the actual estimated relative risks (RRs) and 95% CIs pooled across the categories of milk exposure with the generalized least-squares method and the actual weights. On the x axis, the RR is plotted with a line through the RR (= 1) that indicates no significant association between exposure and outcome. The diamond at the bottom indicates the pooled result, with the RR in the middle and the 95% CI. A test for heterogeneity, the Higgins and Thompson I-squared value, shows how much heterogeneity is due to between-study variation with a P value (if P < 0.05).

FIGURE 3.

Relation between milk (per 200 mL/d) and coronary heart disease: dose-response meta-analyses of 6 prospective cohort studies (n = 259,162, no. of cases = 4391). Shown are author names, reference number, year of publication, country of study, and the size of the association per study expressed in squares (size of square indicates weight of the study to the overall meta-analysis); the horizontal lines indicate 95% CIs. The last 2 columns contain the actual estimated relative risks (RRs) and 95% CIs pooled across the categories of milk exposure with the generalized least-squares method and the actual weights. On the x axis, the RR is plotted with a line through the RR (= 1) that indicates no significant association between exposure and outcome. The diamond at the bottom indicates the pooled result, with the RR in the middle and the 95% CI. A test for heterogeneity, the Higgins and Thompson I-squared value, shows how much heterogeneity is due to between-study variation with a P value (if P < 0.05).

FIGURE 4.

Relation between milk (per 200 mL/d) and stroke; dose-response meta-analyses of 6 prospective cohort studies (n = 375,381, no. of cases = 15,554). This figure includes the data from the Larsson study on high-fat milk. The pooled relative risk (RR) including Larsson's low-fat milk instead of high-fat milk is similar (RR: 0.87; 95% CI: 0.72, 1.05). Shown are author names, reference number, year of publication, country of study, and the size of the association per study expressed in squares (size of square indicates weight of the study to the overall meta-analysis); the horizontal lines indicate 95% CIs. The last 2 columns contain the actual estimated RRs and 95% CIs pooled across the categories of milk exposure with the generalized least-squares method and the actual weights. On the x axis, the RR is plotted with a line through the RR (= 1) that indicates no significant association between exposure and outcome. The diamond at the bottom indicates the pooled result, with the RR in the middle and the 95% CI. A test for heterogeneity, the Higgins and Thompson I-squared value, shows how much heterogeneity is due to between-study variation with a P value (if P < 0.05).

FIGURE 5.

Relation between milk (per 200 mL/d) and all-cause mortality: dose-response meta-analyses of 8 prospective cohort studies (n = 62,779, no. of cases = 23,949). Shown are author names, reference number, year of publication, country of study, and the size of the association per study expressed in squares (size of square indicates weight of the study to the overall meta-analysis); the horizontal lines indicate 95% CIs. The last 2 columns contain the actual estimated relative risks (RRs) and 95% CIs pooled across the categories of milk exposure with the generalized least-squares method and the actual weights. On the x axis, the RR is plotted with a line through the RR (= 1) that indicates no significant association between exposure and outcome. The diamond at the bottom indicates the pooled result, with the RR in the middle and the 95% CI. A test for heterogeneity, the Higgins and Thompson I-squared value, shows how much heterogeneity is due to between-study variation with a P value (if P < 0.05).

FIGURE 6.

Funnel plot for studies of the association between milk intake and all-cause mortality to illustrate the presence of publication bias (n = 62,779; no. of cases = 23,949). Each dot indicates one study with its size and its relative risk (RR). The y axis contains the SEs of the log (RR). The smaller the SE, the larger the study. The RRs are plotted on the x axis. The lines are drawn around the dots to visualize how symmetrically the studies are divided around the true estimate; symmetry indicates no evidence of publication bias.